15 replies to this topic

[Auscar]

Hi all,

 

Looking to re-tune my single su carb for more "Poweerrrr!!!" as new parts have been fitted.

 

Looking for a professional to do it with a rolling road, could anyone recommend someone in the Bournemouth area?

 

or am I being completely dramatic and its a 5 min job you can do by yourself with a screw driver?

[Stiggytoo]

What have you done to the engine? Stage one kit? 

Obviously, to get the best out of it, you'll want to rolling road it, however if you are fairly competent you can get pretty close and save yourself some money.

[KernowCooper]

If its just a standard engine then you just carry out a engine tune and adjust the mixture once all the other items have been done, do you have a workshop manual, you wont get more power by tuning the carb, only what power the engine capable of producing when correctly set up.

[Auscar]

its stage 1 kitted, gonna swap out the needle i think is original for a richer mix one AAA. car is running but i think a tune will really help economy.

 

unfortunately not, the only resources i have/depend on is this forum and a battered haynes manual i use for the wiring diagrams and finding where bolts are hidden.

 

would prefer to rolling road ideally to see the difference, just wanting to find out if anyone could recommend someone. found there are alot of people who THINK they are "experts" on minis, looking to filter out those cowboys.

Edited by Auscar, 13 April 2015 - 08:27 PM.

[ACDodd]

Its a long response but;

 

Many, many vehicles have been fitted with SU (Skinners Union) carbs over the years, and probably the most common being the mini with the beautifully simple A-series engine in all its many guises.  These carbs are quite rightly used in standard and not so standard performance applications which, providing that the instruments are modified to suit that new performance use, provide years of flat-spot and trouble free motoring.

 

The most common SU carb in use today on the mini, is the HIF carb, followed by the HS, and the Twin setups.  HIF’s being more common due to there standardised fitment on most minis from  ’91 – ’92.

 

My day to day workings with the SU carb has given me experience of many problems and common mistakes associated with the use/miss-use of these carbs on different cars (mainly minis). After all these years of magazine articles and the endless needs of owners asking how things are done, I have decided to put pen to paper and write down what the necessary tricks and tweaks that need to be carried out when using SU carbs on your road cars. These are even more important, when we are considering the use of these carbs for performance (Non-competition) use.

All the mods and suggestions here are aimed at maximum performance whilst retaining very good drivability, and the all important emissions and fuel consumption. Any carburettor install for the road needs to be able to deliver legal exhaust emission standards, good fuel economy and very good performance.

 

Needless to say, any carb used on your engine must be in new, near new or freshly rebuilt condition if you expect to get anywhere near its best potential. This is especially applicable to twin setups where even the most seemingly insignificant wear can cause all sorts of running problems.

 

Sizing of SU carbs is not really to much of an issue, people do tend to get hung up on sizing thinking that to large a carb will be detrimental, well, this is not necessarily so. When we think of webers (DCOE’s) for example, they have no means of altering their choke size. Ie they are fixed. They have to have a choke that is large enough to give the required airflow for an engines given maximum air demand at maximum power. This leads to a carb which at lower engine speeds the choke size is far to large to maintain air speed and promote inertial ramming at lower speeds. Not to mention the lack of airspeed over the jets. This means a poorer cylinder fill and less low end torque than that of a carb where by we have a variable choke (good old SU). The choke size on the SU carb adjusts to the engines air demand and maintains the optimum size (when calibrated correctly) right across the rev range. This ensures a high gas speed and good fuel pickup from the jet giving better fuel metering at lower Rpms leading to better fuel economy and part throttle running.

 

Horsepower is a good way of determining carb size, but it does not show the whole picture. A better guide would be if; gear ratio, and capacity were taken into account. In this context vehicle weight would have an effect too, but for simplicity and as the engine ‘Sees’ weight via the reduction effect of gearing it can be disregarded as we are using gearing as a factor in our table.

 

What I have below (Table1) is not a mathematical table showing good carb sizes but one that works the best (in my experience) giving the best overall performance, by performance I mean, the amount you get pushed in the seat when you put your foot down and the economy/emissions. Ie the best all rounder. This table is for Single SU applications. The column headed tune, refers to the power the engine has been built to produce, not what it would produce if the stated carb were fitted.

Engine size                   Tune                 Gearing (final drive)                   Carb

                                                                                               

848cc                           Standard                       3.765:1                         HS4, HIF38

upto +0.060

 

                                    40hp                             3.44:1                           HS4, HIF38

                                                                        3.765:1                         HS4, HIF38

                                                                        3.93:1                           HS4, HIF38

 

                                    50hp                             3.93:1 and lower            HIF44, HS6

 

                                    60hp                             3.93:1and lower             HIF44, HS6

 

998cc

upto +0.060”                 Standard                       3.44:1                           HS4,HIF38

 

                                    50hp Stage 1 kit             2.95 or 3.105:1             HS4, HIF38

                                   

55hp and up                  3.44:1 or lower             HIF44 or HS6

 

1098cc

upto +0.060”                 Standard                       3.44:1                           HS4, HIF38

 

                                    55hp Stage 1                2.76 to 3.105     :1         HS4 to HIF38

                                   

                                    55hp stage 1                 3.44:1 and lower            HIF44 or HS6

 

1275cc

upto 1330cc                  Standard 50hp              2.76 to 3.105     :1         HIF38 or HS4

 

                                    60hp Stage 1                Any FD                         HIF44 or HS6

 

                                    upto 80hp                     2.76 to 3.21:1                HIF44 or HS6

 

1380cc and up             upto 80hp                     2.76 to 2.95:1                HIF44 or HS6

 

 

Ok from this you will probably say, what has gearing got to do with SU carb sizing, good question, well presented.

 

The carbs airflow potential has little to do with the real world driving experience. Sure a HIF44 carb will let an appropriate A-series engine produce in excess of 100hp @ flywheel, but it is far from the optimum carb for the use of that engine, here is why;

 

Think back to when we were talking about variable chokes. It takes time for the variable choke (or piston as it is in the SU) to rise once the throttle is opened. An engine with X Torque with high gearing like a 2.76:1 FD will not be able to accelerate a car of given weight as the same car with a lower final drive. Now there comes a point when the final drive is sufficiently low enough that in the lower gears,1^st, 2^nd etc, that the piston cannot rise fast enough to allow the engine to accelerate at its full potential as suggested by its power/torque figures. This slow rising piston ‘holds back’ the engine stunting its performance in the lower gears.

This is why the power figures look low in the table above. To get around this we remove the piston from the equation, ie fit a weber, or get to grips with getting the best from the SU. To understand how to do this we need to understandard how the SU works when correctly setup.

 

When operating correctly the air flowing through the carb causes a low pressure area above the piston this causes it to rise and draw the needle from the jet. The spring, within the suction chamber assembly, acting on the piston, purely controls the piston position at any given air demand. It should be weighted such that at full power it allows the needle to be lifted to its highest position. Regardless of the power of the engine it is fitted to. As the needle is the correct one in our example, this needle allows the correct amount of fuel to be drawn off from the jet to fuel the engine. When correctly setup this is what happens at any steady state speed at any rpm and at any combination of throttle opening.  As the SU does not have a accelerator pump jet extra enrichment during acceleration is catered for by slowing the piston rise when the throttle is opened. This retardation in the piston rise causes the amount of ‘Pull’ on the fuel leaving the jet to increase and thus more fuel is drawn off and this enriches the mixture during the acceleration phase.   Common misunderstanding is that to gain greater performance you can thin out the oil within the dash pot to allow the piston to rise faster and gain greater acceleration. While true to a point it misses out the fundamental point of fuel economy.

In order to gain the best fuel economy and performance, the needle used must be chosen such that at any steady state running point it is fuelling the engine correctly. Enrichment during acceleration is purely catered for by the Damper assembly within the carb. Modifying the damper by thinning the oil for example in this case would mean that the amount of enrichment would be reduced during acceleration and possibly cause a flat spot. Now using this thinner oil coupled with a richer needle would mean that the fuel enrichment balance during acceleration would now be restored and because the piston rise is quicker we will notice an increase in the acceleration when we put our foot down. This is what usually happens when most minis are rolling road tuned. The needle is a little too rich during normal running and about perfect during acceleration. This is great if you don’t mind paying for the extra fuel!!!

10mpg less is common for this way of setting the SU up! 

 

So from this it can be seen that if the ultimate all round performance is needed then if you are suffering with the damper holding you back when accelerating in the lower gears you either need a bigger carb or more than one, and that’s where twins really come into there own. Below is table 2 for twin carb setups.

 

 

Engine size                   Tune                 Gearing (final drive)                   Carb

                                                                                               

848cc                           Standard                       3.765:1                         Twin H1’s

upto +0.060

 

                                    40hp                             3.44:1                           Twin H1’s

                                                                        3.765:1                         Twin HS2’s

                                                                        3.93:1                           Twin HS2’s

 

                                    50hp                             3.93:1 and lower            Twin HS2’s

 

                                    60hp                             3.93:1and lower             Twin HS2’s

 

998cc

upto +0.060”                 Standard                       3.44:1                           Twin HS2’s

 

                                    50hp Stage 1 kit             2.95 or 3.105:1             Twin HS2’s

                                   

55hp and up                  3.44:1 or lower              Twin HS2’s

 

 

 

 

 

 

1098cc

upto +0.060”                 Standard                       3.44:1                           Twin HS2’s

 

                                    55hp Stage 1                2.76 to 3.105     :1         Twin HS2’s

                                   

                                    55hp stage 1                 3.44:1 and lower            Twin HS2’s

 

                                    65hp                             3.44 and lower               Twin HS2’s

 

                                    75hp                             3.44 and lower               Twin HS4’s

 

1275cc

upto 1330cc                  Standard 50hp              2.76 to 3.105     :1         Twin HS2’s

 

                                    upto 75hp                     Lower than 3.105:1        Twin HS2’s

 

                                    upto 100hp                   Any FD                         Twin HS4’s

 

1380cc and up             70hp to 100hp               Any FD                         Twin HS4’s.

 

 

 

Now we have sizes sorted out, we need to think about calibrating the carbs correctly.

 

There are 4 things to consider about your correctly sized carbs in order to get them dialled in correctly, regardless of size or the number fitted;

 

1. Correct Damper
2. Correct spring
3. Correct needle
4. Correct oil.

 

 

1. Dampers, basically very straightforward.

 

Piston dampers need to be of the fast drop type. Easy ID of the HIF type. Part Number LZX 2085. This is the only damper that should be fitted.

 

Incidently, this is a good point to mention, it is very worthwhile mod on and HS4 or HS6 carb. That is, to use the HIF ball bearing suction chamber and damper assembly on the older carb. In doing this mod is endows the older HS carbs with the same fast drop characteristics of the later HIF carbs when fitted with the LZX 2085 Damper rod.

The reason for having ‘Fast drop’ piston is to allow the piston to drop during gear changes. If the piston does not drop fast enough during a gear change the needle will not be in the correct position to fuel the engine when the throttle is opened again after the next gear is selected. The result of this is a huge flat spot between gear changes.

 

Dampers can be modified to fast drop type by modifying the return valve as shown here;

 

                        (img 003. Showing Modded fast drop damper)

 

The damper fitted to older HS and H  carbs is of the fast drop design, but due to the fit of the piston in the suction chamber the rate of drop is somewhat hampered by the oil used. Hence using the Ball bearing suction chamber decouples the oil from system, and allows the drop speed to be independent of the oil grade used.

 

 

2. Correct spring.

 

Easy one this, whatever carb you have, ensure that it is not over sprung. In fact a little under sprung will help in the performance stakes, not too much or the fuel economy will suffer.

As stated above aim to spring the carb such that it is fully open as you reach maximum power.

 

 

3. Needle/s.

           

There are a number of ways of getting the correct needles. Forget the ones which tell you to use public roads and a stop watch and look at the plugs. Its dangerous and can lead to melted pistons, and large bills due to high loads at lean mixtures.

 

Far better is to use a proprietary rolling road with exhaust gas analysing equipment.

Using a rolling road will allow the operator to assess carb spring requirement, check damper operation and sort out the needle and oil requirements for your SU.

 

To determine the correct needle requirements for your vehicle, the operator should not only drive the engine on the rolling road at full power from as low an rpm as the engine will pull cleanly up to peak power, but he/she should also drive the car at  varying speeds. Paying particular attention to part throttle cruising at the most used speeds will enable the operator to monitor the exhaust gas CO% under load at these points. Remember on the road, most of your driving is using part throttle and progression (using more than part throttle to accelerate slowly) so it makes more sense to get this area of fuelling spot on. This information can then be used to determine where and how much the needle needs to be made richer/leaner.

The best rolling road operators will custom make needles to suit any engine spec, meaning at any throttle opening and any engine speed the engine will get what fuel it needs. AVOID rolling roads that file needles in carbs with have spring bias needles, it causes premature jet wear as the needle rotates, and quickly negates any setup gains that were realised. Proper custom needles will use either a needle lathe, or some form of rotating machinery, and the needles will have there diameter reduced and polished after the work has been carried out to ensure no longer term side effects.

 

When we talk of the correct mixture strength for road use, I aim for 1.25 to 2.5 CO% at cruise, that’s a constant figure and not varying. This should rise another % or so as you accelerate and as the revs build to full power aim for a minimum of 5% better still 5.5 to 6% sustained across the largest part of the power band at full throttle.

If your rolling road operator has no means of loading the engine to sustain a load, use the highest gear to hold the engine back, this will give time for the analyser to settle and give accurate results.

Another way of monitoring which is becoming very popular (due to engine management) and the need for real time mapping, is the use of a wideband lambda sensor. Again using one of these on a rolling road it give the readout much faster than that of the Gas analyser. It s readout is in fuel air ratio, ie 12:1 13:1 etc. You can also use these permanently fitted to your vehicle so that as you drive the fuelling is always monitored. Any discrepancies will show up straight away. Aim for 12.3 to 12.5:1 full power, and around 13.7:1 to 14.0:1 cruise with conventional ignition systems.

 

Needle modifications need to be carried out with care and in a methodical manner.

 

H1, HS2 and HS4 carbs all use a jet of 0.090”. This means that they can all use the same range of needles.

Needles are sized from top to bottom in 1/8” increments, number 1 being at the thickest part of the metering taper right under the shoulder.

 

There are 2 types of needles, that being spring bias (later type) and fixed. The fixed type being the best in terms of long term reliability due to the fact that there is very little wear in use. It does not touch the jet in normal operation, so inconsequence holds its calibration for a much longer period of time. The bias needles, while not having to have the jet centred as per fixed needle variants suffer with much increased wear. The needle is lightly sprung such that it constantly contacts one side of the jet. Over time, this gradually increases the sizes of the jet. There comes a point where no amount of leaning the idle mix will reduce the emissions to an acceptable level. At this point new jet(s) will have to be fitted to restore performance. The needle will also wear, increasing the fuel delivered right across the rev range. Not necessarily a bad thing on a standard engine as these are usually setup a little lean by the factory. But for your performance engine which relies on its fuel being delivered accurately and in the correct amounts right across the rev range, performance can and does become impaired. The check for this is regular rolling road checks to get a handle on any likely changes that are required.

 

At this point it is worth mentioning that engines power output drops over time. So for example if you take a new engine that has fresh valve seal and good piston/bore/ring seal, it will produce its highest power output. Having setup and engine after 500 miles, testing again after 5,000 miles will usually show a loss in peak power of anything up to 10% depending on the level of tuning and the use it has been put to. This is normal and to be expected. The remedy for this loss, is to remove the head and reseat the valves. This in 9 out of 10 engines will restore the seemingly lost performance.  However if you run the engine on the rolling road and notice the power has dropped you will also notice that the engine is most probably running richer as a result. Do not get this confused with needle and jet wear. If the engine is not producing as much power it needs a little less fuel, meaning your perfectly setup needle is a little too rich. By doing the above mentioned valve job will see the correct calibration returned.

 

4. Correct oil.

 

This is easy for road use, this is the grade of oil which gives the best all round performance. If the needle selected is one which fuels correctly at every steady state speed, then the oil required will be the grade at which produces the required enrichment during acceleration. There is no ‘the best oil for all SU’s’ as the tolerances and wear found within a particular SU’s piston and damper assembly will influence the grade of oil to be use. However it can be noted that using a mono grade oil, will allow the piston rise in colder temperatures to become harder, thus allowing for further enrichment on acceleration during the warm up phase. This tendency has the marked effect of reducing the flat spot tendency during the first few minutes of cold running on cold and frosty mornings.

 

 

 

Needle Modifications

 

Using suitable equipment it is perfectly possible for you to make up your own needles.

 

Modifications are made by polishing needles with wet & dry paper at the required points to reduce its diameter and make an increase in the fuel delivered from the jet.

 

                       

The best way to understand this is to show you an example. This one is common to a lot of mini owners who have installed a Standard MG Metro engine into there mini, removed the standard MG filter and installed a typical free flow cone type together with the other Stage 1 kit components. The standard Car uses an HIF44 carburettor with a BDL or BFY needle and RED spring.

 

 

Needle Dimensions;

 

    BDL                                                          Modified BDL;     

 

  1 . 0.099”                                                          1. 0.099”

  2.  0.0955”                                                         2. 0.0955”

  3.  0.0924”                                                         3. 0.092”

  4.  0.0893”                                                        4. 0.0888”

  5.  0.0865”                                                         5. 0.085”

  6.  0.0825”                                                         6. 0.081”

  7.  0.079”                                                          7. 0.077”

  8.  0.076”                                                          8. 0.0745”

  9.  0.0735                                                          9. 0.071”

10. 0.071                                                           10. 0.0675

11. 0.068”                                                          11. 0.064”

12. 0.065”                                                          12. 0.061”

13. 0.063”                                                          13. 0.059”

 

As the full open position of the Piston in the 1.75in range (HS6/HIF44 carbs)  is needle point 12, only these area’s need to be modified to complete a successful custom needle. 14 to 16 are just smaller than 13.

Incidentally, for 1.25in carbs point 9 is the full open position, and for the 1.5 in carbs it is needle point 10/11. 12 through to the end, again just made smaller than 11.

 

The modified needle is typically what is needed to fuel the engine correctly after such a swap has been carried out. This represents a considerable change from the stock needle. Of course the dimensions above are only a guide and are intended to get you running. As previously mentioned, proper setting up using the methods mentioned earlier is ‘THE ONLY WAY’ of getting this spot on.

 

As to why the fuelling requirements have changed I will explain.

The Airfilter has been changed for a unit with considerably shorter ram length. The intake manifold has now changed (typical stage 1 kit)  and the exhaust system is also different to the original metro.

Some people seem to consider the SU a self adjusting unit. I.e. if the needle is correct for a certain airflow, then increasing the airflow will allow the piston to rise and lift the needle further from the jet and keep the mixture ratio in proportion. This however uses the assumption that the airflow is constant and average. To understand this more easily, think of the air flow as a series of pulses. Bigger engines produce bigger pulses than Smaller engines. The piston rise responds not to these pulses, but the average of these pulses as its action is damped by the piston damper/oil combination. The pulses are much faster moving than the piston can react to so these go unnoticed. However, the fuel in the jet is very sensitive to these pulses so the fuel being drawn from the jet is for want of a better description, pulse sensitive.

So for a given engine size, using X needle, then increasing the size of the engine while all else remains the same, can mean a needle being leaner than X being needed to fuel this engine correctly. Ie the bigger engine produces a larger induction pulse, this being applied to the fuel jet means a larger ‘pull’ being applied to the same effective jet size meaning more fuel is pulled from it. To correct this a thicker or leaner needle would be needed in this case.

With our example with less ram length the there is a little less average air flowing through the carb at low speeds, it seems that the ratio of fuel being delivered drops off more than the average airflow, thus meaning a richer needle needed at that point in the rev range even with the engine producing  little less torque. Add to this the usual factory settings are that which usually tend towards the lean side of running, mean that even a standard engine could do with a little richening to get maximum performance and smooth throttle response with no flat spots and good fuel economy.

My testing has shown that having a needle too lean in the part throttle positions will actually increase fuel consumption in the urban and cruise cycles. Again if the needle is too rich, the same applies, though even more so.

 

Below are a few more typical examples of standard needles verses the typical actual requirements I have seen when dyno tuning.

 

 This is based on the humble high compression A+ 998 mini, fitted with the usual stage 1 kit, consisting of  Taper filter(K&N or Pipercross or similar), LCB manifold, alloy intake manifold, and performance exhaust. Standard RED spring.

 

 

  Needle Dimensions;

 

  AAC                                                           Modified AAC;     

 

  1 . 0.089”                                                          1. 0.089”

  2.  0.085”                                                          2. 0.0855”

  3.  0.0835”                                                         3. 0.0825”

  4.  0.0811”                                                        4. 0.079”

  5.  0.0788”                                                         5. 0.076”

  6.  0.0765”                                                         6. 0.0735”

  7.  0.0742”                                                         7. 0.071”

  8.  0.072”                                                          8. 0.068”

  9.  0.0698”                                                         9. 0.065”

10. 0.0676”                                                        10. 0.0625

11. 0.0665”                                                        11. 0.0605”

 

 

 

Adding a performance cylinder head, such as one described as stage 2 or stage 3 to the MG metro example will see the needle change from the modified example to a richer profile. Still using the RED spring.

 

 

 Modified BDL;                                     Modified BDL Plus Head

 

  1 . 0.099”                                                          1. 0.099”

  2.  0.0955”                                                         2. 0.0955”

  3.  0.0924”                                                         3. 0.092”

  4. 0.0888”                                                          4. 0.0885”                   

  5. 0.085”                                                           5. 0.0848”

  6. 0.081”                                                           6. 0.0805”

  7. 0.077”                                                           7. 0.077”        

  8. 0.0745”                                                          8. 0.073”

  9. 0.071”                                                           9. 0.0685”

10. 0.0675                                                         10. 0.065”

11. 0.064”                                                          11. 0.061”

12. 0.061”                                                          12. 0.057”         

13. 0.059”                                                          13. 0.055”         

 

 

These are to be treated as examples, and whilst the needle profiles will get you in the right ball park they WILL need tailoring to suit every vehicle with the same modifications.

 

Armed with this information, hopefully it will allow you to understand a little more about what to do and expect regarding the performance issues regarding the SU carb in a ‘road’ use guise.

There is NO SUBSTITUTE for a proper setting up session where the exhaust gasses can be monitored and intelligent needle changes made to get the best from whatever engine/carb configuration you are running.

 

 

©ACDodd 2015

Edited by ACDodd, 13 April 2015 - 08:38 PM.

[nicklouse]

Nice bit of reading.

[Project_Yoshi]

That's a great bit of reading but for the traditional look i went down the route of twin HIF38's rather than a 45

 

I am not sure what vehicle my carbs were sourced from before i got them, but they currently have ACD needles - seem to have good power but way too rich

 

My car is booked in for a rolling road session but i thought it would be wise to purchased a couple of sets of needles beforehand

 

I could really do with some advise as to what needles i need as a baseline as at £20 pair it could become rather costly and as i am am from Yorkshire i can't have that!!

 

My engine is a 1380 with ported head, 1.5 rockers, 544 cam, vizard book ported SU's, maniflow LCB & system

 

Cheers Jeff

 

Any thoughts or other folks running twin HIF's??

[ACDodd]

Take a pair of lean needles like some AAC's and then get the rolling road operator to make you a pair, thats how its done.

AC

[petey81]

Exactly what i was going to say! Ac beat me to it ha ha.

[Project_Yoshi]

Thanks for the advise, when you say 'make me some' - do you mean narrow the needle profiles with wet n dry as this seems a little heath robinson?

[ACDodd]

It's not Heath Robinson, when done correctly it is perfection.

AC

[klivins]

Very nice read indeed. I wish I had this 5 years ago when I discovered for me the unknown depths of the SU carburettors. In a country where there are no rolling roads, I played with cheap Gunsons gas analyser first, and now I had fitted the AFR meter with lambda in the exaust. happy with the results, finally:)

[grumpy dad]

Hi 

AC Dodd can I ask 

if I am right in thinking that a HIF 44

is only good for about 120 horse power ?

 

Thanks 

[ACDodd]

You'll be very lucky to see that!

 

AC

[PACINO]

Its a long response but;
 
Many, many vehicles have been fitted with SU (Skinners Union) carbs over the years, and probably the most common being the mini with the beautifully simple A-series engine in all its many guises.  These carbs are quite rightly used in standard and not so standard performance applications which, providing that the instruments are modified to suit that new performance use, provide years of flat-spot and trouble free motoring.
 
The most common SU carb in use today on the mini, is the HIF carb, followed by the HS, and the Twin setups.  HIF’s being more common due to there standardised fitment on most minis from  ’91 – ’92.
 
My day to day workings with the SU carb has given me experience of many problems and common mistakes associated with the use/miss-use of these carbs on different cars (mainly minis). After all these years of magazine articles and the endless needs of owners asking how things are done, I have decided to put pen to paper and write down what the necessary tricks and tweaks that need to be carried out when using SU carbs on your road cars. These are even more important, when we are considering the use of these carbs for performance (Non-competition) use.
All the mods and suggestions here are aimed at maximum performance whilst retaining very good drivability, and the all important emissions and fuel consumption. Any carburettor install for the road needs to be able to deliver legal exhaust emission standards, good fuel economy and very good performance.
 
Needless to say, any carb used on your engine must be in new, near new or freshly rebuilt condition if you expect to get anywhere near its best potential. This is especially applicable to twin setups where even the most seemingly insignificant wear can cause all sorts of running problems.
 
Sizing of SU carbs is not really to much of an issue, people do tend to get hung up on sizing thinking that to large a carb will be detrimental, well, this is not necessarily so. When we think of webers (DCOE’s) for example, they have no means of altering their choke size. Ie they are fixed. They have to have a choke that is large enough to give the required airflow for an engines given maximum air demand at maximum power. This leads to a carb which at lower engine speeds the choke size is far to large to maintain air speed and promote inertial ramming at lower speeds. Not to mention the lack of airspeed over the jets. This means a poorer cylinder fill and less low end torque than that of a carb where by we have a variable choke (good old SU). The choke size on the SU carb adjusts to the engines air demand and maintains the optimum size (when calibrated correctly) right across the rev range. This ensures a high gas speed and good fuel pickup from the jet giving better fuel metering at lower Rpms leading to better fuel economy and part throttle running.
 
Horsepower is a good way of determining carb size, but it does not show the whole picture. A better guide would be if; gear ratio, and capacity were taken into account. In this context vehicle weight would have an effect too, but for simplicity and as the engine ‘Sees’ weight via the reduction effect of gearing it can be disregarded as we are using gearing as a factor in our table.
 
What I have below (Table1) is not a mathematical table showing good carb sizes but one that works the best (in my experience) giving the best overall performance, by performance I mean, the amount you get pushed in the seat when you put your foot down and the economy/emissions. Ie the best all rounder. This table is for Single SU applications. The column headed tune, refers to the power the engine has been built to produce, not what it would produce if the stated carb were fitted.
Engine size                   Tune                 Gearing (final drive)                   Carb
                                                                                               
848cc                           Standard                       3.765:1                         HS4, HIF38
upto +0.060
 
                                    40hp                             3.44:1                           HS4, HIF38
                                                                        3.765:1                         HS4, HIF38
                                                                        3.93:1                           HS4, HIF38
 
                                    50hp                             3.93:1 and lower            HIF44, HS6
 
                                    60hp                             3.93:1and lower             HIF44, HS6
 
998cc
upto +0.060”                 Standard                       3.44:1                           HS4,HIF38
 
                                    50hp Stage 1 kit             2.95 or 3.105:1             HS4, HIF38
                                   

55hp and up                  3.44:1 or lower             HIF44 or HS6

 
1098cc
upto +0.060”                 Standard                       3.44:1                           HS4, HIF38
 
                                    55hp Stage 1                2.76 to 3.105     :1         HS4 to HIF38
                                   
                                    55hp stage 1                 3.44:1 and lower            HIF44 or HS6
 
1275cc
upto 1330cc                  Standard 50hp              2.76 to 3.105     :1         HIF38 or HS4
 
                                    60hp Stage 1                Any FD                         HIF44 or HS6
 
                                    upto 80hp                     2.76 to 3.21:1                HIF44 or HS6
 
1380cc and up             upto 80hp                     2.76 to 2.95:1                HIF44 or HS6
 
 
Ok from this you will probably say, what has gearing got to do with SU carb sizing, good question, well presented.
 
The carbs airflow potential has little to do with the real world driving experience. Sure a HIF44 carb will let an appropriate A-series engine produce in excess of 100hp @ flywheel, but it is far from the optimum carb for the use of that engine, here is why;
 
Think back to when we were talking about variable chokes. It takes time for the variable choke (or piston as it is in the SU) to rise once the throttle is opened. An engine with X Torque with high gearing like a 2.76:1 FD will not be able to accelerate a car of given weight as the same car with a lower final drive. Now there comes a point when the final drive is sufficiently low enough that in the lower gears,1^st, 2^nd etc, that the piston cannot rise fast enough to allow the engine to accelerate at its full potential as suggested by its power/torque figures. This slow rising piston ‘holds back’ the engine stunting its performance in the lower gears.
This is why the power figures look low in the table above. To get around this we remove the piston from the equation, ie fit a weber, or get to grips with getting the best from the SU. To understand how to do this we need to understandard how the SU works when correctly setup.
 
When operating correctly the air flowing through the carb causes a low pressure area above the piston this causes it to rise and draw the needle from the jet. The spring, within the suction chamber assembly, acting on the piston, purely controls the piston position at any given air demand. It should be weighted such that at full power it allows the needle to be lifted to its highest position. Regardless of the power of the engine it is fitted to. As the needle is the correct one in our example, this needle allows the correct amount of fuel to be drawn off from the jet to fuel the engine. When correctly setup this is what happens at any steady state speed at any rpm and at any combination of throttle opening.  As the SU does not have a accelerator pump jet extra enrichment during acceleration is catered for by slowing the piston rise when the throttle is opened. This retardation in the piston rise causes the amount of ‘Pull’ on the fuel leaving the jet to increase and thus more fuel is drawn off and this enriches the mixture during the acceleration phase.   Common misunderstanding is that to gain greater performance you can thin out the oil within the dash pot to allow the piston to rise faster and gain greater acceleration. While true to a point it misses out the fundamental point of fuel economy.
In order to gain the best fuel economy and performance, the needle used must be chosen such that at any steady state running point it is fuelling the engine correctly. Enrichment during acceleration is purely catered for by the Damper assembly within the carb. Modifying the damper by thinning the oil for example in this case would mean that the amount of enrichment would be reduced during acceleration and possibly cause a flat spot. Now using this thinner oil coupled with a richer needle would mean that the fuel enrichment balance during acceleration would now be restored and because the piston rise is quicker we will notice an increase in the acceleration when we put our foot down. This is what usually happens when most minis are rolling road tuned. The needle is a little too rich during normal running and about perfect during acceleration. This is great if you don’t mind paying for the extra fuel!!!
10mpg less is common for this way of setting the SU up! 
 
So from this it can be seen that if the ultimate all round performance is needed then if you are suffering with the damper holding you back when accelerating in the lower gears you either need a bigger carb or more than one, and that’s where twins really come into there own. Below is table 2 for twin carb setups.
 
 
Engine size                   Tune                 Gearing (final drive)                   Carb
                                                                                               
848cc                           Standard                       3.765:1                         Twin H1’s
upto +0.060
 
                                    40hp                             3.44:1                           Twin H1’s
                                                                        3.765:1                         Twin HS2’s
                                                                        3.93:1                           Twin HS2’s
 
                                    50hp                             3.93:1 and lower            Twin HS2’s
 
                                    60hp                             3.93:1and lower             Twin HS2’s
 
998cc
upto +0.060”                 Standard                       3.44:1                           Twin HS2’s
 
                                    50hp Stage 1 kit             2.95 or 3.105:1             Twin HS2’s
                                   

55hp and up                  3.44:1 or lower              Twin HS2’s

 
 
 
 
 
 
1098cc
upto +0.060”                 Standard                       3.44:1                           Twin HS2’s
 
                                    55hp Stage 1                2.76 to 3.105     :1         Twin HS2’s
                                   
                                    55hp stage 1                 3.44:1 and lower            Twin HS2’s
 
                                    65hp                             3.44 and lower               Twin HS2’s
 
                                    75hp                             3.44 and lower               Twin HS4’s
 
1275cc
upto 1330cc                  Standard 50hp              2.76 to 3.105     :1         Twin HS2’s
 
                                    upto 75hp                     Lower than 3.105:1        Twin HS2’s
 
                                    upto 100hp                   Any FD                         Twin HS4’s
 
1380cc and up             70hp to 100hp               Any FD                         Twin HS4’s.
 
 
 
Now we have sizes sorted out, we need to think about calibrating the carbs correctly.
 
There are 4 things to consider about your correctly sized carbs in order to get them dialled in correctly, regardless of size or the number fitted;
 

• Correct Damper
• Correct spring
• Correct needle
• Correct oil.

 
 

• Dampers, basically very straightforward.

 
Piston dampers need to be of the fast drop type. Easy ID of the HIF type. Part Number LZX 2085. This is the only damper that should be fitted.
 
Incidently, this is a good point to mention, it is very worthwhile mod on and HS4 or HS6 carb. That is, to use the HIF ball bearing suction chamber and damper assembly on the older carb. In doing this mod is endows the older HS carbs with the same fast drop characteristics of the later HIF carbs when fitted with the LZX 2085 Damper rod.
The reason for having ‘Fast drop’ piston is to allow the piston to drop during gear changes. If the piston does not drop fast enough during a gear change the needle will not be in the correct position to fuel the engine when the throttle is opened again after the next gear is selected. The result of this is a huge flat spot between gear changes.
 
Dampers can be modified to fast drop type by modifying the return valve as shown here;
 
                        (img 003. Showing Modded fast drop damper)
 
The damper fitted to older HS and H  carbs is of the fast drop design, but due to the fit of the piston in the suction chamber the rate of drop is somewhat hampered by the oil used. Hence using the Ball bearing suction chamber decouples the oil from system, and allows the drop speed to be independent of the oil grade used.
 
 
2. Correct spring.
 
Easy one this, whatever carb you have, ensure that it is not over sprung. In fact a little under sprung will help in the performance stakes, not too much or the fuel economy will suffer.
As stated above aim to spring the carb such that it is fully open as you reach maximum power.
 
 
3. Needle/s.
           
There are a number of ways of getting the correct needles. Forget the ones which tell you to use public roads and a stop watch and look at the plugs. Its dangerous and can lead to melted pistons, and large bills due to high loads at lean mixtures.
 
Far better is to use a proprietary rolling road with exhaust gas analysing equipment.
Using a rolling road will allow the operator to assess carb spring requirement, check damper operation and sort out the needle and oil requirements for your SU.
 
To determine the correct needle requirements for your vehicle, the operator should not only drive the engine on the rolling road at full power from as low an rpm as the engine will pull cleanly up to peak power, but he/she should also drive the car at  varying speeds. Paying particular attention to part throttle cruising at the most used speeds will enable the operator to monitor the exhaust gas CO% under load at these points. Remember on the road, most of your driving is using part throttle and progression (using more than part throttle to accelerate slowly) so it makes more sense to get this area of fuelling spot on. This information can then be used to determine where and how much the needle needs to be made richer/leaner.
The best rolling road operators will custom make needles to suit any engine spec, meaning at any throttle opening and any engine speed the engine will get what fuel it needs. AVOID rolling roads that file needles in carbs with have spring bias needles, it causes premature jet wear as the needle rotates, and quickly negates any setup gains that were realised. Proper custom needles will use either a needle lathe, or some form of rotating machinery, and the needles will have there diameter reduced and polished after the work has been carried out to ensure no longer term side effects.
 
When we talk of the correct mixture strength for road use, I aim for 1.25 to 2.5 CO% at cruise, that’s a constant figure and not varying. This should rise another % or so as you accelerate and as the revs build to full power aim for a minimum of 5% better still 5.5 to 6% sustained across the largest part of the power band at full throttle.
If your rolling road operator has no means of loading the engine to sustain a load, use the highest gear to hold the engine back, this will give time for the analyser to settle and give accurate results.
Another way of monitoring which is becoming very popular (due to engine management) and the need for real time mapping, is the use of a wideband lambda sensor. Again using one of these on a rolling road it give the readout much faster than that of the Gas analyser. It s readout is in fuel air ratio, ie 12:1 13:1 etc. You can also use these permanently fitted to your vehicle so that as you drive the fuelling is always monitored. Any discrepancies will show up straight away. Aim for 12.3 to 12.5:1 full power, and around 13.7:1 to 14.0:1 cruise with conventional ignition systems.
 
Needle modifications need to be carried out with care and in a methodical manner.
 
H1, HS2 and HS4 carbs all use a jet of 0.090”. This means that they can all use the same range of needles.
Needles are sized from top to bottom in 1/8” increments, number 1 being at the thickest part of the metering taper right under the shoulder.
 
There are 2 types of needles, that being spring bias (later type) and fixed. The fixed type being the best in terms of long term reliability due to the fact that there is very little wear in use. It does not touch the jet in normal operation, so inconsequence holds its calibration for a much longer period of time. The bias needles, while not having to have the jet centred as per fixed needle variants suffer with much increased wear. The needle is lightly sprung such that it constantly contacts one side of the jet. Over time, this gradually increases the sizes of the jet. There comes a point where no amount of leaning the idle mix will reduce the emissions to an acceptable level. At this point new jet(s) will have to be fitted to restore performance. The needle will also wear, increasing the fuel delivered right across the rev range. Not necessarily a bad thing on a standard engine as these are usually setup a little lean by the factory. But for your performance engine which relies on its fuel being delivered accurately and in the correct amounts right across the rev range, performance can and does become impaired. The check for this is regular rolling road checks to get a handle on any likely changes that are required.
 
At this point it is worth mentioning that engines power output drops over time. So for example if you take a new engine that has fresh valve seal and good piston/bore/ring seal, it will produce its highest power output. Having setup and engine after 500 miles, testing again after 5,000 miles will usually show a loss in peak power of anything up to 10% depending on the level of tuning and the use it has been put to. This is normal and to be expected. The remedy for this loss, is to remove the head and reseat the valves. This in 9 out of 10 engines will restore the seemingly lost performance.  However if you run the engine on the rolling road and notice the power has dropped you will also notice that the engine is most probably running richer as a result. Do not get this confused with needle and jet wear. If the engine is not producing as much power it needs a little less fuel, meaning your perfectly setup needle is a little too rich. By doing the above mentioned valve job will see the correct calibration returned.
 
4. Correct oil.
 
This is easy for road use, this is the grade of oil which gives the best all round performance. If the needle selected is one which fuels correctly at every steady state speed, then the oil required will be the grade at which produces the required enrichment during acceleration. There is no ‘the best oil for all SU’s’ as the tolerances and wear found within a particular SU’s piston and damper assembly will influence the grade of oil to be use. However it can be noted that using a mono grade oil, will allow the piston rise in colder temperatures to become harder, thus allowing for further enrichment on acceleration during the warm up phase. This tendency has the marked effect of reducing the flat spot tendency during the first few minutes of cold running on cold and frosty mornings.
 
 
 
Needle Modifications
 
Using suitable equipment it is perfectly possible for you to make up your own needles.
 
Modifications are made by polishing needles with wet & dry paper at the required points to reduce its diameter and make an increase in the fuel delivered from the jet.
 
                       
The best way to understand this is to show you an example. This one is common to a lot of mini owners who have installed a Standard MG Metro engine into there mini, removed the standard MG filter and installed a typical free flow cone type together with the other Stage 1 kit components. The standard Car uses an HIF44 carburettor with a BDL or BFY needle and RED spring.
 
 
Needle Dimensions;
 
    BDL                                                          Modified BDL;     
 
  1 . 0.099”                                                          1. 0.099”
  2.  0.0955”                                                         2. 0.0955”
  3.  0.0924”                                                         3. 0.092”
  4.  0.0893”                                                        4. 0.0888”
  5.  0.0865”                                                         5. 0.085”
  6.  0.0825”                                                         6. 0.081”
  7.  0.079”                                                          7. 0.077”
  8.  0.076”                                                          8. 0.0745”
  9.  0.0735                                                          9. 0.071”
10. 0.071                                                           10. 0.0675
11. 0.068”                                                          11. 0.064”
12. 0.065”                                                          12. 0.061”
13. 0.063”                                                          13. 0.059”
 
As the full open position of the Piston in the 1.75in range (HS6/HIF44 carbs)  is needle point 12, only these area’s need to be modified to complete a successful custom needle. 14 to 16 are just smaller than 13.
Incidentally, for 1.25in carbs point 9 is the full open position, and for the 1.5 in carbs it is needle point 10/11. 12 through to the end, again just made smaller than 11.
 
The modified needle is typically what is needed to fuel the engine correctly after such a swap has been carried out. This represents a considerable change from the stock needle. Of course the dimensions above are only a guide and are intended to get you running. As previously mentioned, proper setting up using the methods mentioned earlier is ‘THE ONLY WAY’ of getting this spot on.
 
As to why the fuelling requirements have changed I will explain.
The Airfilter has been changed for a unit with considerably shorter ram length. The intake manifold has now changed (typical stage 1 kit)  and the exhaust system is also different to the original metro.
Some people seem to consider the SU a self adjusting unit. I.e. if the needle is correct for a certain airflow, then increasing the airflow will allow the piston to rise and lift the needle further from the jet and keep the mixture ratio in proportion. This however uses the assumption that the airflow is constant and average. To understand this more easily, think of the air flow as a series of pulses. Bigger engines produce bigger pulses than Smaller engines. The piston rise responds not to these pulses, but the average of these pulses as its action is damped by the piston damper/oil combination. The pulses are much faster moving than the piston can react to so these go unnoticed. However, the fuel in the jet is very sensitive to these pulses so the fuel being drawn from the jet is for want of a better description, pulse sensitive.
So for a given engine size, using X needle, then increasing the size of the engine while all else remains the same, can mean a needle being leaner than X being needed to fuel this engine correctly. Ie the bigger engine produces a larger induction pulse, this being applied to the fuel jet means a larger ‘pull’ being applied to the same effective jet size meaning more fuel is pulled from it. To correct this a thicker or leaner needle would be needed in this case.
With our example with less ram length the there is a little less average air flowing through the carb at low speeds, it seems that the ratio of fuel being delivered drops off more than the average airflow, thus meaning a richer needle needed at that point in the rev range even with the engine producing  little less torque. Add to this the usual factory settings are that which usually tend towards the lean side of running, mean that even a standard engine could do with a little richening to get maximum performance and smooth throttle response with no flat spots and good fuel economy.
My testing has shown that having a needle too lean in the part throttle positions will actually increase fuel consumption in the urban and cruise cycles. Again if the needle is too rich, the same applies, though even more so.
 
Below are a few more typical examples of standard needles verses the typical actual requirements I have seen when dyno tuning.
 
 This is based on the humble high compression A+ 998 mini, fitted with the usual stage 1 kit, consisting of  Taper filter(K&N or Pipercross or similar), LCB manifold, alloy intake manifold, and performance exhaust. Standard RED spring.
 
 
  Needle Dimensions;
 
  AAC                                                           Modified AAC;     
 
  1 . 0.089”                                                          1. 0.089”
  2.  0.085”                                                          2. 0.0855”
  3.  0.0835”                                                         3. 0.0825”
  4.  0.0811”                                                        4. 0.079”
  5.  0.0788”                                                         5. 0.076”
  6.  0.0765”                                                         6. 0.0735”
  7.  0.0742”                                                         7. 0.071”
  8.  0.072”                                                          8. 0.068”
  9.  0.0698”                                                         9. 0.065”
10. 0.0676”                                                        10. 0.0625
11. 0.0665”                                                        11. 0.0605”
 
 
 
Adding a performance cylinder head, such as one described as stage 2 or stage 3 to the MG metro example will see the needle change from the modified example to a richer profile. Still using the RED spring.
 
 
 Modified BDL;                                     Modified BDL Plus Head
 
  1 . 0.099”                                                          1. 0.099”
  2.  0.0955”                                                         2. 0.0955”
  3.  0.0924”                                                         3. 0.092”
  4. 0.0888”                                                          4. 0.0885”                   
  5. 0.085”                                                           5. 0.0848”
  6. 0.081”                                                           6. 0.0805”
  7. 0.077”                                                           7. 0.077”        
  8. 0.0745”                                                          8. 0.073”
  9. 0.071”                                                           9. 0.0685”
10. 0.0675                                                         10. 0.065”
11. 0.064”                                                          11. 0.061”
12. 0.061”                                                          12. 0.057”         
13. 0.059”                                                          13. 0.055”         
 
 
These are to be treated as examples, and whilst the needle profiles will get you in the right ball park they WILL need tailoring to suit every vehicle with the same modifications.
 
Armed with this information, hopefully it will allow you to understand a little more about what to do and expect regarding the performance issues regarding the SU carb in a ‘road’ use guise.
There is NO SUBSTITUTE for a proper setting up session where the exhaust gasses can be monitored and intelligent needle changes made to get the best from whatever engine/carb configuration you are running.
 
 
©ACDodd 2015

GREAT !!