52 replies to this topic

[carbon]

The two connections on the HS4 float chamber are likely to be for:

- fuel feed pipe

- chamber vent pipe

 

Just make sure you get them the right way round. Also recommend putting a fuel filter in the line between tank and carb.

[Spider]

SU recommend the Single 1-1/4" Carb for engines up to 72 HP and the Single 1-1/2" for engines up to 88 HP.

 

They can be internally modified to increase the Air Flow Volume Rate, however, it is not as simple as it first appears as the 'natural' air balance of the carb is usually lost, which usually results in incorrect mixture ratios on the cruise if tuned for maximum power over the rev range of the engine (or viki-verka).

 

Normally, you'd just go up a carb size rather than mess with them.

Edited by Moke Spider, 25 July 2017 - 08:50 PM.

[Cooperman]

The standard head is always the limiting factor with A-series engines.
The 12g295 head is so much better than the standard one.
It is necessary to measure and machine to get the CR correct.

[jabos7]

I like Moke Spider's idea to just slide up to the HS4, as my HS2 that came on the car is shot (Repair on float chamber had big blob of KwikWeld--stayed dry though!)

 

The 12g295 head is probably hard to find?   I'm stuck waiting on anyone to get just a stock recon back on the shelf to send me.  I'm weighing the costs

of having my old head re-done, but machine shop estimate is big money!  -And then buying all the valves and springs after that...cha-ching!

I just can't get it done for the straight price from Spares or Sport on a stock 998 head.

[carbon]

SU recommend the Single 1-1/4" Carb for engines up to 72 HP and the Single 1-1/2" for engines up to 88 HP.

 

They can be internally modified to increase the Air Flow Volume Rate, however, it is not as simple as it first appears as the 'natural' air balance of the carb is usually lost, which usually results in incorrect mixture ratios on the cruise if tuned for maximum power over the rev range of the engine (or viki-verka).

 

Normally, you'd just go up a carb size rather than mess with them.

Like Moke Spider says, most of the modifications which improve the air flow in an SU are also going to upset the air/fuel metering balance, and it will take a lot of tweaking to get this right across the full operating range.

 

The only modification which is probably not going to upset the SU too much is thinning the throttle shaft and using butterfly screws without the 'legs'. But still likely to need a different needle.

[Minigman]

The stock head won't give much more power despite increased compression ratio. Get yourself a 12g295 head. The std 1100 head (12g202) is marginally better than a std 998 head but not worth fitting if you have to rebuild it first. An HS4 is fine upto 70hp so fine for a mildly tuned 998.

[jabos7]

A mild tune is where I'm heading.   I'd love a 12g295 but wouldn't know where to look for one other than ebay.  Right now I can't even procure

a stock 998 head--everyone is out of stock.

 

A follow-up to "carbon" on the HS4--  where does that chamber vent pipe get hooked to?  Is it meant for a return if I'm running

an electric/higher psi pump?  Or just an air venting?  My HS2 only had one line on my MK3 and the HS4 I'm looking at is the same.

[Retroman]

Sorry to disagree with Carbon and Moke Spider but in my experience of modifying Su carbs since 1979 of all sizes to DV spec it doesn't upset the air flow, exactly the opposite, it allows the engine to suck more air and increases throttle response at all revs in all gears. It just enhances the whole thing and gives much better smoother running at all revs. Instead of the air being battered around inside the body creating all sorts of flow problems on all the sharp corners and screws the air has nothing to worry about other than mixing with fuel. 

I did an HIF38 recently for the road with a 270 cam 1107cc (998 taken to 68mm bore) and a standard 940 head, it made 81 on the rolling road, with a virtually flat torque curve from 3 to 6 K the guy loves it.

Its basic stuff, but seriously overlooked, people spend hundreds on heads, cams and pistons but without a modified SU carb/manifold and a stubstack or rampipe you are not getting the best out of any engine as there is huge potential for improvement. I have seen an gain on a circuit racing Mini of 2.3 seconds per lap.

Another that is over looked is the dyno (rolling road) always have a dyno run to get the fueling set up and ignition, without it you are giving it best guess, and thats not good enough.

Richard Longman and David Vizard knew what they were doing.

An engine is basically an air pump, and anything you can do to get more in (or out) without going tooo big makes it more efficient.

All of the above applies to any engine from a 848cc standard road engine to a 1330cc 220BHP Turbo.

[carbon]

Sorry to disagree with Carbon and Moke Spider but in my experience of modifying Su carbs since 1979 of all sizes to DV spec it doesn't upset the air flow, exactly the opposite, it allows the engine to suck more air and increases throttle response at all revs in all gears. It just enhances the whole thing and gives much better smoother running at all revs. Instead of the air being battered around inside the body creating all sorts of flow problems on all the sharp corners and screws the air has nothing to worry about other than mixing with fuel. 

I did an HIF38 recently for the road with a 270 cam 1107cc (998 taken to 68mm bore) and a standard 940 head, it made 81 on the rolling road, with a virtually flat torque curve from 3 to 6 K the guy loves it.

Its basic stuff, but seriously overlooked, people spend hundreds on heads, cams and pistons but without a modified SU carb/manifold and a stubstack or rampipe you are not getting the best out of any engine as there is huge potential for improvement. I have seen an gain on a circuit racing Mini of 2.3 seconds per lap.

Another that is over looked is the dyno (rolling road) always have a dyno run to get the fueling set up and ignition, without it you are giving it best guess, and thats not good enough.

Richard Longman and David Vizard knew what they were doing.

An engine is basically an air pump, and anything you can do to get more in (or out) without going tooo big makes it more efficient.

All of the above applies to any engine from a 848cc standard road engine to a 1330cc 220BHP Turbo.

What I said was about the effect modifying the SU has on the air/fuel metering balance.

 

In my (limited to HS2 and HS4) experience most of the modifications which improve the air flow in an SU also upset the air/fuel metering balance, and it takes a lot of tweaking to get this right across the full operating range.

 

For example, pretty easy to get mixture correct under full load. A lot trickier to get mixture correct (twin HS2 carbs in this instance) under light load such as small throttle openings at 30 - 50 mph when I have often had lean surge on a heavily modified SU set-up in fast road engine.

 

And running a mildly Vizarded HS4 on a pretty much otherwise standard 998 I managed to burn out an exhaust valve in pretty spectacular fashion. At the time I didn't have the money to fund a rolling road session...

Edited by carbon, 28 July 2017 - 05:41 PM.

[Spider]

 

Sorry to disagree with Carbon and Moke Spider but in my experience of modifying Su carbs since 1979 of all sizes to DV spec it doesn't upset the air flow, exactly the opposite, it allows the engine to suck more air and increases throttle response at all revs in all gears. It just enhances the whole thing and gives much better smoother running at all revs. Instead of the air being battered around inside the body creating all sorts of flow problems on all the sharp corners and screws the air has nothing to worry about other than mixing with fuel. 

I did an HIF38 recently for the road with a 270 cam 1107cc (998 taken to 68mm bore) and a standard 940 head, it made 81 on the rolling road, with a virtually flat torque curve from 3 to 6 K the guy loves it.

Its basic stuff, but seriously overlooked, people spend hundreds on heads, cams and pistons but without a modified SU carb/manifold and a stubstack or rampipe you are not getting the best out of any engine as there is huge potential for improvement. I have seen an gain on a circuit racing Mini of 2.3 seconds per lap.

Another that is over looked is the dyno (rolling road) always have a dyno run to get the fueling set up and ignition, without it you are giving it best guess, and thats not good enough.

Richard Longman and David Vizard knew what they were doing.

An engine is basically an air pump, and anything you can do to get more in (or out) without going tooo big makes it more efficient.

All of the above applies to any engine from a 848cc standard road engine to a 1330cc 220BHP Turbo.

What I said was about the effect modifying the SU has on the air/fuel metering balance.

 

In my (limited to HS2 and HS4) experience most of the modifications which improve the air flow in an SU also upset the air/fuel metering balance, and it takes a lot of tweaking to get this right across the full operating range.

 

For example, pretty easy to get mixture correct under full load. A lot trickier to get mixture correct (twin HS2 carbs in this instance) under light load such as small throttle openings at 30 - 50 mph when I have often had lean surge on a heavily modified SU set-up in fast road engine.

 

And running a mildly Vizarded HS4 on a pretty much otherwise standard 998 I managed to burn out an exhaust valve in pretty spectacular fashion. At the time I didn't have the money to fund a rolling road session...

 

 

I found the same as Carbon.

 

I should also clarify that many internal mods can be done to improve air flow volume (eg, as Carbon suggested, the spindle and screws) however once the Bridge is modified, the characteristics of the carb - for street use - are lost.

 

It will provide results when seeking all out power, as Retroman has talked about, and for achieving good numbers on the Rolling Road, but were it does loose out / become 'up-set' is at part throttle when there is low volumes of air flowing through the carb. It very much appears that it needs the sharp shapes of the standard bridge to create a 'balanced' depression over the Jet. In standard form, this is how the same Needle can provide correct A/F Ratios on W.O.T. as well as suitable ratios that are not too lean or too rich (usually held close to 17 - 18:1) on part throttle.

 

I know we have all seen the Needle Diagram that suggests the thicker parts of the Needle are for Idle, the Thinner parts for W.O.T. / Max Power and Cruise (part throttle) somewhere in between. This is partly right, only that the entire Needle is used for W.O.T., not just the thin end. The SU Carb works by measuring Air Flow Volume and then accurately metering the fuel in quite precise quantities directly related to that Air Flow Volume. The Air Flow Volume needed at say 1200 RPM at W.O.T. is going to be substantially different to that at 5000 RPM with W.O.T.. This is why it uses the entire Needle for W.O.T. and what it's normally calibrated against. By design, the natural 'balance' (as I've put it) of the carb then provides an A/F Ratio of 17 - 18:1 at part throttle. Modifying the Bridge disturbs this 'balance'.

 

I don't disagree at all (agree completely) that at all out W.O.T., gains can be found, but what's it like at part throttle?

 

<EDIT: It appears the forum has an auto-check / correcter for spelling? I've now twice typed in W O T (without spacers) only to have it appear as what. Edited to W.O.T. (Wide Open Throttle)>

Edited by Moke Spider, 28 July 2017 - 09:34 PM.

[Spider]

For example, pretty easy to get mixture correct under full load. A lot trickier to get mixture correct (twin HS2 carbs in this instance) under light load such as small throttle openings at 30 - 50 mph when I have often had lean surge on a heavily modified SU set-up in fast road engine.

 

And running a mildly Vizarded HS4 on a pretty much otherwise standard 998 I managed to burn out an exhaust valve in pretty spectacular fashion. At the time I didn't have the money to fund a rolling road session...

 

 

That's quite similar to what I found too, the mixture generally leans out on part throttle.

[Retroman]

Ok guys you a basically right about the bridge as such it should not be messed with greatly. When I do any carb I only take the sharp front edge off the bridge ( about 1mm filter side), we are not talking about boring them out or stuffing them full of epoxy. Its totally unneeded I have never done that, never will, even on a racer using twin 1 3/4 carbs its not needed, as you both said it upsets the mixture too much and probably only works at what. 

     As you have both found when doing any mods to an Su it leans the mixture out which will cause flat spots etc. (as would other mods) Its not the mods that cause the problem, you can't do the mods and expect it to work with out setting the mixture up correctly. That usually entails a needle change and then maybe a tweek on the dyno to ensure the mixture is good. You will also find you need the jet to be new or newish as they wear, and the throttle spindle bushes and shaft need to be good or new.

   When I modify carbs they will give better throttle response in all gears at all revs, feathering it or what or anything in between, regardless of weather its a standard road car or any competition car. Sharp edges in any airflow situation create turbulence which is not good for air flow.

   An Su carb is a variable venturi, the bridge is there to create the venturi effect and works with the piston to suck the fuel out, its the restriction in size and the pressure drop that does that, not the sharp edges, they are airflow and powers enemy, and most of them are only there due to the way the bodies are cast / machined.

 

Ive tried to put some images in here but can't...???

 

There are gains to be found right through the rev range not just what by modifying an SU correctly. But as with any engine mod  the mixture HAS to be correct, otherwise you will not see the full benefit and or do some serious damage. Not having a dyno run is false economy, even if you have spent many hours setting the mixture changing needles and tweeking the timing and maybe even filing the needle, used a colourtune and think its OK, you will always make gains and know its not gunna do any damage even for a standard road car.

   Sods law always works against you and there are no shortcuts, if you want a good reliable engine with usable power it takes attention to detail, time money and effort, choosing the right parts that work well together is a bit of an art, takes thought and experience and that includes choosing a needle and carb on any engine.

   Once you start modifying any engine the the extra power will exploit and exaggerate any weakness more, be it the clutch, gearbox, crank, valves or fuel mixture....

Fast and reliable is never cheap or easy, just fun...

[Spider]

Cheers Retroman.

 

I think too this shows - in part - a fundamental difference between an SU and a 'Performance' carb like a Weber. SU always seem to win out on low RPM Power and overall economy, where as the Weber always has the edge in regards to power.

 

Regrettably, the aspect of the SU that makes it work also hinders it's ability to perform and that is, that it's a Constant Depression Carb, meaning that there must be a flow restriction (and pressure drop) across it in order for it to work.

[ukcooper]

[Retroman]

Hi Moke Spider not quite sure why you have a downer on performance Su's. Like me they are simple and they really take some beating, have a look a DV's yellow book, page 123 onwards.

   When it comes to comparing SU and Webers they are two entirely different animals, but a modifed 1 1/2 inch Su will perform equally or better at lower revs and then should equal a 40DCOE Weber higher up. The modded Su will flow just a tadge under 170cfm and a 40 will do 175cfm maximum with 36mm chokes, and despite having 2 barrels will not out-perform the Su by much if any,...other than in making the best noises known to man...

  Economy wise too a Weber can be just as economical, its down to having it setup correctly and driving style, and the tendancy is when power is on tap its used, I have had Minis with Webers and got 30mpg driving quietly (no pun intended), the same car could also do 12-15 with a lead foot style.

   ALL carbs including Webers of all sizes and types, ALL have a restrictions in the form of venturi which causes the pressure drop needed to draw the fuel in just like an Su. The difference is that the venturi (chokes) in a Weber are fixed (but can be changed when rebuilt) and the Su it is variable by the movement of the piston.

   Su carbs are a variable venturi constant depression carb. The constant depression (sounds like a moody teen hhaha) refers to the way the vacuum from the engine lifts the piston in an effort to keep the pressure drop (which is lower hence a depression) constant, which means a theoretically stable pressure drop across the bridge, this allows the fuel to be accurately metered.

  There has to be a restriction in all carbs to create a pressure drop and draw the fuel in, if there is not it will not work.

When rejetting Webers for a different engine the chokes have to be at least 4mm smaller than the carb body (max 36mm in a 40DCOE) if they are bigger than this the pressure drop signal is weaker and they are not as responsive.

 

  I would have to disagree about the lack of ability of an Su to perform, maybe in standard guise but modified correctly they can and do out perform Webers. The green Mk1 (my logo) was a 999cc S with a fully sorted HIF44 Su and it would get 2nds and 3rds in the 1300 hillclimb classes and did 16.45 seconds on the dragstrip with 165 road tyres...regularly beating 1293s with slicks and Webers. It was less than 1.5 seconds slower than Jimmy Macraes 2wd Cossy at Harewood, and I drove it to and from events....

I guess I am arguing that a modified single Su or twins will have an edge over a Weber but ultimately they should both allow any engine to produce its power....and going back to the start of this thread they look standard too.