20 replies to this topic

[Military84]

I was just wondering if anyone had any tips on tuning the carb on my 998cc Mayfair. It just doesn't seem to be running quite right. Also, any maintenance tips would be great too. Like I said when I introduced myself, I'm new to classic minis but I'm far from new to tuning, so feel free to throw any terminology at me that you see fit. Thanks in advance.

Josh

PS. For those who may be wondering why my avatar pic looks strangely familiar, it's because I bought the car from Motion.

[siggy]

Buy yourself a manual which will go into more detail than we can here, then ask specific questions.

Siggy

[Military84]

I've already ordered the manual, I was just wondering if there were any specific things that I should watch out for that aren't mentioned in the manuals. I think I've IDed it as being an HS6.

[Dan]

Extremely unlikely, the HS6 is a fairly rare carb anyway and it would run terribly on a 998 unless it was in a mentally high state of tune.
More likely is an HS4, that was standard on the vast majority of 998 Mini's.
HS4 is 1 1/2" choke, whilst the HS6 is 1 3/4".
The only tips are to make sure it is in good condition before tweaking it really. Make sure the throttle spindle seats aren't worn (there should be no radial lash on the shaft) as this is a good source of air leaks on used carbs. If they are worn you can get bushes to fit in the body, but it is quite an engineering job to fit them and get them reamed.
Make sure the jet isnt worn (the hole in the jet needs to be exactly round, the biased needle tends to wear a groove in the forward side of the jet which means the fuelling is unpredictable).
Make sure all the linkages are clean and free to move (don't gum them up with loads of heavy grease).
The float chamber fuel level is very important so spend some time checking that, and make sure the needle valve is in good condition. It is likely that a good used needle valve will work better than a new one as the new type tend to seal poorly.
Also make sure that the chamber is horizontal (viewed from the side) when in position on the engine. If all the parts are standard then it will be but since these carbs were fitted to many cars there are different angle spacers available so it is possible that it has the wrong one fitted. Along these lines check that all the parts are as they should be, the dashpot spring is something which gets overlooked a lot.
Make sure the piston is a good fit in the vacuum chamber and seals nicely and there is the right amount of the right oil in the pot.

[Military84]

After looking at again I misread some other information and you were correct about it being an HS4 (not that I doubted you). I checked everything out and it appears as though the dashpot damper (what looks like a dipstick into the dashpot, right??) is a really tight fit. Difficult to put back in. What is the proper oil for the dashpot?? The dashpot itself seems tilted towards the front of the car, but the air inlet and air box assembly appear to be level. There is a lot of red gasket "making" material around all the joints so I assume it has been taken apart and possibly rebuilt at least once. I have noticed after I've driven for long periods of time (approx. 20min) without stopping it idles poorly and sometimes stalls when I do eventually come to a stop. Is this a symptom of poor carb tuning or is there something else I should check out??

[Jordie]

the carb should look like this



As you can see, it is not vertical and does tilt forward as normal. However, i run a cone filter....but the standard filter uses a bend at the rear (or air intake) of the carb, to turn the filter through 90 degrees.

The poor running of car may be down to many things, but winter doesnt help.

Check the oil in dashpot and the damper shud have some resistance when u put it back in. if u need to top up using a quality 10/40 oil.

Adjusting the idle atouch higher may help your idle and stalling problem.

Since cold air is alot more dense you may need to run your carb abit richer over winter to overcome the air/fuel ratio.

Hope this helps

[Guess-Works.com]

SU provide a bespoke oil fo rthe dashpot, but is rather expensive, If you can get 3 in 1 oil in Germany, use that as it's pretty much the same viscocity... Use engine oil as a last resort as in cold weather it can get too thick and will prevent the damper from rising...

If you want to get into tuning you will need a few basic bits of kit which will make life a lot easier, A timing light ( strobe ), a multi meter, and a glass spark plug... The latter enables you to see the colour of the combustion, and hence the richness of the mixture...

You'll also ( with an HS carb ) want a stubby spanner which will allow you to adjust the mixture screw at the bottom, without too much restriction...

[Jackman]

What oil should really go in the dashpot?

[Guess-Works.com]

depends who you ask...

[Jackman]

Currently i have break fluid in mine... well it was when the mechanic did it.

[Military84]

What level should the dashpot oil be at?? The guy I bought the car from has been using the same oil as the engine in the dashpot(Castrol 10/30). I think I want to change it out for something better, what's the easiest way to drain it??

I have recieved one of my manuals that I ordered, Vizard's Tuning the A-series engine. I've been doing some reading in it and I think I'm going to try a few things suggested in it to fix my driveability problems. I did raise the idle a bit last night and it idled much better today, but now it has a tendancy to diesel so I think more idle tweaking is going to follow. I probably increased it too much.

Thanks for all the input, I appreciate the help.

[Dan]

The dashpot oil should be 1/2 inch below the top of the damper tube (that's the tube that you can see inside the pot when you remove the damper piston, not the threaded outer tube that the piston screws into). It's there to work against the piston to control the reaction rate of the piston as it rises and falls in the vacuum chamber. There is no ideal oil to fit as the required viscosity depends on other tuning issues, and selecting the right oil for your setup is part of tuning an SU properly. Having said that, 3 in 1 or similar is good for most applications in the UK, or SU or Penrite carb damper oil. Some people say that cycle oil is the best but it's too thin really, this is a myth that started because it was the right oil for the old HD type carb but it's no good for the more modern carbs.
Your engine oil is probably too heavy for this application and this may be causing some of your problems. Remove the old oil by taking off the vacuum chamber and pouring it out, there isn't much in there. If you do add too much, don't be overly concerned as the excess will get sucked through the carb body eventually. Don't make a habit of it though or the residue will build up in the vacuum chamber and gum up the piston.
It's the float chamber which needs to be horizontal, that is the fuel canister next to the carb.
Sounds to me like your carb is due a good overhaul, a full rebuild kit including throttle spindle is worth considering.

[TimS]

Depending on the tune of your engine and how quickly it accelerate, changes the viscosity of the damper oil as a car with a light weigh flyhweel accelerating alot quicker than a std engine would need a thinner oil to alow the dash pot to rise quicker, but there there are also different springs!

[Military84]

Thank you for the info. I think I've solved the studdering/hesitation issue...It wouldn't start today so I checked a few things and noticed the bracket holding the ignition coil (what I assume supplies the ground) was loose. I hand tightened it to get it home and it no longer had the same drivability problems.

I'm sure it's running too rich, but I'll wait till I get the clear spark plug to try to tune that.

I checked throttle spindle "play" and it seemed to be tight. I checked the dashpot oil, which was at a good level, but definitely seemed too thick. I'll change the oil out this weekend. I do plan to buy a rebuild kit for the carb.

I am having a hard time figuring out a problem with the throttle cable itself. The pedal is terribly stiff and is very hard to press down to a part throttle degree. I can hear a grinding type noise near the bulkhead when I press the pedal in. I've checked to make sure the cable isn't contacting the bulkhead and it's not. It's a brand new cable, which is why this is very surprising. The brass fittings at either end of the liner appear to be secure. Any suggestions as to what it might be??

Here are some pics of my carb and a front pic of my engine. If you guys happen to spot any really noticable problems I may have missed please point them out. Thanks again.

[Purple Tom]

your master cylinders are on the wrong side of the car!!!!

oh, hang on a minute, no its ok! :wink:

Everything looks ok to me, can't imagine why the thottle cable would feel so stiff, unless there is something actually blocking the movement of the pedal near the bulkhead?