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MG MGB Technical - carb mixture

I have SU HS4 carbs on 1975 BGT. MOT tester said mixture too rich so I have checked it out using gunston colour tune. Now when I lift the pin on the front carb revs rise a little then settle so OK. But on rear carb the engine tries to stall when the pin is lifted even when the mixture screw is turned right down. The carbs were recently rebushed and checked by local MG expert. Any ideas on what the problem is? I have not yet checked the float bowl level.


Rod
R E Merrall

Well, Rod, probably checking the float levels is the first thing to do. I find it easier to check by pulling the dash pot and piston and seeing how far down the jet tube the level is. Sorry, I don't remember what the distance should be. You may need to pull the "choke" to see the level more clearly.

Then, while the pistons are out, see if both needles are mounted with the shoulder flush with the bottom of the pistons. You might want to check to see if both needles are the same number, but I believe I have read that at idle they are all about the same.

And, are you lifting the piston the correct distance? If you lift it as far as the pin will lift it, that is too far. There is probably some good info in Paul Hunt's web site that will have the details I can't remember.

Then, it is also probably worth looking for a vacuum leak.

Charley
C R Huff

Well, as you state, it's rich and if you continue winding the jet down it will get even richer, bog down and stall. As a guide, look where the jet is on the front carb and replicate it on the rear and use that as the starting point. Also check that, without the dampers, the pistons easily fall at the same speed and "clunk" on the bridge. If they don't, the jet might need centralising. An off centre jet will hold the piston up causing it to run rich at, and even just above idle.
Allan Reeling

When you say 'mixture screw', on HSs the mixture is controlled by the large nut on the jet underneath the carb.

And as Allan says if that is screwed right down it will be grossly rich.

The lifting pin will rise a significant amount before it reaches the piston, you can feel it get slightly heavier then, the piston itself should only be lifted 1/32". That sounds impossible to measure, but in practice it is a case of barely lifting the piston once you reach it.

With HSs it is relatively easy to remove the float chamber lids and compare the fuel levels.

Sounds to me like they should be set up from scratch i.e. balancing airflow and mixture. Tinkering individually with the carbs is rarely successful.
Paul Hunt

To me, it sounds like the main problem is that the carburetors aren't balanced with each other. One of them is doing most of the work, while the other one is trying hard to catch up. As Paul stated, starting from scratch is probably the best way to get them running correctly. RAY
rjm RAY

Thanks for the advice. I compared float levels and the level in the rear carb was lower than the front although still half full I tried to adjust the level in the rear carb but it didn't seem to make any difference. What next?
R E Merrall

The float is lower on the carb you say is running rich? That's opposite to what would be expected as the fuel level in the jet would be lower and therefore a weaker mixture. You need to get these both the same (as near as dammit) before moving on. Don't forget it could be the front one is wrong but adjusted to compensate! So check both from scratch. Check the float valves are operating correctly or replace with new if you have any suspicions. Check the jets are the same number and, as Alan says, centered correctly. Check for wear in the jets if the needle has been running off centre. Then do the full set-up eg balance, mixture etc after rechecking the timing etc.

Or bring it over next weekend and we can go through it together :)

Best of.....
MGmike
M McAndrew

Whilst the level being low in one carb would normally contribute to a weak mixture is correct, but that assumes the rest of the adjustments on both carbs are the same. That seems highly unlikely in this case, hence the recommendation to set up from scratch.

As the level is different, that would be the first place to start. How did you try and adjust the rear carb? Early floats had a metal tab to operate the float valve that could be bent, later all-plastic floats cannot and the valve itself has to be shimmed with washers. However that is not normally needed, unless that float valve already has excess washers adding them will lower the level still further. Both need to be checked as maybe the front one is high. The float height on HSs is supposed to be such that, with the float chamber lid upside down, the float should just rest on a 1/8" to 3/16" round bar placed across the middle of the lid parallel to the hinge pin.

Faulty float valves may be contributing to different heights, as could faulty lids, or faulty floats.

Having said that I've never found float height made that much difference - in normal driving at least. At one time I had a sunk float in my V8 which was flooding in a major way. But as I had fused both the fuel pump and OD circuits I was able to cross-connect the two circuits and use the OD switch to turn the fuel pump on briefly to recharge the carbs, then off, only turning it back on again when the engine started to splutter many seconds later.
Paul Hunt

If the rpm is rising you may be able to weaken it a little, if the revs stumble that implies too weak. As a starting point drop the jets to 60 thou from the bridge for both carbs. Ensure the carbs are set evenly in terms of how much each butterfly is open, maybe visually from how much the lever is lifted from closed and/or hiss of airflow. Worn jets go oval and can fool you for settings in terms of jet depth!

Peter
Peter Burgess Tuning

Took your advice and set up from scratch and now much better. Only odd thing is that the revs are a little high but on the front carb I can back the throttle adjustment right off and the revs are still too high. Don't understand that. Rear carb OK.
R E Merrall

Rod,

The carb linkage is tied together, so if you back off one, the other can hold both open.

Part of matching them is to get it set so that the butterflies are the same, then lock the shafts together, then always move the throttle stop screws the same amount if adjustment is the same.

You can try backing both of the all the way off, then turn each till it just touches the stop, then turn them both the same until the idle is correct. It actually should have been done when you had the link between them released.

Charley
C R Huff

Rod,
Do you know if the throttle plates have the poppet valves fitted? Is so I would replace them with solid ones.
As Charley points out, the interconnecting rods need to be loosened off when adjusting the throttle stops. What are you using to balance them? I find the old tube in the intake difficult and therefore use a gunsons unit, dead easy to use.
If you can close off one of the throttles completely it has something still not quite right.

Best of....
MGmke
M McAndrew

The carbs need to be set up with the interconnecting shaft clamps loosened. Then at the end you tighten them, and check that the air-flow is still balanced when just off idle as well as at idle - after all, being balanced off idle is far more important.

Finally check that there is a little free play in the throttle cable before the pedal starts opening the carbs. If the throttles are hanging on the cable you won't be able to set the idle speed correctly.

You should always be able to stall the engine by backing both idle screws off. If you can't, either the butterflies are hanging on the cable, or there is some other problem like stiffness in the linkages, or a butterfly not seating correctly in the carb throat. It can also happen with severe wear in the throttle shafts and bushings, such that fuel mixture can still get past a fully closed butterfly. Faulty poppet valves can also cause it, but personally I've opted for soldering them shut. The only benefit of a plain butterfly are a fractionally increased air-flow at full throttle, but you have to balance that against the job of replacing it and getting it correctly seated, screws locked etc.
Paul Hunt

Done as suggested and disconnected linkages and checked that with everything backed off the butterflys are closed. I then started up and the tickover is 1200 revs with the butterflys closed! With my balancer the front carb reads 10 and the rear 8. What is going on?















1
R E Merrall

You must be getting air in somehow, and it is likely that the butterflies are not completely closed, even though they appear to be. This is why fitting new butterflies is a job that requires a certain amount of care. It is very easy to have them very slightly off centre and then a tiny amount of air can bleed past them. Look at them very closely with a bright light and a mirror if it helps. They should fit the throat of the carb exactly with no gap at all around them.

If they do fit properly, you have to look at how air is getting into the inlet manifold on the engine side of the butterflies. Are the butterfly spindles a loose fit in the carb body? Air can get in there. What about the vacuum take-off for the servo? Is that air tight?

Good to meet you the other evening at the Caledonian Centre AGM. Sorry I can't be more help but its a quite a few years since I had to mess around with SU carbs.

Mike
Mike Howlett

Thanks mike. The butterflies are new as are the throttle spindles which were rebushed by local mg experts who also checked out the carbs. Good to see you too.

Rod
R E Merrall

I would remove the carburetors and check the gaskets, phenolic spacers and the intake manifold for possible leaks. The intake/exhaust manifold gasket should also be inspected for any minor flaws that might allow air to enter the mixture. RAY
rjm RAY

Rod,

Any chance that the fast idle part of the choke linkage is holding the butterflies open a bit?

Charley
C R Huff

I think Mike may be close to the mark. Try forcing the butterflies shut with a screwdriver against the linkages and see which carb is causing the most problem and if it does drop the idle when you force it. If it does you need to take the carb/s off and relocate the butterfly/flies. Poor positioning to start with including not slackening the idle screws completely off can cause the problem. You can always test the carbs/manifold for air leaks using a penetrant spray such as wd40 when the engine is running.

Peter
Peter Burgess Tuning

"The butterflies are new"

In that case I suspect the butterflies weren't correctly installed or are the wrong ones.

While a vacuum leak on the manifold side of the carb will cause an increase in idle for a given butterfly opening, compared with no leak, if the butterflies are completely closed there is no way a vacuum leak can cause the engine to continue to run. An engine needs fuel as well as air to run, so if it is running fuel must be getting through - somehow.

The balancer showing 10 and 8 confirms they are effectively open, i.e. actually open, or fully closed but air getting past somehow. There are different butterflies, HIF for example have notches on one edge.
Paul Hunt

Also the butterflies have chamfered edges which accommodates their slight angle at rest. But when installing they need carefully centring before the bifurcated screws are tightened.
Allan Reeling

Hey! It's nice to have PB say I might be right. Can I ask Peter, what effect spraying WD40 into the carb throat would have? It's something I've never done.
Mike Howlett

I wouldn't spray it in the throat Mike (poor use of English language on my part) I meant spray everywhere except down the throats which is where the air should be entering the system.

Peter
Peter Burgess Tuning

Thanks for all your help; if I had adhered to it precisely all would have been well, but alas I am an idiot. I backed off everything except the fast idle screw on the front carb. Once I realised that all was well.
R E Merrall

Don't be hard on yourself Rod, we have all done silly things many times over. Every day is a school day.
Mike Howlett

Perfection is a goal, not a reality. RAY
rjm RAY

This thread was discussed between 08/04/2016 and 14/04/2016

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