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MG MGA - Carbutetion Issues

This week's project was to add exhaust wrap to the exhaust manifold and an enlarged heat shield for the carburetors. I will post some pictures of the project, but I think it went well overall.

I am however very frustrated withe poor performance of my engine, since after putting everything back at it was, the engine runs very rough!!!
It backfires, hesitates on acceleration, misfires, you name it. These symptoms are mainly visible at low RPMs. Under load and driving moderately fast it works fine.
It also shakes are lot at around 1200rpm.

I spent most of this morning trying to re-setup the carburetors but I was unable as per the instructions in the SU Tuning book.
Basically I am not able to get to a point where the by lifting the pin the rpm increases. In all cases when I lift the pin, the engine will stumble or stall.
I when from jet full screwed-up to open all the way, with no noticeable change. In all cases the engine stalled when the mixture goes richer.

I don't know what I am doing wrong.
I am thinking when I performed the modifications maybe the vacuum pipe got damaged?
I also thought wonder if the fitting on the exhaust manifold with the exhaust is not airtight, so the back-pressure is affected? but I don't see how.

Could there be something wrong with the distributor/electrical settings?
I doubt it because it was working fine before I started to work on it.
I also know that dynamic timing on my distribution is very advances since once with Terry we measure it and it was high. I never changed the timing in my car since i've had it cos it has always worked well.

Any ideas of what do to next?
Maybe I just need to do the carb setup once again starting from scratch...?

Thanks in advance!
Gonzalo Ramos

Do you have oil in the dampers? Have you installed the rear air cleaner correctly? It can be installed upside down which blocks some air passages that control the vacuum to the piston and needle assembly.
Ed Bell

Sounds like an air leak to me. Are you sure you got all the carb gaskets in place and seated well?
Jeff Schultz

If you've not touched anything other than the carbs I suggest you have one more go at adjusting jets as per manual. I had the problems you describe after following the manual so did it again with much improvement. Never found out what I did wrong first time. Try screwing the jet adjusting nuts up tight and then unscrew 2 and 1/2 turns back- this will be a reasonably starting point. Make sure the jets are up pushed up tight. Check the colour of your plugs - this may indicate if your running excessively lean or rich.
J H Cole

My old favourite, did you remove the filter boxes while you worked on the heat wraps etc? If so, you may have put the rear filter box on up side down (very commonly done resulting in similar symptoms). If fitted upside down, the rear carb balancing air hole to the piston becomes blocked, stalling the piston. Effectively your car then runs on the front carb and whatever fuel/air ratio is stalled at on the rear carb.

Have a look at the photos on my website to see what I mean:

Steve Gyles

Note that to check the correct mixture setting, the piston lift using the pin is only 1/32nd of an inch or 0.8mm. That's about 30 thou, i.e, not the full travel of the pin. Later, you can lift it 1/4 inch or 6mm to check for rich mixture.
Also, when adjusting the jet nuts up or down, make sure that the jet is always in contact with the nut - you can screw up the nut, and the jet will not move up with it.
P. Tilbury

I did not remove the air-filters so I don't think it is the issue comes from there.
I will try to do then the carb set-up again before I move onto other things.

Peter, indeed I think I may have been lifting the pin more than 0.8mm. Hence the stall!

Will try again an report.
Gonzalo Ramos


Check that the forks in your float chambers are correctly adjusted as per WSM. You can use a drill bit of the appropriate diameter to adjust them, making sure they're only bent at the point shown in the manual. If they're not right, the fuel is not at the right height in the jet, so winding the jet up and down has little effect. Also make sure your needle valves are not badly worn. If in doubt, replace them with Grose jets.

Malcolm Eades

Have you tried disconnecting the plug leads one at a time to see if problem relates to a specific cylinder? (do you have the plug leads in the right firing order?).
Mike Ellsmore

This thread was discussed between 29/05/2011 and 01/06/2011

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