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MG MGB Technical - Idle Problem

What would be the reason for my car running well without the aircleaners and then running poorly with them installed? Would this mean the mixture is too rich? I have a 1970 B.
Shell

Air cleaners should make very little difference, if it does it implies they are either clogged or if non-standard too restrictive. But if they are the originals make sure the bases are installed the right way up with the passages though the base lining up with the ports in the carb flanges.

A weak mixture is usually characterised by an intermittently varying idle, rich mixture with a regular variation. Using the lifting pins (preferable, and one at a time) or adjusting the mixture nuts/screws (only every adjust both carbs by the same amount in the same direction) should also tell you whether the mixture is weak or rich with the air cleaners on. Running for a while with a rich mixture will also turn the plugs black if not fouled.
Paul Hunt 2

Paul,

Thanks for you reply. I checked to make sure the bases of the air filters were installed correctly and they were. I have the original style air cleaners which don't seem to be clogged and are relatively new.

I can get the mixture correct (using the lifting pin method) with the air cleaners off. It is when I install the air cleaners, the car will start but won't run well.

Maybe I am doing it wrong butI will try starting from scratch and see if I can find what I am doing wrong.
Shell

Shell, the air filters are quite restrictive, especially if your engine has been modified. I have the same problem in my own car. If it gets worse when you apply the choke, then yes, you are too rich.
Jeff
Jeff Schlemmer

Ok, I reset the carbs using the lifting pin method and still had the same problem. With the air cleaners off, the car runs fine but when I put the air cleaners back on, the car would barely run. I leaned the mixture and the car ran fine. Maybe I don't understand the lift pin method? Do you push the pin all the way up? Does this give you the 1/32" lift on the piston.
Shell

No, it isn't pushed all the way up, it is barely lifted to check for correct mixture. 1/32" is about the thickness of a credit card.

Clifton
Clifton Gordon

The lift pin is handy in a situation such as yours. You can adjust the mixture with the air cleaners on. But it sounds like they're a bit restrictive.
Tom

Remember that the lifting pins go up quite a way before they start to lift the carb piston. It is the carb piston that you lift 1/32", the lifting pin will go up more than this. Unscrew the damper cap and leave it lying loose in the damper, then you can see (with the air cleaners on) when the piston moves.
Paul Hunt 2

This thread was discussed between 18/08/2006 and 24/08/2006

MG MGB Technical index

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