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MG MGB Technical - twin carbs question

could any one tell why only one of the pistons moves up (Left one)when i give the choke a quick pull (air filters removed ) yet when i pull on the gas cable both pistons rise up together I should point out that engine does seem a bit lumpy on tickover but not awful and since i changed the head gasket only seems to need a very little choke for about 10 mins my carbs are the twin su type on a 1971 gt
thanks
Richard
Essex
England
r henderson

Richard
Look closely as you pull on the choke cable under the bonnet and you will see the fast idle cams contacting the fast idle adjustment screws. That's where you adjust to balance the fast idle on each carb.The screws are thin with a fine slot in the end. Normally anticlockwise screws them down and increases fast idle do that on the right carb until it matches the left carb and it should be OK.
Dave
Dave Wellings

I'm going to take a chance and offer the observation that your choke linkage probably is not enriching the carbs equally. I wouldn't be too concerned about this. If the car starts OK--then it's not really a problem. Sometimes we can get so bogged down by details, that we lose sight of the true objective. No where in the shop manual does it address the test you've performed. It's more important to make sure the carbs are properly adjusted and synchronized. Just make sure that your linkage is opening both jets by about the same amount. Those linkages wear and can work unevenly.
R. L Carleen

The choke link between the carbs is either way out of adjustment or (as I suspect) loose. As RL above noted, as long as the car starts it isn't a huge problem, fix it as you get time. The choke should be taken off as soon as possible anyway and I have never needed it in the summer. The throttle motion is only at the later part of the choke movement ("fast idle" adjustment) and is similarly rarely necessary.

The engine must be running for the pistons to rise.

In general, the pistons should always rise together. This is called "syncronized." Only one is rising when the choke is pulled because they are not syncronized. Since the pistons move more or less equally under throttle, the non-synconization is caused by only one carb actually enriching the mixture and/or opening the throttle plate as Dave mentions above. This causes two cylinders to "demand" more air and the piston rises to accomodate.

Mike!
mike!

Thanks Chaps for the advice
R Henderson

This thread was discussed between 02/07/2004 and 03/07/2004

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