Hi everyone, I've been trying to tune my 1098cc engine which has a 1.5inch SU carby. Currently it is idling very high and seems erratic (seems to idle between 2000-2800RPM).

When I try to decrease the idle speed it will continue very erratically, hence when it's idling around 1000rpm it will sometimes stall (as it jumps between idling at 1000-300rpm and stalls ).

The carby was supposedly off a working car and I gave it a quick clean etc before attaching it.

Anyone have any idea where im going wrong? Any idea where to start (you'll have to dumb it down for me)?

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By what you say, it appears it's something to do with the carby. However, it could be any number of things, ignition, carby, no oil in dashpot, mixture, loose jet, loose manifold - sucking in air, distributor, fuel pump, fuel mixture, too numerous to go into detail here. Basically you need to eliminate one thing at a time till you find the problem. If you do too many things before testing, you won't know what the problem was. If you're up to doing the work yourself, it may pay to get hold of a worshop manual.

From what you describe, is the throttle spindle worn? This is a round brass bar that can be seen on both sides of the carby body and it is connected to the accelerator cable mechanism which turns it to open and close the butterfly, controlling air entering the engine. When badly worn, it lets in too much air at idle but can also let the butterfly disc to move to various positions to close off all air (stall) or get stuck open slightly to idle high. If you can hold both sides of the spindle there should be virtually no sideways movement.