Curing a Sticking Piston

When the piston fails to fall freely on to the jet bridge in the carburettor throat, the fault may be due to grit or gummy deposits on the piston and the interior of the suction chamber, or, more usually, to an out-of-centre jet or a bent needle. As an aid to diagnosis, make another test with the jet lowered to its fullest extent by the starting control. If the piston falls freely, one of the two latter faults can be suspected.


To remove the suction chamber, withdraw the dashpot piston and unscrew the two retaining screws. Lift the chamber and jet piston care¬fully, to avoid bending the jet needle. The interior of the suction chamber and the sliding surfaces of the piston should be cleaned with petrol and a lint-free cloth. On no account should emery cloth or an abrasive polish be used. The parts are designed to have a small working clearance which provides a controlled air leak. They should not be lubricated.

Before refitting the suction chamber and piston, make sure that the small screw that retains the jet needle is tight. It is most important that the shoulder of the needle, or the point at which the gradual taper suddenly becomes pronounced, should be just flush with the face of the piston; otherwise the needle will not be withdrawn from the jet to precisely the right extent to match any given combination of engine speed and throttle opening and the mixture strength is bound to be upset.

Do not stretch the piston-return spring, which should be fitted with the closed coil downwards, resting on the thrust washer.