In what way does a worn throttle shaft and bore upset tuning?
In the SU carburetor achieving good idle mixture quality is achieved by moving the jet in relation to the needle. This determines the mixture to a lesser degree over the entire operating range. Any air which enters the intake tract without passing over the bridge under the piston (false air) produces a leaning affect, which must be compensated for by lowering the jet orifice. This may correct the idle quality, but it will upset the mixture at higher engine speeds even though the ratio between false air and venturi air is far smaller. Worn throttle shafts and bushings also produce a variation in idle speed, as the two components rarely sit in the same relative position each time the throttle is closed. A correct idle speed may suddenly be too fast or too slow depending on this position. Also setting the idle speed may be troublesome as one slows the engine gradually only to find a sudden drop in RPM. Achieving the correct idle speed, and having it repeatable over all throttle closings m
