SU Carb Throttle Shaft Wear
Unstable and erratic engine idle speed, (IE: idling at 800 one time, 1,200 rpm after the next throttle opening), can be traced to an unacceptable amount of wear on the front SU carb throttle shaft. Most of the wear occurs on the forward throttle shaft/bearing area due to the linkage operating arm being at the very front of the dual carb arrangement. The wear results in the throttle plate (butterfly valve) not aligning with the carb bore and this binding the valve against the body, This gives inconsistent throttle plate opening/clearance and hence varying throttle speeds. Air leakage along the throttle shaft due to wear will compound this condition and cause idle fuel mixture inconsistencies. As the butterfly will also wear badly in the shaft area, throttle plates and shafts must be replaced together. Re-bushing of the carb body may be necessary in extreme cases of wear.