You must realize that the idle speed is controlled by the timing adjustments, not by carb butterfly openings.
So, if all butterflies are absolutely closed, and I mean closed, the high idle could be caused by abnormal idle timing due to a failed
timerbase, or powerpack.
You could try to get a timing reading which would verify if the timing was way advanced for some reason. The timing at idle should be at or around 4*ATDC.
Look for way off value like maybe 10*BTDC, which would cause a very high idle. All these numbers are just examples.
If for example if you are seeing TDC timing at idle instead of my suggested 4*ATDC, that would not cause the problem. The value has to be
significantly way off the spec.
Alternatively, look under the flywheel from the side, and try to determine if the timerbase is sticking. It should freely rotate as the
throttle is advanced to WOT and then returned to idle. Old sticking grease is the general culprit.
Sticking timerbases are a known problem, some linkage designs are more prone to the problem than others....
