The secondary plate return spring is something many folks overlook. This spring is wrapped around the secondary throttle plate shaft. If it has loosened up, it does not fully close the secondaries and increases the idle by about 500 RPMs.

Simply remove the screw holding the linkage on, and pull the spring off slightly, wind it one more turn around the throttle shaft, then put it back together.

To make sure that was the problem, take the carb off the engine and inspect it yourself, you may see it not returning 100% of the time, but maybe 1 out of 10 times it does not close fully.
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