I had an issue with the carburetor gaskets.
Two years ago I installed 2 new gaskets and was certain the carburetor mounting bolts were tight. But the 951 motor still had a tendency to rev high. Just recently, I rebuilt the carbs again, and after removing the gaskets to inspect, I found there was a small area of the gasket where the gasket appeared to have no impression. There was my small air leak.
I located another set of carb gaskets that were a bit thicker and more surface material. Instantaneously, the idle settled right down to normal....actually had to adjust to increase idle speed as it was too low.