Re: Surging in NOT carb
Try holding the throttle vane steady. If surging problem goes away do a static governor adjust. If it engine revs or slows down then it either the carburetor, intake, or intake valve problem.
Just the carburetor works fine on another doesn't there is not problem with it. Engines are slightly from engine to engine.
Sometimes the static governor adjustment has to be tweak a little to condensate for fly weights wear. Also be aware the governor spring must be the correct one and in good condition.