If it runs better soon after you turn off the fuel, it sounds like either the problem is in the governor, or it is running rich whenever the fuel is on. Is the choke opening completely? Does the float bowl overflow (flood) when you leave it standing, not running, with the fuel turned on?
If it doesn't flood, you might look for a mixture adjustment screw. If it has one - and only one - you could go through a simple adjustment procedure. Do this with the air cleaner installed, or you'll be wasting your time. First, find out the current adjustment setting: note where the mixture screw-slot is pointing, then gently turn the screw clockwise until it bottoms lightly, noting how far it rotates. Write this down somewhere. Then put the screw back where it was in the first place, start the engine, run it at a moderate above-idle speed, and turn the mixture screw clockwise slowly. If the engine speeds up, keep turning the screw until it reaches maximum speed and begins to slow down. Note the position of the screw. Then turn it anticlockwise until it begins to slow down. Set the screw halfway between the first position (lean limit) and the second position (rich limit). Make a new note of the screw position.
If it doesn't speed up when you start to turn the mixture screw clockwise in the first test, chances are the mixture is not the problem, or not the only problem. Tell us what happened, and we'll go from there.
