Have you tried to de-carbon the engine? It's pretty easy and with most people running an extremely rich 20/1 mix it's likely got a ton of carbon on the piston.
Remove your head, and turn the engine until it (the piston) is at the top. Use a wooden dowel or other piece of wood to remove the most of it, then finish it up with a brass brush. The kind that you put on a Dremel tool work great. DON'T use a scraper or screwdriver!
Use a shopvac to keep it all clean as you go.
It should help.
