FIRST do EVERYTHING that Cruiser54 recommends!
Here's is something simple "fix" I do on many different types of engines that are down a little on power. It has worked very well for me for many years. I've noticed over the years, in many engine rebuilds, that the oil drain-holes in many piston deigns are inadequate and create problems. Since the "oil- squirter" holes, in the connecting rods, are always throwing oil on the cylinder walls, a problem develops when the piston oil drain-back flow, to the crankcase, is reduced. The oil then "pools" around the rings and then solidifies/carbons-up and locks the piston rings in a "closed position" on the piston. Since the piston rings are locked closed (i.e. no tension against the cylinder walls) the compression gases blows by the rings and slight power losses occur. The "fix" is simple and inexpensive to do. If the piston oil drain-back hole is a very poor design, you just continue to do the "fix" on a regular maintenance basis.
Here's what I do:
1. Buy some Gumout; which has solvents for oil and gasoline lacquers.
2. Pull all the spark plugs and spray the WHOLE can of Gumout equally into ALL the spark plug holes. Let it set overnight.
3. The Gumout will dissolve all the lacquers and dried oil, from the piston rings, and then drain-back into the crankcase where the solvents will evaporate. The rings will then snap-out and contact the cylinder walls, as they were designed to do, to reduce compression gas blow-by and restore power.
4. The next day, replace ALL the spark plugs and fire it up. You'll get a little white smoke at first then it will clear up.
Best regards,
CJR