the spark plug in my flymo petrol mower is very sooty and the engine revs rise and fall whilst it is running.

Kevin g, May 2004

All 2 cyle engines are designed to run at full rpm so the fuel can burn efficiently. Air fuel mixture is the problem. Check and clean or replace your air filter. Other than that it is your carb.The air fuel mixture screws have to be adjusted correctly. That should be done by a technician at a shop.

JOHN, September 2004