The most common problem if it has been sat for a while is rust on the ignition timing flywheel. If you take the pullcord housing off (recoil) then you will see a big flywheel with what can only be described as a big forked prong with a couple of wires on it. This flywheel should be stainless steel and rusts when sat around for a while without use. Its job is to complete the circuit to the spark plug every time the flywheel passes the forked prong at a given spot on the flywheel, the faster the wheel turns the faster the spark hence the engine runs quicker. Clen the rust off with a releasing agent (wd 40) hey presto!!!!!!!!!!!