Thanks both,
I stripped it down and the cylinder, piston, rings and muffler look in good order.
It is a Stihl FS85 and has seen a lot of work. I tested it yesterday and noticed that it runs well up to about 30 minutes. Then as you press the throttle the power drops away, also if you raise the cutting head the engine runs well again. In effect if the cutting head is down in cutting position the fault persists, if raised so the cutter is more or less horizontal it runs well. I also changed the S/plug which had a white coating on the electrode. The fuel filter looks in good order but there seems to be a slight leek where the line enters the tank. Also seems to be some 2 stroke oil present on the air filter elements. I'll take some photos today and post, but any ideas in the mean time???
