Perfectly happy with this until last week, when started losing power. Took it into to local garden machinery shop where they "tuned" it for £10, which took about 2 minutes. It ran ok for an hour or so, then started losing power again. So yesterday I washed the air filter, which had some oil in it. Today I checked the spark plug, which was a bit sooty, and gave it a clean. It managed to cut for about 30 minutes, but it is still running very roughly, and now when I try to increase to full throttle it speeds up but then starts to die, only recovering when I release the throttle. It doesn't have enough power to turn the chain on the pole-saw If I remove the air filter it roars away with loads of power. I am wondering if it is running rich, but I can't see any way of adjusting the mixture. Any thoughts?