I went out practicing today and after a couple of hours I occasionally felt a rumble our unbalance in the engine. It wasn't constant and only at low revs. I'll have a look during the week, but in the mean time any ideas where to look?

Classic symptom of worn main bearings in a two stroke.

Hi barfy, almost certainly magneto side crank bearing.

Thanks guys. Is it a main bearing even if it isn't happening all the time and just (sometimes) just about 1/8 turn off tick Over?

Barfy. Check motor mounts.

You can pull the mag cover & check for movement.

Those engines have very robust bottom end.

Larry

I would agree that it could be mains on that old bike. But mains are constant. Check the rear hub bearing as it can be going bad and feel like the motor.

also check the exhaust system for loose or broken mounts or touching something where it should not

Thanks for the ideas. I'm going to start looking for the problem tonight.

I'm favourite on something loose. Just before this outing I took the clutch off and washed it out so I hope that I didn't tighten something properly. There again it's probably wishful thinking.

The bolts at the rear of the bash plate were loose because the thread had stripped. Also one of the engine mounts was loose. Seems to have lost the rumble (around the garden anyway).

Give a proper go in anger next weekend.

That should hopefully account for it. I had an engine bolt come loose on my Sherpa once on the road and the rumble was bad enough for me to think the clutch flywheel weight had come loose - or worse. Rumble disappeared when we tightened the bolt.... Also had the same affect on another Sherpa when a badly mis-shaped bashplate was touching the underneath of the engine. The rumble disappeared when the bashplate was removed