When I start my OSSA MAR the engine revs just keep on climbing until I kill engine. Carb slide working OK.

Any ideas.

sounds like an air leak somewhere 

crank seals / gaskets etc.

what carb have you got fitted ?

have you recently worked on the bike before this happened ?

 

Crank seals replaced. First time bike has been run in at least ten years. (Not by me). After several tries starting, poured a little petrol in plug hole. It then started but just proceeded to rev until I hit the kill switch. Left it for half hour or so, started again with same result.

Amal carb fitted. Needle was on bottom setting. I've now moved it to middle but bike won't start now. Not sure if it's because I've moved needle position or just that it's a pig to start anyway.

Air leak to the crank. Get some silicon sealer, smear to all round the joins on the outside and start it. If there's no revving, there's your issue, new crank seals.

 

Are there any cracks or leaks on the inlet manifold?  I've had that before, it only looked small but caused big problems with starting and over-revving.

For what its worth, one problem that can occur with some 2 strokes, particularly older types, is that if the carburettor is flooding (even slightly), then when the engine is stopped, fuel can drain into the crankcase. Then (after standing for a while) when the engine is started, this causes a very rich mixture, and because it is liquid fuel (not atomised by the carburettor), it may take a while for all of it to be cleared out of the crankcase, thus causing the engine to over-rev for a while on start up. Of course, once this exess fuel is used up, the engine runs normally, until the next time it's started after standing for a while. This is why many older 2 strokes had a crankcase drain plug. There may be another reason for your engine's misbehaviour, but I hope this helps. M.S.

I would certainly check the float level, and also the action of the float valve, and the condition of its seat. M.S.