Stone saw

[fixitflav]

My brother has a Partner stone saw with 2-stroke petrol engine. When it's started it's about as likely to go the wrong way as the right. His first thought was - why isn't the exhaust coming out through the carb when going the wrong way? I had a think and convinced him that on a basic 2-stroke, no valves, the ports work the same way whichever way it's turning. Only difference is if it's meant to fire say 5° before TDC, when the wrong way it's 5° after.
I assume it fires before TDC when cranking, and sometimes the advance overcomes the forward momentum, kicking it the wrong way. It does sometimes give quite a nasty jerk on the starter cord, for a small engine. It has electronic ignition, and the on-line operating instructions don't give the timing point, and there's no way of adjusting it.

Anyway, I thought it was interesting, anybody comment?

 

[Burnerman]

Indeed, it is only the ignition timing that determines which way the two stroke motor will spin.....it’s the inertia of the recoil starter turning the flywheel that spins it in the direction we want.
It sounds like this engine has slightly advanced ignition timing but of course it isn’t adjustable as the flywheel is keyed onto the crankshaft.....maybe it’s just one of those manufacturing tolerances.
I’d recommend the use of the decompressor if this engine has one when starting.
John

 

[Tigercubrider]

A mate had a russian motorbike that would occasionally decide to run backwards.
You could never quite be certain when you started it if it would go forwards or backwards when selecting first.

 

[fixitflav]

Thanks gents. The manual shows a trigger unit with contact points, but that must be diagrammatic as there's nothing mechanical which could open them. And of course not adjustable.
It doesn't have a decompressor.

 

[Burnerman]

Modern engines have an igniter unit, with no contact breaker points but initially the cb points lurked behind the flywheel.
The gap between them - typically 15 thou/inch - would slightly alter the timing.....the dwell angle. In fact the stator plate they were fixed to often had elongated slots for adjustment.
I’m going back to the 70’s here!
John