Saw running backwards

Got a McC 610 late model built for Montgomery Wrads. I don't run itr much any more since getting bewtter saws. Something has been nagging at me to post about it here. What will cause it to run backwards after starting it. You go to start it and when it fires the crank will spin counter clockwise on the clutch end. It has happen about 5 times in about 200 starts. Kind of strange when you go to cut and the chain is going the wrong way.
Just wanted to know if others have ran into this or is this saw just a freak.
Thanks all
Bob
P.S.
Should change my sig at the bottom, fouond a bunch more MIP (Missing In the Pile ) saws. Count now is up to about 85. And of all things got to clean out another shed on the farm soon. The wife is getting madder about all the finds now.

2 strokes with the timing set very near tdc will run backwards just as happily as they will forwards. What's happening is that when you pull the rope you are getting all the conditions right for normal operation then running out of "oomph" on the starter cord. The compressed mixture in the cylinder acts as an air spring and pushes the piston down. If this happens before the piston got to tdc then the engine is given a kick in reverse. If there is enough momentum the saw will run backwards. The biggest danger mechanically is blowing the starter to smitherines as the dogs were not designed to slip in reverse.

Does happen with:
> Cheaper electronic ignitions as module begins to fail.

> Flywheel magnets loses power.

> Coil-Flywheel air gap wrong.

Baas akwards

This phenomenum occured on occasion,with kart engines,during the early 60's.Before the use of portable starters[long shafted Ford starter,and v belt].the karters normally removed the recoil,and used a simple starting cup,and rope.It was a real hoot,to see a double engine kart,try to go with 1 engine running backwards.It would"zero turn",like a dozer.

One of the reasons you don't see this with later two-stroke designs is that the later stuff all have Schneurle-loop (or some variation of same)inertia scavenge type porting, which does not work well with the crank spinning backwards. Older macs still had a 'air dam' or deflector on top of the piston-a sign that it is not a Schneurle loop ported motor.
I remember older RC glow motors did this too. When I got my first Schneurle motor (Super Tigre X60) if found that it would only run backwards very, very poorly, quickly load up with fuel and stall out.

Jimbo

The only glow motors I'd had that would run backwards (actually run the same as forward) were the reed-valve models.  Those with the rotary valve intake (through the drive shaft) were 99.99% reliable in not even starting backwards.  That was the problem, no doubt, with your ST, jimbo, and the reason it ran poorly; it had a rotary intake "valve".

Transfer timing is exactly the same in either direction, the porting style has nothing to do with it, and piston-ported intakes behave exactly like the reed-valve type in being totally symmetrical in either direction.

Glen

Transfer timing is the same but the scavenge port flow is highly sensitive to crankcase windage; they flow poorly when the thing is spinning the wrong way. All of the glow motors I've ever owned (with the exception of some Cox stuff) was rotary(sleeve) valved through the crank snout. The several Fox motors I had ran *perfectly* bass ackwards with a rotary valve. Ditto for the old Enya. Not the ST with Schneurle. Very good forwards! Just not backwards.

Jimbo

I did the same with an 86 Kawasaki KX250, Gave me a hell of a fright the first time I dumped the clutch. Roosted like a MF though.

Originally posted by jimbo1490
...they [transfer ports] flow poorly when the thing is spinning the wrong way. ... The several Fox motors I had ran *perfectly* bass ackwards with a rotary valve. Ditto for the old Enya. Not the ST with Schneurle. Very good forwards! Just not backwards.

Click to expand...

I'm particularly familiar with some older Fox motors (from the mid-fifties) which have the intake valve open (basically) only while the piston is traveling away from the crank in the normal direction of rotation.&nbsp; When running the reverse, the crankcase is open to the atmosphere through the "carb" (basically) only while the piston is traveling <i>toward</i> the crank.&nbsp; They don't run worth a crap (if at all) backwards, regardless the transfer port and/or piston crown configuration.

Nevertheless, the thread is about two-stroke-cycle engines with spark plugs, which require a timing mechanism of some sort which is typically one side or the other of TDC by enough to make a difference.

Glen

back in the day the old rock trucks in the strip mines with detrout diesels were stopped and started backwards for the reverse gear
very cool and simple setup.

sawguy

That must have been a wild ride on that KX250. Would have paid admission to see that. The Chrysler two strokes used on tampers were nasty for pulling that stunt if the timing was retarded. They were designed to be timed for rotation in either direction.