Old snowmobile engine information OMC-Evinrude Skeeter opposed twin

[Diy89]

I was sitting on the sofa thinking i should be building when i got to thinking about this old snowmobile i had long while ago.
The sled was an OMC Evinrude, Skeeter wide track model. The engine was the opposed twin. What boggels me, then and now, was that it appears that both cylinders fired together. At first, i thought one spark went out with the exhaust. But during the project of getting it running, i found only one set of points. It had two coils connected directly to the plugs. The manual showed the timing to be set on either cylinder with a strobe light. The crank journals were directly stacked on top of each other. Unless one rod was shorter, both pistons would be at tdc the same time.
I never took the engine apart, once i had it running, i just rode the heck out of it. ;D
it had gobbs of torque, and a really unique sound. Top speed though was about a good walk on hard ground. :'(
Anybody have info, insight, thoughts about how i would build the crank and time the ignition?

 

[steamer]

I can only assume it is a 2 stroke?

The set up would make sense if that is the case, as the charge must be ported through the crankcase, and there wouldn't be any pumping action to pull a charge into the crankcase otherwise.

Dave

 

[vlmarshall]

Unless one rod was shorter, both pistons would be at tdc the same time.

Even the short-rod piston would still be at TDC, just a slightly lower TDC. ;D



Dave's right about 2-stroke and crankcase pressure... and if it were a 4-stroke, the extra, 'unused' spark at the top of the exhaust stroke is quite common.

 

[LADmachining]

A quick google search shows that this engine is a horizontally opposed two-stroke twin.

By necessity, the engine must have a 180 degree crank, i.e the two crankpins are mounted 180 degrees apart around the crankshaft periphery. This ensures that the pistons move in and out together, which is necessary to provide a pumping action in the crankcase. The pistons move out together, resulting in an increasing volume crankcase volume, and hence a drop in pressure which allows the mixture to be drawn in. As the pistons move together, crankcase volume decreases, resulting in an increased pressure to force the mixture into the cylinders through the transfer ports.

As both cylinders reach TDC at the same time, the coils fire together, which is why there were only one set of points. It produces one power stroke per revolution, the same as a single cylinder engine, but with two cylinders the force exerted on the crankshaft is doubled. That is why it was a very torquey engine!

Edgar T Westbury designed many different types of engine over the years. His Craftsman Twin is an example of a flat twin two stroke.

See http://www.modelenginenews.org/gallery/p3.html

Anthony

 

[RobWilson]

This may bee what your after ,THE SIAMESE BEE ENGINE ,

plans here http://www.billreichart.com/engines.shtml#gyro

i have one under construction ;D

regards Rob

 

[Diy89]

Thanks for the info! I found an old scan of a manual from google once i knew what i was looking for. I was in the shower and realized i have a bunch of old string trimmers in the shed. My plan is to merry the cranks and build a crankcase. Cheating i know, but i'll shoot some pics as i start putting it together!