Spark, Fuel, Air, Compression. The four basics to check prior to going on a deep dive. Agreed that it’s not the carburetor nor the ignition with your swap testing. You installed a known working carburetor and magplate on the problem motor and it still doesn’t work.

Have you verified that fuel is actually getting to the carburetor? The sintered tank filters in the OMC classic motors are notorious for getting clogged when they get stored with fuel mix in the tank, then sit for decades. Removing/cleaning the tank filter, fuel lines, and of course the tank, is a necessity on these 3 hp motors.

Tubs has some good advice regarding the rings possibly being stuck. If you do a compression test and the compression is 70 psi or better in both cylinders then compression is ok. Removing the exhaust cover will likely cost you a gasket, but a new gasket, and resurfaced exhaust cover is a wonderful thing.

Dennis Drake wrote:

seakeye12,

I looked over the split of the 2 crankcase pieces and there’s no visible opening at the crack between the 2 pieces. But it’s probably not visible. I know it doesn’t take much of an opening to allow a significant leak of air during vacuum or pressure.

Do you know of a way to test for a possible leak?

I’m thinking I could fashion a plate that will seal the throat of the carburetor, but has a tire valve in the center that would allow air to be introduced. Then position the pistons so they are both at their half-way position, allowing them to both be blocking their exhaust ports and their ports to the crankcase, but which leaves open their ports to the carburetor. Then pressurize the crankcase through the carburetor, and start spraying soapy water on the outside of the crankcase to look for leaks.

Do you think this would work? Do you have a different suggestion for detecting a leaking crankcase?

Well;  the way you describe is the way it’s typically done on chainsaws and the like.  Not sure it would show much on the Lightwin because there are no seals at the top and bottom of the crank.

Plus;  if the engine has not been split;  I would doubt that there would be enough of a leak anywhere to prevent starting.

I don’t remember….did you remove the plugs and squirt some fuel in the cylinders and then see if it would fire?  If not….I would try that.

Then….if it fires I think I would consider Tub’s comments about stuck rings.  Maybe you could try spinning the motor over with a drill……

From the Long Longer and Longest LongShot Department:

There’s a flywheel key in place ?  The Point Cam in not installed Upside Down?