Did you check to see if you actually have spark or the engine just won't fire? IF the latter, I suspect the flywheel key slipped out of place OR you didn't get the flywheel torqued and it sheared/partially sheared. Gap between coil and flywheel, .010" - .014".

Walt Conner

I have the same issue, but the speed of the starter slows and then stops. The engine flooded due to junk in carb bowl. I cleaned it. Now it is cranking at start speed, but only for about 2 seconds, then slows to a stop. I jumped it from a running car with no change. I also jumped from battery directly to starter. No change.

Maybe it is still flooding or excess fuel in oil or combustion chamber

It starts up fine after cleaning the carb bowl. Nasty, rust-colored, lumpy sediment in carb bowl. No longer flooding. I ran it this morning for 20 minutes just fine. Once I turn it off, it won't crank the motor fast enough to start again. I have to wait about 2 hours before it will start again. Thoughts?