Small Engines (Lawn Mowers, etc.): 3 hp Horizontal shaft Briggs shearing flywheel key, valve lapping compound, lawn edger

Question
I recently purchased an old lawn edger with a 3 hp horizontal shaft Briggs engine that was not working. I figured is was worth the $40 to see if I could fix it.

I found the flywheel key sheared. Once it was changed the unit fired up no problem, but within minutes the engine back fired and died, at this same time the belts (which need changing) jumped off the pulley.

Not thinking the flywheel key would have sheared again, I took apart the carb to look for another cause of the unit not running but found no problems in the carb. I kept the belts off and then found the flywheel key sheared again. I changed it and within seconds the engine started, then back fired....


I pull the flywheel off again and found the sheared key (my last spare). After this I pull the cover off the cylinder and found the valves were dirty and sticky. I cleaned them up with a soft brass brush and WD40, then manually turned the engine using the pulley and the valves seem to be moving freely.

My questions are:
1- Is there something I am missing that could cause the flywheel to shear in a horizontal shaft engine? (I am used to blades on vertical shafts causing this problem by either hitting rocks or being severly bent....but can not think of a cause on this unit).
2- does the pulley need to be loaded (belts attached), or not. My concern is the may be a need for a load on the motor.
3- My only other thought is it could be the piston or the cam, in which case I will the the unit out.

thx rob

Answer
The engine should have a cast iron flywheel which is heavier than the aluminum ones and therefore you do not the belts or any other load on the engine.

I am assuming you have a starter clutch. There should be a washer under the clutch, between the flywheel and the clutch.

The second trick I've used is matching the flywheel and crankshaft taper.  Put a little valve lapping compound on the crankshaft and just spin the flywheel around the crankshaft.  This will ensure the tapers are mated.  Clean off all lapping compound.

Finally, if it is still shearing keys, remove the head and check for carbon build-up.  Had this one time.

Let me know if this helps.
Eric