Well, I got everything back together today and made sure to inspect the meshing of the starter and flywheel gears. Everything looked perfect. The engine turned over perfectly smoothly, and I set it on the equipment, bolted it down, and started it up. Ran perfectly for 2 or 3 minutes. I put the belts back on, drove it around, mowed a little...can't imagine what was giving me the trouble before. I guess I'll just test it again tomorrow, and if nothing else happens, I'll notify the customer that his mower is fixed. Of course, he's not going to know everything about it you guys do!