Well, it turns out that the JS46 that I've had hard starting problems with, has a bent crankshaft. It tries to start, and seems to be hitting, but just never catches hold and starts running, and it often backfires when I'm trying to start it. If I can ever get it started, it runs fine, but is a lot louder than I remember.
How can a bent crankshaft keep an engine from starting, assuming it isn't bent so badly that it locks up the engine???