So, all this talk about piston lifespan seems to have jinxed me; thanks guys!
As I was tooling down the highway yesterday and coming to a light, I let off the throttle and pulled the clutch. My engine purred down and suddenly I hear a single CLUNK, and my engine made sounds no more. I was about a mile from home, so I locked the clutch-lever, and pedaled on. Once home, this is what I found:
The durned nut on my carb's choke-lever fell off, and the washer got sucked into the intake. It took some doing to work the piston back-and-forth to first clear the obstruction and THEN do a little dental-work with the dremel to get the raised damage down for the piston to clear the cylinder-skirt. The piston is toast; the washer caused a large fracture in its side, but the cylinder MIGHT be reusable after some grinding. There was no damage in the combustion-chamber area, and the rings were unscathed as well. Pity. It only had probably 150-miles on it, and it had just broken in so beautifully. I really could have avoided this too. This carb's choke-lever was maddeningly loose to begin with, and I'd tried to tighten it adequately in the past (it had a tendency to engage at speed and would flood the engine). I tried unscrewing the nut early on, but the bolt it's on is designed with a slight peen to keep it on there no mater what...and to prevent exactly what happened to me. I've never needed the choke, but rigged up a linkage to engage and stabilize it anyway. Well, if I HADN'T done that, I'd have probably either replaced the carb, or ripped out the choke assembly before it came to this. ANOTHER lesson learned. New cylinder and piston already installed with the old rings. Time to break it in again...and order a spare upper!