Thank you. I've tried an external capacitor (condenser) and while the engine starts and runs nicely with two condensers in parallel, it still died on the same hill. The timing seems awfully close to the 1-2 mm before top dead center that's specified.
And yes: there is indeed a reed valve. It's not at all clear how these are to be dealt with aside from just leaving them alone. Advice would be appreciated.
Now, I had not associated the two problems, but it seems that my starting clutch is also slipping quite a bit, making it difficult to restart the bike after it dies. Thus I'm disassembling the clutch and while I have noted very little wear, it occurs to me that there is a crankshaft seal right behind the clutch. If that seal is leaky, perhaps it is shooting oil-laden fuel vapor into the clutch.
And so I shall clean and buff the clutch surfaces and replace the crankshaft seal, and then the seal beneath the magneto as well. Does this sound like a reasonable strategy? The carburetor is so simple that I've run out of ideas therein.
Thanks very much for your response.
Mark Kinsler, Lancaster, Ohio (edited)