Thanks once again to everyone for your suggestions. Here is a follow-up report . . .
I picked up a PET 4000 spark tester (very nifty) and tried it while the engine was missing. Sure enough, the spark in one cylinder was deficient. I ordered a couple of ignition modules, and, happily, all...
Thanks very much to all of you for your suggestions.
I don't see smoke of any sort when it is missing. I've tried removing the gas cap to check for a plugged vent. The air filter is clear and it misses even with it removed. .The cooling fins are clean.
Going forward... I've ordered a...