Drove Lucille over to Eddings with '49er parts but the place was locked up. We headed home and at one point I mashed on the gas harder than usual and was greeted with stumbling and a nice backfire up the carb! Eased off the gas, and she continued as if nothing had happened. I tried again and yes, I could repeat the symptom at will. We got home just fine, of course, and I checked the carb mounting bolts and tested vacuum levels. It was a bit low: leak in the setup that goes to the wipers. Capped that and adjusted the timing. Setting it a bit higher than spec. A test drive revealed the stumbling and backfires were cured but now I had ping. I backed off the timing again and the backfires returned, though not as bad. So now I've tweaked the timing so that the backfires are gone but there is still a tad of ping under certain conditions.