Thanks for the several suggestions on how to fix the old Cub. Here's what I did:
1. Pulled and inspected the plugs - all had a grayish deposit around the electrode, but looked clean, dry, and free of oil
2. Verified firing order as 1,3,4,2 with 1 being at the 1:00 position looking at the distributor
3. Pulled the carb apart and inspected it. No dirt in the inlet screen and nothing observed inside. Cleaned with some spray carb cleaner to verify openings were clear.
4. Checked the air cleaner, which was a little low on oil but otherwise OK.
5. Started and observed that after warming up a bit it will run very smoothly, but only at idle. Advancing the throttle quickly immediately results in (brrp brrp) missing and poor running, almost killing the engine. Advancing slowly seems to allow it to speed up, but still seems to miss a bit periodically or a lot if under load.
6. Adjustments to the idle screw have little effect, so I left it about 1 1/2 turns unscrewed.
Could this be a problem with the governor? Is there an advance mechanism in that housing? This has a 12v conversion and I have never touched the timing or opened the governor in 20 years...just set point gaps by feeler gauge when doing a tune up. It almost always is used at just about full throttle and had been running well this summer, with an occasional stumble when pulling a load and turning uphill while the regulator catches up. I appreciate the advice as it is barely usable at the moment.