Thanks for the several suggestions on how to fix the old Cub. Here"s what I have done. Recall it was running fine until taken to a shop to replace a busted throwout bearing. Since returning it doesn"t run well enough to be used for anything:
1. Pulled and inspected the plugs - all had a grayish deposit around the electrode, but looked clean, dry, and free of oil. The distributor cap and rotor are new as are all the plug wires. The coil, points and both condensors were replaced with the plugs last Spring and it ran fine through the summer.
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. Loosened the main metering jet from outside and verified fuel is flowing out the drip hole.
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 a low idle. Advancing the throttle immediately results in (brrp brrp pop pop brrp) missing and poor running, almost killing the engine. Advancing slowly seems to allow it to speed up better, but still seems to miss a bit periodically or a lot under even a light load.
6. Adjustments to the idle screw have little effect, so I left it about 1 1/2 turns unscrewed.
7. Pulled the sediment bowl and everything appears clean. Reinstalled it and opened the valve, and pulled the fuel line off the carb and verified fuel runs freely through the hose.
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 not usable at the moment.