Help with a 292 (1967 vintage)

[massey 410]

I have a 292 Chev in a Massey 410 combine which I picked up last season. It smoked badly but did the job. I replaced 3 broken push-rods, and put in new ignition components, cleaned the carb. Right now it idles beautifully, runs at no-load governed speed smoothly. When i engage the combine the motor runs smoothly for from 5 seconds to 4 minutes, and then starts backfiring and looses power. Disengaging the combine and idling for a few seconds has it running smoothy again, and then it will rev up again.Fuel flow seems all right but this AM am putting a pressure gauge at the carb. The plugs are all burning cleanly and look textbook, and the exhaust doesn't smoke. Points are set at .016 and they contact almost fully. I'm running out of smart ideas-and also dry days to get this grain off. This site looks like a good one to ask for help...Thanx...Karl