77 backfiring

Working on a 1950 77 that had a lot of electrical issues and a fully gunked TSX carb. Had the carb cleaned, points and condenser applied with new plugs. Old plugs were badly fouled. Coil seemed weak so a known good one applied. Starts lovely with about 1/3 throttle and choke on, instant fire up. After a minute or so, it begins an irregular backfire with flame from carb intake. I enriched the main and idle jets a bit, but didn't seem to make a difference. I think there may be some cross firing in the plug wires which I'll address, but I don't want to dink much with the carb without some insight from people who know. What would you all recommend?
 
A back fire like that can be caused by having the points set wrong or timing off a bit or sticking valves
 
After start up is choke pushed in all the way? If so try leaving choke pulled out part way if tractor runs with choke part way out you have carb problem. Let us know if it runs this way
 
if it's a cylinder pull the plug wire to each, one at a time while running (be careful). When you get the to problem it will stop backfiring.

gas is igniting outside of a cylinder with both valves closed...through the carb is an intake problem...retarded ignition time...just wouldn't run right if the carb wasn't set right.

could be a cracked cap effected as it heats up.
 

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