1981 Questions for all other cars Miss-firing

1981 Questions for all other cars

My question is about my 20 year old Sears 16hp, two cylinder garden tractor. Can you help?
It misses, pops, and stalls, and will not run under load. I have replaced the plugs, gas, and adjusted the mixture according to the manual (In untill it misses, then out 1/2 turn).
I observed that both plugs fire acrross the gap. The engine runs well off the left cyclinder however the right cylinder seems to miss-fire or not fire, especially after the engine warms. It runs better when the mixture is very rich, or the choke is on full. Ignition is from a Magnetron. The compression on both cylinders is 35 pounds. The carburetor is a simple down-draft to a two-tube manifold.
What do you think may be wrong?