A bunch of things to check:
Ignition static timing. Must be set to spec or the engine will run poorly. (See manual - I think it's TDC but I'm not positive) .
Vacuum advance. The distributor diaphragm chamber must be connected to the manifold vacuum port with a small (1/8"?) hard line. And the vacuum mechanism must work freely and not leak.
Centrifugal advance. It must not be stuck.
Breaker point gap. Set to 0.020. If the gap's too small or the points are worn it will cause misfiring.
Carb mixture. Fire out of the exhaust can mean a lean mixture. Try turning the power screw out a turn or two and see if this helps.
Carb accelerator pump. The stock Marvel USX carburetor is notorious for accel pump diaphragm problems. A failed diaphragm will give a temporary lean condition on acceleration. And for reasons I don't understand a new diaphragm is not ordinarily included with rebuild kits.
The 1850 gasser is a generally solid, trouble-free tractor. It's only real downside is it's thirst for gasoline!