Mike---My guess is that the "crack" you heard and the resultant "black
smoke out the exhaust pipe" was a good symptom of an engine backfiring.
A temporary lack of spark while the engine continues to pull in fuel,
then ignition takes place, as it should. The unburned fuel stored in the
exhaust manifold and pipes then cause the backfire and smoke.
If it continues to happen, you'll for sure need to find out the reason.
Start with any loose wire in the ignition system, including the points
setting.
If the black smoke is still continuing out of one exhaust, disregard the
above, and look at the carburetor and valve train that mostly sends down
this one pipe. Keep us posted on what you find!
Dick T.
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