Don't know if you have one carb or two, but this scenario is more likely
with one.
It's possible that your float valve is stuck shut and a sharp rap on the
carb will jar it loose.
It is also possible that your fuel filter is partly clogged and will pass
some, but not much, fuel.
I don't think it is very likely that your fuel pump is not being activated
by the cam but you can open the oil filler and look in while cranking and
see if the valves are working. If so, the cam is turning. Timing gears DO
fail on B18, B20 and B30 engines. They usually get pretty noisy first, with
a knocking sound from vague locations near the front of the engine. Easily
mistaken for rod knocks.
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George Downs Bartlesville, Heart of the USA!