What could cause an engine to backfire?

[Doc]

Issue with son in laws ski boat. A ski natique with a ford 351 inboard engine.
Tried adjusting carb to no avail.
Thought it was timing so go the timing light and timing is right on.

So thinking it is still a carb issue but ...looking for suggestions. We might have blinders on and missing the obvious.

 

[Phillbo]

Pull the distributer and look for water intrusion and burning.. could be a crack...

 

[Denny]

Common causes of backfire are running rich (too much fuel going into cylinders) or faulty ignition, possibly a fouled (dirty) spark plug, coil, or plug wire. Pop-backs are usually caused by problems with timing

 

[Iggy]

Common causes of backfire are running rich (too much fuel going into cylinders) or faulty ignition, possibly a fouled (dirty) spark plug, coil, or plug wire. Pop-backs are usually caused by problems with timing

Yep!