GM Delco Voyager EST kit for the win. It’ll come with the coil and wiring as well as spark plug wires for about $450 per side.

I think ebasicpower.com has them for $430.

Agree Delco EST

i would check the fuel system from the carb back to the tank,it is easier for the fuel pump to suck air than fuel.through the years i have seen loose fittings on both sides of the screw on fuel filter,cracked fuel hoses,cracked fittings at the tank&fuel selector valves.to me it sounds like a lean condition backfire.add in,i just worked on a 39 sea ray that had your same problem,the seal in the fuel selector valve dried up and allowed air to be sucked past it but it never dripped a drop of fuel.

Valve sticking or rocker arm loose? Pop the valve covers?

only 1 tank that both motors draw off. the port runs fine. Swapped carbs and still have the problem. Swapped distributors and the problem follows the distributor

looks like you found the problem.

Probably timing sensor if a thunderbolt ignition