So I looked up the engine serial on the store and ordered a replacement distributor cap for my 78 mercruiser 228. New one looks like it clamps on. Old one has holes. Pics attached. Help me with part #?
We can see that... The new cap you have is the 'right' cap for that engine based on the year. Have you considered that the distributor in the engine may not be the original? Which appears to be the case.
The distributor on the engine looks like it might be (god forbid!) automotive. In which case you need to remove it and put a properly 'ignition protected' distributor in...
So, a little digging has turned up a cap (2 actually) that may match what's on your engine. It's a cap from a Prestolite distributor.
Here's the rub... It's either a distributor from a 1987-1995 5.7 inboard (ski), cap part number 392-9766; or a distributor from a 1963 to 1969 327, cap part number 392-33708... Either way, it's not the right distributor (for a 1982 305 4bbl aka '228')...
That to me indicates that distributor could also carry a cap for reverse rotation, meaning, inboard.... If I was a betting man, I'd pick the 392-9766... But it's not my money....
Update to this, I think this engine model used the old resistance wire. I suspect I'm overheating the coil. If I were to get an internal resistor, what wiring modifications would I have to make? Should I try the new coil and clean the contacts, keeping a resistance wire?