That was a problem I used to see when I was working on old cars. I also encountered it once on an inboard-outboard boat with a GM 6-cyl 235 engine. The owner (Dad) had replaced the coil, but didn't know he needed either a no-external resistor coil, or he needed to install a 6v ignition resistor on the + side of the coil. Regular 12v coils actually need 6 volts. They will handle 12 volts for a while, but will eventually get weak then fizzle out. We found out his error about 1/2 mile from shore in the Sea of Cortez. He said he had replaced that darn thing twice and it keeps going out! That was when I explained to him about the resistor thing.
You can use a regular 12v coil, but you have to buy an ignition resistor (a couple bucks), which is simply a voltage drop resistor that drops the battery voltage to 6v at the + side of the coil. Much simpler to buy a no-external resistor coil, which apparently has the resistor built-in, but either way is no big deal.
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Tony
Stafford, VA
1650, 682...
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