Cleake1

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I’ve got a 1973 Chevy Crew Cab with a 12-valve p-pump Cummins swap, and the motor has around 90,000 miles on it. Lately, I’ve been dealing with a misfire that only happens during cold weather. The issue is entirely temperature-related because it goes away completely once the engine is warmed up.

When the engine is cold, I can drive it really easy, and it won’t misfire. However, if I press the pedal with any decent force, it starts misfiring, and white smoke comes out of the exhaust. Interestingly, the misfire seems to clear up once the turbo spools up a little bit. It almost feels like it’s getting too much fuel and not enough air until the turbo starts building some boost.

Has anyone else experienced this kind of issue with a 12-valve? Could this be tied to the injection pump (timing advance, etc.), or is there something else I should be looking into? I’d appreciate any suggestions or advice for diagnosing and resolving the problem. Let me know if you need more details about the setup.
 

Black dawg

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Is this a single cyl missfire, or does it blubber like an over revved gasser floating valves?

Most likely the timing is a little slow, causing it to blubber like that when revved up.
 

Black dawg

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All of the p pumped motors will do that if it is cold enough, unless the timing is advanced a bit.
 

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