1957 312 V8 stumbles accelerating

mkmskm

mkmskm

Senior Member
The car starts fine, idles fine and at higher speeds runs great but when I come to stop and start out again it wants to stumble and die and giving it more gas doesn't help but letting off and them popping the throttle seems to help it over the struggle point. Once it gets over about 30 mph then its fine. Thoughts? Bad vacuum advance?
 
At speed, from closed throttle to part throttle how is it? Any other work done lately, or did this just develop out of the blue?
 
A bad vacuum advance usually just makes the engine feel a little sluggish but not stumble, most people don't even realize it's not working.

With it not running remove the air cleaner and see if you can see the accelerator pump squirt fuel when you move throttle. If it's not squirting fuel that would make it fall on its face under acceleration for just a moment but then the carb will catch up with the fuel supply and it will smooth out. If your foot is steady and you're having problems, then it's not the accelerator pump.
 
It started doing this some a month or two ago but not as bad. Then the car just quit idling when it was cold. If I took my foot off the gas it would die until warmed up. I could see choke issues so I rebuilt the carb, Holley 4160 and that solved the choke, and cold idle issue but the stumble at part or heavier throttle when starting from a stop when warmed up seems worse. When I apply throttle or part throttle it just lags and won't accelerate until it gets going. Giving it more throttle doesn't help. Lifting off and then applying throttle seems to work the best until it gets going. And the issue persists for longer than I would expect for an accelerator pump problem as it can persist 10 seconds or so before it gets going. Once it's rolling at higher speeds then no issues.
 
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I had the same issue with my 57 312 and it turned out to be the Pertronix ignition. That was my third Pertronix failure. I'm done using them and went back to points
 
Dave V said:
I had the same issue with my 57 312 and it turned out to be the Pertronix ignition. That was my third Pertronix failure. I'm done using them and went back to points
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Conversion kit, or changing the entire unit (system; dist, coil, wires) over from new?
 
mkmskm said:
Still running points and stock distributor
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Ya know, that entire plate in the distributor rotates on ball bearings, right?

Get out a flashlight, lean over the engine and look down the carb as you ease into the accelerator. Does it squirt right off? If it does, it takes you to 1) vac leak, 2) distributor assembly.
 
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