Mailbag: What Causes My Engine to Shake After Cold Startup?

Q: I have a couple of problems I hope you can help me solve. I have a 1969 383 Mopar that shakes right after the first cold startup, but only if it’s been longer than 24 hours since the engine was last running. I was told this might be because the electric choke on my Edelbrock carburetor might be set “too aggressively.” Does that make any sense?

When I’m driving the car every day, it will start cold with one pump or no pump of the gas pedal. But if it sits longer than a day, I can pump the pedal three times and turn the ignition key, but it won’t start until I press the gas one more time. Does the diaphragm in the carb’s accelerator pump have anything to do with this?

A: The shake on the cold startup is because the engine has been flooded. You might try leaning the choke out one notch and set the fast idle to 1,600 rpm. The Edelbrock/AFB-style carburetors have been known to have a fuel drainback problem when used on big block Chryslers. You might want to install a small electric fuel pump along with the mechanical pump to help reprime the carburetor on cold startup.

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This is another in a series of weekly Q&A Mailbag sessions with Summit Racing‘s tech department, in which there are hundreds more. Click here to see them all.