Backfiring when back driven

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Nov 12, 2008 | 08:18 AM

yukgoon

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Joined: May 2003

Posts: 61

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From: Savannah, GA

Backfiring when back driven

I recently replaced the intake and exhaust manifold, carb and the mix on my 300 with a Clifford 6=8 and the Holley 390 carb. The timing is set to 6 off TDC and the truck runs great. However, when ever I get off the gas after a quick acceleration or am coasting down hill the engine backfires like a mad man and seems to be back driven by the transmission. This only happens with its in gear. Could this be a vacuum advance problem off the distributer or is there something else that could be causing this problem?

The truck is an 82 F100. The transmission is a 3 w OD on the floor. The transmission was just rebuilt and the engine has 230,000 on it and is still as strong as ever.

Nov 12, 2008 | 10:24 PM

Ford_Six

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Joined: Oct 2002

Posts: 18,488

Likes: 22

From: The Big, Oregon

You have a rich idle mixture, and a leaky exhaust. The strong negative pulse of the exhaust while under engine braking draws air in, which allows the extra fuel to burn, making a bang.