1968 Pontiac GTO 400 Engine Back Fire Out Tailpipe On Deceleration

Reader Question Hi Austin,

I have a 68 GTO with a 1970 Pontiac 400 engine. I installed a new manifold on Holly 770 carburetor this winter. I have noticed that when driving at higher speeds >50mph and take my foot off the accelerator the car will backfire through the exhaust.

I have read that it could be a multiply of causes and that timing could be one of them. What is your opinion on what it could be and the checks I can do to nail down the problem.



Howdy there Tim,

Timing could be a culprit here, but must likely it is due to the carburetor running RICH. I would make some carb adjustments and try and lean out the mixture to see if that helps.

Backfire back through the carburetor is usually due to a firing order or timing problem, or head/valve problems. Backfire out the exhaust is usually due to a rich fuel mixture allowing too much unburned fuel into the exhaust system.


Austin Davis

Posted in: Running Problems

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