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Pop, pop, pop on throttle lift!


jeffdgale
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I have a '74 Avanti with the 400 c.i. SBC engine. The exhaust is straight through (no muffler or CAT) and I I have an open free breathing air cleaner to maximize overall breathing. When I lift my foot off the throttle driving down the street at high RPM, I hear a sort of low pop, pop, pop exhaust note which then dies down as the car decelerates or if I get back on the throttle. Further if I maintain even just the slightest throttle the noise does not occur. The noise is sort of a small explosion, not a backfire. I’ve heard this kind of noise before on high performance cars such as at drag strips. This might be normal cylinder ignition but the way that it occurs puzzles me and I don’t think it’s normal or good for the engine. I am hoping that one of you experienced car guys car give me some ideas as to what this is and what I should do if anything. Thanks, JG.

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Is the sound coming from both tail pipes or just one side? Without actually hearing it it's difficult to say, but I can think of several possibilities.

It could be running rich and loading up and unburnt fuel is igniting in the exhaust pipes. Pull some spark plugs and look at them. If they're wet and gas covered, you're running too rich. If so, your carburetor needs adjustment or maybe even rebuilt.

If it's from just one side, you might have a burnt valve.

You say you have a straight through exhaust. Is it recent? Sometimes the noise might be normal but a quiet type muffler eliminates the sound but a straight through design makes it audible. Also, any engine requires a bit of back pressure for proper exhaust scavenging and full combustion. There's the possibility your engine has too little back pressure when backing off the throttle, and allowing unburnt fuel to exit the exhaust and combust in the pipes.

Studebaker used glass packs which were straight through, but also four barrel carburetors on a high compression engine or a supercharged engine to pressurize the induction system. That helped a great deal on scavenging. Your 400 cid engine is bigger displacement and low compression by comparison, plus I believe originally a two barrel carb. That can make for driveability issues of not precisely tuned due to its larger venturi's that can feed large amounts of fuel when you don't need it...like when backing off the throttle.

You could have other issues, but these are the first that come to my mind. I'm guessing your issue is either carburetor related or you need mufflers to introduce a bit of back pressure for full combustion in the cylinders, not in the pipes, or a combination of the two not calibrated to work together.

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I agree with Gunslinger.

Look for a nice grey coating (not black) in the tailpipe ends (after an extended run on the road) as a check on proper fuel mixture.

Also, a bad spark plug wire/connection could cause the symptom (one cylinder not firing, but the engine will still run pretty smoothly on 7 cylinders and you might not notice it) so as Gunslinger suggests, pull the plugs and look for any wet or oil-coated electrodes.

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