exhaust popping

Grumpy

The Grumpy Grease Monkey mechanical engineer.
Staff member
" grumpy, when I let off the throttle at the end of the 1/4 mile , the exhaust gurgles and pops? any ideas"

thats all too common, its basic cause is un-burnt fuel/air mix igniting and burning in the exhaust manifolds
this can be the result of faulty ignition timing, but its more frequently the result of an intake manifold vacuum leak or a leaking exhaust or header gasket.
when you let of the throttle at the end of the 1/4 mile and use the engine as a brake, your still causing a great deal of vacuum or negative air pressure in a rapidly revolving engine, this sucks fuel/air mix through the engine an some fails to ignite on the reduced power stroke with the throttle closed,and exits the exhaust, where the heat in the exhaust or a subsequent pulse of burning exhaust gas from an adjacent cylinder ignites it, causing it to ignite in the header or exhaust manifold. defective/leaking header or exhaust gaskets, exacerbate,s the problem as the leak provides additional fresh oxygen laden air allows easier combustion.
if your ignition timing is off the ideal and your using very little throttle as in a car idling a similar condition, can occur if you have leaking exhaust gaskets or an intake manifold or leaking vacuum line on the intake.
do a vacuum leak check and its generally a good idea to replace header or exhaust manifold gaskets if this happens
yes do a compression test, theres a slight chance it the result of a leaking or burn exhaust valve

When the throttle of an engine is closed at higher RPMs, a very strong vacuum is created in the intake manifold - this causes a lot of fuel to be drawn in via the idle circuits of the carburetor, and even exhaust gases drawn back from the exhaust manifold due to valve overlap.

This mixture does not burn fully and goes into the exhaust system, where it may get enough air to actually combust fully. so it explodes with a small pop and sometimes flames maybe seen.

It is harmless for the most part. Even a perfectly tuned engine can pop its exhaust.


http://garage.grumpysperformance.com/index.php?threads/locating-vacume-leaks.882/#post-45944

http://garage.grumpysperformance.com/index.php?threads/header-gaskets.1045/page-2#post-55418

http://garage.grumpysperformance.com/index.php?threads/distributor-gear-wear.1701/
 
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Happens if you use a,Quadrajet carb also Grumpy.
Huge amounts of fuel being dumped in and the large secondaries are slow to close.
 
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