77Dream

Does anybody know what cuases this? I have mostly stock engine with edelbrock 1903 q-jet carb (could use rebuild). I think I am running a little rich because I can smell gass sometimes.

Milest

Not sure what causes it, could be unburnt fuel. H-pipe will make it go away and cut the drone at certain RPMs.

I have a stock 72 L-48 with factory exhaust + h-pipe

BB wowbagger

Could be a leaking headergasket.. :smash: :cheers: :cheers:

TedH

sounds like raw fuel (over-rich condition or carb leaking into intake).

If this is the carb, Lars may know...

Surfer69

Usually leaking exhaust gasket or possible lean condition caused by choke not opening fully.

cardo0

Correct me if I'm wrong but isn't the purpose of the vacuum advance to provide enough advance at idle and low speed to eleminate pre-detonation? That's it's only job as far as I know. Sometimes an adjustable canister will help. Most ignition "how to do" books would be a better source for this. :yesnod:

V8yunkie

Try something else: Disconnect the vacuum system from the manifold feed and plug the fitting with a rubber plug. Then drive. If the blob is gone, then you have a severe leak in the vacuum system, causing air to come into the manifold through the fitting. Thus the closest cylinders to the fitting run too lean causing pre-detonation (the leaner the mixture the earlier it ignites).

Regards,
Thomas

Jvette73

Have you checked or changed the plugs recently? Poor combustion could be due to bad plugs which would lead to raw gas smell in the exhaust. Also a mis-firing plug can cause crossfire inside the dist cap making another cylinder fire out of time while the exhaust valve is still open, igniting the unburnt fumes, causing the blop blop pop pop in the exhaust. Plugs can be bad even when the look good, even new ones. Exhaust leaks can also be the culprit.

70ZZ3 96LT4

Does your 77 still have the air injector pump in operation? My 68 is now having a problem making the combustion noises in the mufflers when I let off the gas. I don't think that the air injector pump is supposed to pump air when you let off the gas pedal. On my 68 there is a valve that directs the injector air out a mini muffler attached to the control valve when I let of the gas, I think my valve is either sticking or needs replaced. I just need time to check it out.

Lee74C3

I had the same problem on my 1974 Big Block. It is usually a very common issue on A.I.R. pump equipped engines. There is a "diverter valve" bolted to the A.I.R. pump. When this goes bad, the symptom is just what you describe. As a short term test, unplug the vacuum line from the carb to the diverter valve (stick a screw in it to stop the vacuum leak), and drive the car. If the noise stops, it's the valve. Easy to replace, sometimes very difficult to find and expensive. They can't be rebuilt. Hopefully, yours may be a common part to many other GM engines and be readily available for about $75 list price. Mine was a one year only application and cost me a lot. There are a few sources in Hemmings and NCRS Driveline that have original parts.

goofygrin

Um...

I'm confused... is this different from the normal sound you get when compression braking?

I always loved to downshift to slow down with my 240z (with 3 2-barrel carbs and glasspacks) to hear the pop-pop-pop from the exhaust...

stock70SB

As previously noted, is this occuring at high rpm deceleration or low rpm? I get a similar sound when decelerating from high rpm in my '70 small block Vette or my 2001 Durango R/T, neither have any mods. To me, sounds normal.

:cool:

Aaron-74

I get my Vette to do that under bridges all the time...6,000 RPM and let off...POP POP POPOP!! It's just backfiring threw thew ehxuast.

My 2001 V6 Silverado will do that If I rev to 6,000 and let off in 1st..I would call it normal.

After I put an Hpipe on it went away... :(


Don't worry though..it's pretty normal. I dont know an old 350 powered car that doesn't do that.

Aaron