[fsted]

car is a 76 trans am trying to finish restoration, but i have this noise i cant figure out. Its has a flapping sound in the exh. I will post a video of it running, in the video it sounds like a backfire but its not. Its sounds just like a leaking exh valve. The engine is a .030 over 455 96 heads ram air iv cam. hooker headers 2.5 exh back to dyno max super turbo muffler 2.25 tailpipes. everything is brand new. the engine runs great and the noise goes away just off idle. I have checked compression no problems, put air on the cylinders nothing leaking. tried mutiple carbs and dist plugs wires etc. I dont think its a misfire. reset the valves nothing to tight valve train is quite as a mouse. also no crossover so its both side of the engine, and the exh heat risers are filled. the torque conv is tighter than i would like and the car idles pretty slow in gear 550 but you can still hear it out of gear at 750 just not as bad. anybody ever had a problem like this ? i have put a couple of hundred miles on it, i would think it would be getting worse if it were wiping a camlobe? http://www.youtube.com/watch?v=tymN0...H2V753kRDxgE7g

[400 Lemans]

Kinda sounds like your running manifold vacuum advance instead of ported. Mine will sound like that also if i dont run ported.If not limited, manifold vacuum pulls way to much timing in at idle. All the pontiacs that i have owned are much happier on ported. They rev cleaner and cruise better also. Mine is also limited to add only 10 deg more timing.Hope this helps.

[fsted]

good call i had manifold advance to start with really ran like crap then! its ported now 10 initial 34 total. I dont know why i was trying to use maifold advance to bring the idle speed up but it was dumb move. I found a post where cliff r was going back and forth with a guy over this, my car was acting just like he said so i went out reset the timing and carb mixture and major improvement.

[zeebo]

so did that stop the popping sound?

[fsted]

no the noise is still there. when i started the car for the first time i was using manifold vacuum advance to bring the idle up. Thats a bad plan car idled bad misfired dropped the rpms too low in gear. It was pretty much undrivable. Thats when i reset base timing and adj carb mixture, now the car runs and drives fine plenty of power. If i could get that noise out of it i could move on with other projects on the car.

[tpssonic]

A fluctuating vacuum gauge reading would tell the story.

[400 Lemans]

Try 14 deg initial timing with that cam. 10 deg is a little low for that combo. Factory setting on RA-IV cars with that cam were actually 16 deg initial so with your heads at about 9.5-1 compression you will be fine. Watched the video again and it sounds like unburned gases in exhaust from initial timing being to retarted.

[fsted]

vacuum gauge is steady. I will bump initial timing in the morning and see what happens.

[Blued and Painted]

Is it an HEI with Autolights or Champions? Sounds like a cracked or fouled spark plug. An induced cross fire due to plug wire routing. Worst case, tight and sticking exhaust guide. Severely worn guide with valve moving around on seat. But I'm still thinking ignition.

[fsted]

hei with autolites. 2 complete different dist 2 sets of plugwires multiple carbs, compression test, i put air in the cyliders listened for leaks in the exh. several different new sets of plugs. I considered sticking valve guides, zero noise from the valvetrain. The heads were done by a reputable guy, and all the parts were new. However there is still a problem that normal troubleshooting has not solved.My gut instinct says leaking exh valves, but the compression test was good and the vacuum gauge is steady. I think i am gonna drop the exh and air the cylinders up again and listen at the headers. I have a couple hundred miles on it and hasnt gotten worse.

[Bill Eveland]

I had a similar type noise in my 400 with 60243 crower cam. This video was with open 3 tube headers , I added extensions to the headers that had a few sharp bends that helped clear up a lot of the noise . Maybe the camshaft timing caused some of this somehow, anyway adding in some extra back pressure helped for whatever reason .
http://youtu.be/P5VTYG6P89g

[Formulabruce]

Originally Posted by Bill Eveland

I had a similar type noise in my 400 with 60243 crower cam. This video was with open 3 tube headers , I added extensions to the headers that had a few sharp bends that helped clear up a lot of the noise . Maybe the camshaft timing caused some of this somehow, anyway adding in some extra back pressure helped for whatever reason .
http://youtu.be/P5VTYG6P89g

This is the most reasonable answer. Its probably a latent fuel explosion due to cam timing/fuel quality/mixture at idle. Could be the cam was installed well advanced? Adding back pressure will increase torque and possibly make the chances for a more complete combustion to occur. Just my 2 cents...I find the alcohol requires a bit richer at idle, and that means some needed attention elsewhere usually

[Rocky Rotella]

Try taking a rag (not oil or gas soaked!) and stuffing up the front outlet of the exhaust splitter on each side. I think you'll be surprised at the result...

[fsted]

Thanks for the ideas i am going to try to get to the bottom of it today. The cam was installed at 112 and the car has a full exh system not sure how i would add backpressure. Rocky i tried what you suggested but i didnt do both sides i will try it again.

[69fireburger]

I had a noise almost exactly like that in a small block Chevelle many years ago. Is that coming from one side and not the other? It took me a long time to track down but my problem ended up being one of the (brand new) mufflers. There was a spot weld that was supposed to join the muffler case to one of the internal baffles but the weld didn't hold. The muffler case was "oil canning" at idle and sounded exactly like the video you posted. At anything above idle there was sufficient pressure to hold the muffler case out so the noise went away. Then back to idle and...pop...pop....pop. I corrected my problem with a hammer and added a crease to the muffler case to prevent it from flexing.

-Bob

[zeebo]

Originally Posted by fsted

no the noise is still there. when i started the car for the first time i was using manifold vacuum advance to bring the idle up. Thats a bad plan car idled bad misfired dropped the rpms too low in gear. It was pretty much undrivable. Thats when i reset base timing and adj carb mixture, now the car runs and drives fine plenty of power. If i could get that noise out of it i could move on with other projects on the car.

so you run ported vac advance now..ok..where did you hook up the ported vac line? i need to do this to my car..im currently running man vac and have a miss at idle..its not a popping sound, but its a miss/stumble

[track73]

I could be wrong but, think some of those mufflers have valve in them that opens up when the exhaust pressures exceeds a certain point. It happened to my friends 383 Corvette stroker motor I built. They make a flapping noise if there isn't enough pressure at idle. He got different ones and I got his old ones.

[fsted]

Originally Posted by Rocky Rotella

Try taking a rag (not oil or gas soaked!) and stuffing up the front outlet of the exhaust splitter on each side. I think you'll be surprised at the result...

ok here is a new video trying what Rocky said to do. Thanks very much for the tip. i would love to know how you figured that out and what causes it. Now the big question how to fix it.
Would a balance tube help? http://www.youtube.com/watch?v=1mlKw...H2V753kRDxgE7g

[Rocky Rotella]

It looks as if your car is setup very much like my own '76 Trans Am was when I had the same issue. As best as I can tell, the uneven pulses created by a high-duration, high-overlap cam travels down each individual exhaust bank, and when it reaches the first outlet of the splitter, it creates a phenomenon much alike blowing over the top of a pop bottle (or beer bottle if you're celebrating the New Year already!). Once you increase engine speed above idle, the pulsations are so close together that they bypass the first pipe and exit through the rear eliminating the noise.

I think a balance tube would help equalize the pressure in each bank, but I have a feeling an x-pipe would synchronize the pulsations best. I also believe a larger diameter pipe would help the situation by giving the pulsation more area to spread itself over, thus relieving pressure slightly. A balance tube would likely be the cheapest approach for you.

I'm guessing Pontiac noticed this in later years because when I first discovered the issue on my car, I begun looking at original splitters on other cars and found some where the front pipe is fully open, partially open, and fully closed. I considered welding mine up to keep the noise down, but I since went with a Pypes crossflow setup with an x-pipe and 2.5" splitters and I have no discernible "pop" from the open splitters.

Here's an idle clip of the 244/252 hydraulic flat-tappet:
http://www.youtube.com/watch?v=PsrTK6DpjUE

Here's an idle clip of the same setup but with a 236/248 hydraulic roller:
http://www.youtube.com/watch?v=_77ELMcojhU

[fsted]

Thanks for help. I was really struggling with this and could not force myself to work on other things until it was fixed. My friends all said dont worry about it just drive it, but they didnt have to listen to it at the redlight! I am not sure how i am gonna fix it, right now its on too wet sanding and reclearing then wetsanding and buffing. Thanks again you saved me alot of work and time.