SanDiegoPaul

My car pops big time through the exhaust if I decelerate in gear after getting on it. Like an exhaust leak upstream causing exhaust explosions.... but there are no leaks. Smog pump is not pumping ... and on normal decel there is no problem.

But when I get on it, it will pop.pop.pop. all the way down unless I upshift to prevent it. What is causing this?

Michael Tucker

Paul,
did it do this before the new motor? are you using the same carb? i think it must be leftover fuel in the exhaust if your sure there is no exhaust leak.
you should contact forum member lars.
Mike

Chuck Harmon

Paul,

Even my ZO6 does this a little. For the most part it is because there is very little back pressure in the exhaust. Still, if you are running too rich, and/or there are exhaust leaks, you will get some extra popping. My L88 had the problem real bad until I leaned up the jets about 6 numbers. Just make sure you still have a mixture rich enough for full throttle operation. With Holley carbs, this gets into idle feed restriction size, power valve selections, etc. All very tuneable.

Chuck

OkeeGuy

are you running Flowmaster mufflers?
they are notorious for that...
its the reason i hate my Flowmasters...they sound good until you let off as you described...

Ganey

Add x-over pipe if not already on- has helped some.

SanDiegoPaul

did it do this before the new motor? are you using the same carb? i think it must be leftover fuel in the exhaust if your sure there is no exhaust leak.


No the old motor didn't do this and it had the same mufflers. But the carb is new now, and it has headers on it that were not on the old one.

The engine's not running rich (corrected that) during the power band. I'm using Dynmax Magnaflows with 2.5 inch exhaust and thru a free-flow cat. The car does NOT have true duals, so there is no need for a crossover.

Any more suggestions? :hat

cardo0

Had a Olds 350 for a short time that I had to retard the ignition to <0 to pass emissions and would back fire like yours.
Just a guess but maybe your vacuum advance too slow (not enough). Could try an adjustable vacuum canister on dist. also. Also condition of dist.? Original equipment? :skep:

Chuck Harmon

Paul,

Just because the car is not running rich during a power run doesn't mean that it isn't rich at part throttle or idle. If there wasn't any unburned gas in the exhaust pipes there wouldn't be any popping. Unburned fuel can also be ignition related. i.e spark plugs too cold, timing off, bad plug wires, etc.

Chuck

SanDiegoPaul

Had a Olds 350 for a short time that I had to retard the ignition to <0 to pass emissions and would back fire like yours.
Just a guess but maybe your vacuum advance too slow (not enough). Could try an adjustable vacuum canister on dist. also. Also condition of dist.? Original equipment?

The dist is OEM but we rebuilt it on the dyno. It has a new shaft and of course it has been re sprung and new weights. I have initial timing at 12, and total advance is 38 degrees@3500 RPM.

This isn't much of a problem but with all that $$$ spent on new stuff it should be perfect! :hat :blueangel: :yesnod: :lol: :lol: :lol: :lol: :lol:

Hershey

Paul, the only time that I have had such a problem was back when they first put smog pumps on vehicles. The first thing us Hot Rodders did was to disconect those "power robbing things" Since you said yours is not operative, making it operative might help you. The noise that poping makes can be agrevating. My son just got an 03 C5, and the first thing we did was put on a Borla Stinger and it also pops. As the other posts said it is probably a combination of unburned fuel and free flowing exhaust. Hershey :chevy

Vetterodder

I also suspect that you're hearing unburned fuel being ignited in the exhaust. This is actually a normal condition but even a slight increase in unburned fuel, combined with the changes you've made in your exhaust system can make it much more noticable. Stock manifolds and stock mufflers quell the sound much better than headers and low restriction mufflers (and that's why they are called mufflers). Many (and I mean MANY) years ago, making an engine backfire through the exhaust was considered "cool". You turned the ignition off for a coupe seconds while holding the throttle open. letting fuel build up in the cylinder. When you turned the ignition back on the fuel in the exhaust would ignite and make a big bang. I think it was supposed to impress the girls in the next car but I don't think it was any more effective than burping or farting in front of them :). It also tended to blow holes in mufflers and, sometimes, in your exhaust manifolds :). Personally, I love the sound of an exhaust backing off but not if there are booms and pops involved.

Surfer69

Check choke and make sure its functioning right otherwise running rich gives the pop noise and leaks at headers, something I had to deal with for a while.