"Popping" on deceleration! Why?
 

Help! My 289 has significant "popping" on deceleration at all times. A friend advised it was due to too restrictive an exhaust. I am running a stock original type under-car exhaust, with glass pack type mufflers that I am sure have no "glass pack" left in them. The engine is a 347 stroker with Webers. Any suggestions? The car is way too loud the way it is, also. I attract enough attention without the noise. They won't let me autocross anymore because I exceed their dB requirement rules.
What should I do (besides sell the car to you for $100 to get rid of my problems--I've already considered that offer!:3DSMILE::confused:

Most often it is low back pressure. Combine with rich condition coming off the gas makes for header back fire.

I've had sever popping on decel due to exhaust system leak and lean idle mixture.

I richened up the mixture screws, Holley, about 1/2 turn and most of the problem went away. Gone completely when I replaced header gaskets.

Mike

1. Too rich
2. poor ignition (if gas is not burned it will mimic rich condition)
3. leak in the exhaust system
4. blown header gaskets

At one time or another I've had all of the above (sometimes in combination) with the resultant pipe POP upon deceleration.

Good luck, Pete C

If you have an exhaust leak this will happen.

Ditto on MikieC. Pull a plug and look for signs of lean mixture and use your hand to CAREFULLY feel for an exhaust leak. If it's been there a while, you will probably see the black soot around the leak.

Come on you guys I bet 99% of ALL Cobras have SOME popping on deceleration. This is because there is no exhaust pipe connecting BOTH sides together. OK, so maybe the popping in question here is "severe" but my bet is it will NOT go away completely, EVER with separate side pipes.

Since I "blew out" my baffles on my side pipes the popping is WAY more noticable than before. The car is WAY louder also. I wear ear plugs when I race! So far I haven't been "banned", but I HAVE been worried about the possibilty.

5.0 EFI (with no leaks a perfect mixture and NO smog equipment).

Ernie

Ditto on the mixture/exhaust leak. Both the Snake and the 67 BB Vette had exhaust pop. Snake fixed by new header/sidepipe gaskets/sealer. Vette had a mixture problem.
Got to admit though when the Snake is really hot (200deg) I get some pop on decel. Hey I'll give you $101 and take all your problems away from you !!!

$101!!! Do I hear $102? Going , going.....

Thanks for all the advice. I'll look for exhaust leaks.

Jim, mikie c is correct. If there is no exaust leak, then you have a lean misfire causing the pop. The reason is, when you close the throttle, there is no fuel going to the cylinders other then what your idle mixture allows. You could also raise the idle and richen the idle mixture. A vaccuum leak could cause this also. This lean condition on part throttle at higher rpm's is why the cup guys constantly rev and let off the throttle on yellows. If they didn.t, they would cook a piston on restarts as a cylinder just got too hot from a lean condition. Good luck, scott.

MIne was a too rich condition. Upon rejetting down from 77/78 to 71/75 on my 750 double pumper most of the popping went away!
Dana

Part of the noise is caused when you slam the throttle shut on decel, the resultant high rpm, high vacuum condition causes the engine to suck a lot of fuel through the carbs, putting a lot of raw gas in the exhaust, a lot of oem carbs have a dashpot on them to stop the throttle from closing completely on decel, giving time (1 or 2 seconds). for the high vacuum to decrease before fully closing the throttle. dont know what you could do about it on webers, though.

This happened to me once and it turned out to be loose header bolts.

However, Demon carb tech sheet says backfires out of exhaust can be remedied by one or more of the following:

Lowering float level
Decrease fuel pressure
Decrease jet size
Readjust primary and secondary butterflly position

Roscoe

Hey guys Jim isn't running a Holley here...the short runners on the webers will cause popping if you have too much cam in the engine...and getting the jetting just right with 7-8 things that you can move around is a B*#$%.....what are the specs on the cam you are running Jim?

I have noticed an increase in the amount of popping I get from my car on decel from the webers over what I had previously with a holley and single plane but I did have popping with the holley too. This is a known problem with Webers...you can have "NO" exhaust leaks or they will pop!!!!!

There are a couple of things to consider
- I had alot of popping just driving around, at cruising speed ont decelerating, with a lean mixture...I also have to get a laser pyrometer to get the mixture on all cylinders correct to make sure that isnt' some of the problem....
- I can't remember but does the undercar on your 289 connect the same way as a comp car or an ERA? Do the headers slide into the exhaust collector pipe 1 header at a time? This type of connection is a bear to seal up...I still haven't figured out how to get mine to seal yet but am open to suggestions...

My bet is there isn't a header leak but a leak at the collector where the header meets the exhaust and you are sucking cold air in on decel.....best way I found to avoid it was to take it out of gear and coast when I was going to slow down ...no downshifting unless I am flat out on the accel :)

Matt - Ill give $200...bid high and scare the other bidders :3DSMILE:

I second the comments of 427Aggie. I have heard that most Webers will pop a little bit on decel, more so when the engine is hot. However, if you are really having major crackling, an exhaust leak would be a good bet.