Popping through the Exhaust (Webers)

[jabb]

Hi:

I have a 3.0 running Weber 40’s Runs smooth pulls strong. I get popping through the exhaust. The popping is noticeable at about 3 to 3500 rpm’s when cursing along in 4th or 5th gear.

Also get some exhaust popping on deceleration. No popping during acceleration.

Anyone got any ideas….. I am running 60 idle jets 32mm Venturis, 145 main jets
F3 Emulsion tubes and 180 air correctors. Running headers with stock muffler (2 in 1 Out)

Can the pooping just be caused from a bad muffler ?

When I installed this muffler (used) lots of metal flakes fell out….

Thanks….


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• Joe A.
• 84 911 Targa
• 75 914/6 3.0

[Superman]

Here we go again. I've been told by trustworth and experienced mechanics that exhaust popping means a leak in the exhaust system. Period. So far, I think I believe them. But I'm watching for instances where something else has caused this. So just let us know.

Their guess (the trustworthy and experienced mechanics) would be an exhaust leak.

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'83 SC

[Formerly Steve Wilkinson]

If you mean actual little explosions in the muffler rather than just burbling, and it happens on the overrun with the throttle closed, it means your idle mixture is a bit rich. Has nothing to do with exhaust leaks; it happens on my SC with PMOs and the entire exhaust system (and engine) are brand-new, SSIs and a Fabspeed, if I get the idle-mixture screws a bit too far out, although teh car runs fine otherwise.

stephan

[Superman]

I do appreciate the info, Stephan. This condition caused by a rich mixture... is this the burbling, or the banging/popping? What are your specific symptoms, with a rich mixture?

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'83 SC

[CamB]

To back up Superman,

I have plenty of exhaust leaks (don't ask...) and a LOT of backfiring because of it. This is NOT burbling.

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Cameron Baudinet
1975 911S (in bits)
1969 911T

[Formerly Steve Wilkinson]

A high-performance carbureted car will pretty inevitably burble ("blubble-blubble-rumble-dumble...") on the overrun, but if what you're hearing is the sound of, well, little firecrackers, or sharply clapped hands (WOP. WOP.) like actual mini-explosions in the muffler, that's an excess of fuel vapor being ignited after it has passed uselessly through the combustion chamber.

Stephan

[Don in Redondo]

I agree with Stephan, that your setup is probably too rich, you might also want to advance your timing alittle for the same reason, but not to the point of pre-ignition at full throttle.

[Superman]

Well, I believe everyone now. I believe that cars might burble with no exhaust leaks. And I am still thinking that these "trustworthy and experienced" mechanics are correct in that out-and-out explosions are the result of exhaust system leaks.

I'm not sure how hard I would try to get rid of the gentle burbling. I like it, I guess, if I understand what you're describing. My exhaust makes explosions. Not huge, exterior BANGS, but rather noteworth pops that occur in what used to be the CAT. At least one of the two gaskets coming into the CAT are leaking and I may seal that up just to see if it stops the popping.

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'83 SC

[jabb]

Super....

That is a good way to explain it not bangs but Note Worthy pops. On deceleration ...

But also the same pops when cruising along
between 3 - 4 k rpms at steady throttle

Just got to figure out what it is and get rid of it....

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• Joe A.
• 84 911 Targa
• 75 914/6 3.0

[Superman]

Well, I don't know what's causing that. For the record, I am just kind of doing an experiment. the guys I trust seem to insist that popping...real popping means an exhaust leak. Even noteworthy popping.

So, I'm just watching to see if they are perhaps wrong. Or right. So, I appreciate the discussion.

Vacuum leaks will make popping and so will a lean mixture (same thing, really), but the popping is happening in the intake. If I don't find a confirmed instance of actual exhaust POPPING taht was caused by something besides an exhaust leak, I'll believe those guys.

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'83 SC

[cegerer]

Have you cleaned out the idle jets? Mine was doing a similar thing at steady speed although there was also a noticeable momentary 'stumbling' when the throttle was opened. ONce accelerating everything was fine. Cleaned the idle jets and ran some Techron and everything is now perfect.

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1971 911T - 2.5L Big Bore

[autobonrun]

My car is going back on the lift tomorrow for the same issue. I've got both possibilities going right now; a rich mixture and a known exhaust leak. The problem was reduced by tighening the bolts. We'll try one more time to stop the popping sound completely by changing the gaskets and retightening the bolts. If that does not work, I'll probably have to remove the SSIs and have the uneven flange machined.

Lastly, I'll play with the fuel mixture, but I'd like to leave it as is since the car is running well other than the popping sound on decel.

[john walker's workshop]

if air (oxygen) is added to hot, unburned fuel in the exhaust system, it can cause it to ignite (backfire) you already have the fuel, and the ignition source. you just need oxygen to make a fire. enter, the hole in the exhaust system. granted, there are other reasons, like the mixture being off.

[This message has been edited by john walker's workshop (edited 09-04-2001).]

[Doug Zielke]

Jim,
Don't lose your mind over this popping-in-the -pipe thing.
You gutted your cat, right? Then you just effectively reduced the muffling on your exhaust system. You *are* going to hear more popping, burbling, whatever, now.

My cat-free SC has *no* exhaust leaks, but does have the mild burble-pop. I know the mixture is pretty close to optimum, too.

If you have a *real* exhaust leak, you'll get some serious backfire. Otherwise you are just hearing the effects of the fuel mixture annomalies.

Now, get some rest, big-fellow....
you have to take on some nasty underworld types tomorrow, don't ya?

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'81 SC Coupe (aka: "Blue Bomber")
Canada West Region PCA
The Blue Bomber's Website

[This message has been edited by Doug Zielke (edited 09-04-2001).]

[RoninLB]

Carbs are great entertainment. I love them. Simple cave man action. Space age fuel injection systems are wonderful, for some one else. Nobody is ever comfortable with their carbs. Carb trauma. It's the universal thing.It's a carb-eat-carb-fuel world.

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RoninLB