De-acceleration popping sound/back firing...

When in doubt and you're positive it's not coming from anywhere else, look for leaks in the exhaust, (got black soot? then you got leak), right from the manifold gaskets to every joint, seam and weld on the whole system.

It doesn't even have to be a leak big enough to hear, but just enough to let a blast of fresh cold air into the system during deceleration (particularly while 'engine braking') and from that moment on, it's plops, snorts and crackles out the tailpipe.

I think its what really free flowing exhausts do. Or you might be jetted just a little bit off. Both of my cars crackle off throttle, the Alfa not as much, but the Datsun sounds every bit like a race car, and from time to time will emit either a nice blue flame kernel, or from time to time a large orange fireball from the tail pipe. If you have a slip fit exhaust, you will have small leaks, with a fully welded exhaust, its still easy to get a leak. Since you have carbs, the weather will also affect the mixture as well, so it might go a little rich, or lean depending on the day.
If it were me, I wouldn't worry about it.

Will

I just replaced my exhaust with an alfaholics sports exhaust and found it had a small crack in the exhaust manifold - the popping has reduced significantly now so i think this thread is spot on

turn the idle air adjust screws out 1/4 turn at a time till the popping stops. works on my 74 with delortos, works on my 80 yamaha, works on my 2006 triumph bonnieville.
cliff

Giorgio68 said:

Does anyone have any idea why I am still getting back firing on de-acceleration on my berlina 2000.
I have replaced the carbs with new Webers 40DCOE sidedraft, new pointer, new plugs, wires, distributor cap, reset timing, almost anything you can think of. The only different thing I have in the car is straight pipe on the back.

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Depends on how severe the backfiring is. I wouldn't doubt you'd get some "popping" without the last muffler. With the last muffler, you'd still have it but it would be silenced. Of course, severe backfiring is another issue.

I've got the same thing on my Spider, which has Sprint Filter sport air cleaners and a stinger tail pipe.

I kind of like it.

Cliff, are you suggesting a too lean mixture?

-Jason

I've always wanted it but could never figure out how to make it happen.

If it's a steady pop-pop-pop. That's most likely normal.
If it's a single BANG, then you're probably a little on the rich side.

Mine's a steady pop-pop-pop when off-throttle. Seems to be a bit louder since I set the idle mixture with a Colortune.

-Jason

I dont know if its rich or lean, I only know it works. and has for every carburetted engine I 've had.
cliff

popping during deceleration means you have set your idle mixture too lean. back out your idle screw 1/4 turn on each carb and go test drive until the popping stops.

For clarification, I meant idle mixture screw, there's four, one for each carb throat. Did it work? Are you good?

La Voce said:

For clarification, I meant idle mixture screw, there's four, one for each carb throat. Did it work? Are you good?

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The car is in the shop, engine, transmission is out for rebuilding. Everything should be fine now when the car is restored.


Giorgio

La Voce said:

For clarification, I meant idle mixture screw, there's four, one for each carb throat. Did it work? Are you good?

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I tried it and it seems to have helped a lot.

-Jason

I noticed you guys are all talking about carburators. I have an 88 Spider with injection and I get the pop-pop-pop on deceleration (engine braking).
I don't understand why this occurs with engine braking, but not on acceleration. Does anyone know?

The idle is a little low. I'll have to try increasing the idle and see if it helps.

Rich
88 Spider

gamebowser said:

I noticed you guys are all talking about carburators. I have an 88 Spider with injection and I get the pop-pop-pop on deceleration (engine braking).
I don't understand why this occurs with engine braking, but not on acceleration. Does anyone know?

The idle is a little low. I'll have to try increasing the idle and see if it helps.

Rich
88 Spider

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I don't know---but i had a problem with the brake booster check valve with the same symptons on carbs----lean in #4 causes poof-backfire if all else is ok and the valve leaks-----

what would the brake booster have to do with backfiring...?

gamebowser said:

what would the brake booster have to do with backfiring...?

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If it's leaking it'll cause a vacuum leak and the engine can run lean.

Are your mufflers good? The Bosch Spiders can have a burbling backfire sound on engine braking. If the mufflers are working properly it's not too noticeable, but when I got my car the rear muffler was shot and it was downright LOUD.

I don't really know if the mufflers are good. No holes I can see atleast.
If the sound is normal though, then its not a problem for me.
I was just wondering if it meant trouble on the horizon

gamebowser said:

I have an 88 Spider with injection and I get the pop-pop-pop on deceleration (engine braking).

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First thing I'd check is carbon buildup around the throttle butterfly (preventing the throttle from fully closing) followed by the Throttle Position Switch (TPS) adjustment.

On closed throttle deceleration at engine speeds above about 1300rpm, the fuel ECU shuts off fuel to the engine. The fuel ECU gets the closed throttle signal from the TPS. If this signal is not present on decel, the fuel ECU will not shut off fuel flow which can result in backfiring.