Deceleration Popping or Backfiring-Cause

[MysticAlpaca]

I suspected as much, thanks for the clarification Steve. Just one more reason for me to get my order in to Steve.

I just have to get the energy to take my seat out to see if I have a 24 or 28 pin chip.

--Bill

[JBO]

...now I'm confused...on a CIS 911, I had always heard that popping through intake means lean (and is bad), but popping through exhaust (assuming no leaks) means a little rich (not a problem). Which is it????

[Steila911SC Cab]

Very good question?

[Joe Bob]

Ya still gots to verify "NO LEAKS".....if there is no change on your intake or mixture or anything else and you suddenly get popping on decel.....assume leaks in the exhaust system.

Eliminate the obvious......

[Steila911SC Cab]

I'll check for leaks next, and go from there.

Thanks

[afterburn 549]

In most real world applications the cause will be a lean condition ...from a a whole lot of of diff conditions and most will not be harmful to the lil recip eng. You just do not want lean in the power band....that would be a no no

[sammyg2]

I will not try to argue with Steve, especially about computerized cars.

But..... on my CIS engine, leaning the mixture out reduces the popping in the exhaust, richening it up makes it worse.
My car is not far out of tune either, it is very nearly optimal.

[JBO]

My experience was just like sammy's - had popping through intake and adjusted mixture screw (turned to right to richen) and cleared it up, but went a little to far and had popping through exhaust - a little left turn of the adjustment screw and no more popping through exhaust. Thus my conclusion that popping through exhaust (with no /other problems) meant a little rich.

[overman]

I had this trouble with an early car I purchased, 2.7L with weber's, sport exhaust... during deceleration the car would roar off a 'boom' and scare the hell our of me, any one with me and anyone else within a 100 ft... no amount of adjustment would help this thing. Finally I had a leak down test preformed and found the trouble... the engine heads were leaking all over the place... shoty machine work... Reworking the heads would have fixed this problem... I elected to have a total rebuild preformed.

[udidwht]

In most instances it's almost always a lean AFR that causes this (subtle popping) not rich. It's most notable on high RPM overrun conditions especially when going downhill in gear with foot off gas. It's during this overrun that the AFR can go high enough that one can hear it. But on level street one won't (different engine condition).

I can vouch haven seen it on the LM-2. Once I set the carbs for (LBI ~13.0) if I coast down a hill at say 40mph in 3rd gear (high RPM) foot off throttle the LM-2 shows lean 14-low 15's. Which is normal. Attempt the same thing on level street and the AFR won't go high enough...different engine condition.

Could only hear it or get it to be heard on downhill high RPM condition. This also lead me strongly believe it wasn't an exhaust leak. Since exhaust leaks are also heard during many other driving conditions (heavy accel & shifting points) and are almost always loud.

This lean condition (LM-2) while lean still shows fuel flowing thru the engine. But since the lean mix burns slowly it requires more timing. But given my foot is off the throttle timing is too retarded so the small amount of fuel makes its way into the exhaust (collects until next cycle) where it's heard out the tailpipe (pop, pop, pop, pop) a bit like popcorn being popped. No harm no foul.

I'm also running a Bosch SVDA but since it's connected to ported vacuum as it should be... there is no added advance because of the closed throttle = no vacuum. Hence...no additional advance to help with the lean burn. Timing at idle is too retarded to burn the lean mix fuel.

Initially I thought it was an exhaust leak but I soon confirmed that it wasn't after crawling under and not finding any tell tale signs of an exhaust leak.

An exhaust leak will almost always be accompanied with a loud bang or boom due to an excessive rich mix coupled with an exhaust leak/s. They (exhaust leak/s) would also be readily apparent during other driving conditions aside from just on the overrun. Such as during acceleration & shifting points.

VW's & older Porsches are fairly common to this condition (lean popping) they (VW/Porsche) would come up with ways in hopes of reducing it by adding devices such as the following:

Throttle Positioner
Dashpot
Gulp Valve
Decel Valve


But for me none of the above devices apply because I have none of them. I'm running dual Weber 40 IDF's. The above devices generally allow for more air to be passed to bring the AFR up very high (20:1 or higher) to the point where there is no O2...no O2 no way for the fuel to go pop, pop, pop.


1972 Westy Hardtop/Type-4 2056cc
96mm Biral AA P/C's~7.8:1CR
Headflow Masters New AMC 42x36mm heads w/Porsche swivel adjusters
71mm Stroke
73 Web Cam w/Web solids
Dual 40mm Webers (50 idles/125 mains/180 air corr./F11 tubes/28mm Vents - Float height 10.45mm/Drop 32mm
Bosch SVDA w/Pertronix module
Pertronix Flamethrower coil
S&S 4-1 w/Walker QP 17862
3 rib 002 Trans

[s5uewf]

The exhaust mixture is lean. Check you AFR's and if OK, then don't worry about it.

[boyt911sc]

Are you guys know how to read dates? This post was made 9 years ago!!!!! Where have you been all these years?

Tony

[cabmandone]

Originally Posted by boyt911sc

Are you guys know how to read dates? This post was made 9 years ago!!!!! Where have you been all these years?

Tony

I have GOT to start looking at the thread start date. Now Steve is going to wonder what the hell I'm talking about when I pm'd him about my 3.2 popping under deceleration with a cat delete tube.

I guess I could have "hijacked" this thread since it's older than dirt and it wouldn't have mattered.

The worst part about this thread is the OP never posted if they resolved their problem and how. That drives me crazier (not crazy, I'm already there)

[udidwht]

The date is irrelevant...it isn't a 'closed' topic. No reason for a new thread when this one exists.

[kach22i]

Good threads never die.

[stlrj]

Originally Posted by kach22i

Good threads never die.

Make sure this vacuum hose is connected. It's purpose is to limit the amount of manifold vacuum when decelerating to avoid backfiring/popping and, at the same time, reduce emissions. Excessive maniflold vacuum means insufficient oxygen present to support combustion during deceleration resulting in unburnt fuel leaving the combustion chamber igniting in your red hot exhaust system...pow sputter bing bang boom!

The later DME controlled Carreras have an even better strategy. Fuel delivery is totally interrupted during engine overrun, preventing any possibility of sputter and backfire altogether regardless of how lean or rich they happen to be tuned or even if vacuum leaks are present for that matter.



Cheers,
Joe
82 Targa

[boss11]

sub'd

will have a look at the vacuum hose as in above pic

I too have popping on deceleration after installing cat bypass

[AVDCAV]

The pop is awesome. I would never change it ' my fabspeed is like its engineered to kick ass,
Always perfect , just rev it pop the clutch at 4 or 5 k and whammy ". In the Baltimore tunnel
Its priceless . It's like a bomb went off.

Damn Steve just brought logic into the picture ". Those genius guys can be such a buzz kill