Rich condition??

Does popping from the exhaust on deceleration (throttle closed) indicate a rich condition? The problem is usually worse when the car is colder than operating temperature (180).

Thanks,

Olivier
82 SC

Someone with more knowledge than I have will probably correct me, but I had the same problem. Turned out to be the "shut off valve." Pretty inexpensive fix and mine NEVER backfires now, even though I eat as much gas as possible.

What is a "shut off valve"? Is this unique to MFI? My 1971 911T with Zenith carbs is running very rich as indicated by black soot in the exhaust pipe, poor gas mileage (about 10-12 MPG combined city/highway), and a strong exhaust odor (hence the name "Stinky 911T). My 911T pops a lot on deceleration and sometimes at constant speed, too. The Zeniths have been adjusted by two shops - no change in the exhaust odor or the popping.

Any suggestions?

hmmmmm, with the change over to SSI's and the Monty muffler, I now have burbling and minor popping with my foot off the gas pedal. Not sure, however if it is the new set up alone (with reduced back pressure) or a richer mixture.
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Just before the change over, I had the car (83SC) in for emissions test - of course, to pass the test, the mixture has to be leaned way out. After the test, we went to the new SSI/Monty set up and reset the mixture. Could be richer now than the previous mixture. I chalked up the burbling and popping to the Monty. Anyone else with a similar set up have burbling & popping??
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Chuck

Mine burbles and pops subtly. Engine and exhaust are bone stock.

Burbling and popping are much more pronounced with ignition timing slightly retarded.

On another thread there is a discussion of the Decel Valve's effect on backfiring in deceleration. Don't know if the earlier cars have a Decel Valve.

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'83 SC

Mine is a 70'911 T with also Zenith carbs.
I patiently totally rebuilt them and get a decent 20 mpg all the way around. To lower the backfiring, I had to enrich the mixture from the setup at the Porsche shop, go figure!!!
When I cut the gas before a turn and downshift as well, that's when I get the most popping from the exhaust : I always think of my 911 as six single British motorcycle engines and these guys used to backfire like crazy scary... must be the mechanical design !
Thierry

No experience with Porsches on this (yet!), but when I changed the exhaust on my Peugeot it went from merely noisy to noisy and popping and backfiring on deceleration, and if I am lucky between shifts . Difference was a cat replacement pipe and straight through sports muffler.

So that may explain the change with SSIs and a sport muffler for Chuck.

My Pug should rule out running rich etc, as it has a pretty sophisticated computer in charge of things (it is a 1994 model) - it is purely a free-flowing exhaust thing.

Cam

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Cameron Baudinet
1975 911S

Check out a post labeled "911sc deceleration valve" below. I think the last post date was 7-24-2000.

My 2.7 backfires all the time during deceleration. It has a aftermarket (don't know brand) headers and glasspack exhaust. It's super loud in low rpms. If I can get a flame to come out when it pops it would be super cool and I wouldn't mind the backfiring! I'm not too sure on the mixture part as I have no idea if my fuel system is configured optimally. But other than that, I really don't mind it. It's not bad for the engine or anything, right?

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Jeff
1976 911S

For the MFI guys:
We have a shut off valve for engine over run situations as well. It's called the shut off solenoid. You can see it easily if you look at the mfi unit from the engine bay. It pokes out, and looks like a small cylinder, 1" diameter and 3" long. It has a single wire leading to it. When this wire is energised, the solenoid (which is actually a really simple electro magnetic device) forces a rod to move about 3/16" into the mechanisms of the pump. It moves the control rack a similar amount, thus shutting the fuel supply off, and hopefully preventing unburnt fuel from entering the combustion chambers and causing backfire on the over run.
The operation of the solenoid itself is governed by two things: the RPM transducer, which is attatched to the panel where all the fuses and CDI unit hang on. This tells the system what the engine speed is. It puts out 12 volts above 1600 rpm and zero below it. The other piece in the puzzle is the micro switch. It works off the cross shaft that the accelerator linkages are attatched to. You can adjust this. It has two wires running to it and clicks on and off as the throttle shaft is moved.
To quote Pelican parts tech questions:

"With your foot off the accelerator, the switch is closed, and if the engine speed is above 1600rpm, ie you're coasting, then 12v is applied to the relay and it shuts off the pump. If you're below 1600 rpm, even though the switch is closed, ie, you're idling, the solenoid is not activated because the sensor is putting out 0v, and fuel is pumped to the engine".

Geddit??

For what it's worth, my shut off solonoid has not been working since I got the car (soon to be fixed) and I had assumed the backfiring was part of the fun of MFI. The racers took them off anyway, so you shoud view the system, I think, as a nice refinement to civilised driving.
Hope that helps someone!

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'72 911 TE

Oh- I forgot:
You can check to see if your stop solonoid is working by disconnecting the wire from it and applying 12v from a wire attatched to the red (upper) fuse connection directly to the now exposed spade on the shut off solonoid. The ignition can be off when you do this. You SHOULD hear a fairly stron "click'" noise from the solonoid. This is the sound it makes when the rod moves outwards due to the electromagnetic field having been generated within the solenoid, repelling the rod.
The solenoid is grounded, so that's the only connection you need to make.
The other checks for the speed switch and rpm transdeucer are available from the postings on Warment's site..(well worth a visit).