'Popping' on deceleration; 72 911S (2.7)

[JD911]

Hello to all. This is my first posting - hoping someone can perhaps explain the bizarre.
I've owned my 72S for 27 years and 17 years ago had it taken to RS Spec. I basically had a factory trained 911 specialist build a brand new engine - all parts new except the heads which were re-built by well known 911 head specialists.
When I say new everything, that's what I mean - Case, crank, rods, camshafts 'Mahle' pistons and barrels - even a brand new RS injection pump I bought during a factory dealer 'close out' sale. Throttle bodies rebuilt by 'Eurometrix' - then in Calif.
This engine, even after 60K miles (oil changes every 3K) has a HEAP of power just recently showing 123KW on a dyno.
Road performance is flawless (and fast!) except when I back off (typically in 3rd gear) where I might have been pushing say 4500rpm, as soon as the revs drop to 3500rpm I've been getting a popping (sometimes it's a 'light' backfire) at/in the exhaust which Ive put up with for years (10+) because most people say its a 'feature' of the car, even producing a 'Porsche' bulletin stating - 'Backfiring in the exhaust will occasionally occur and cannot always be completely avoided even by the most careful tuning (Source; page 38 from some official factory publication)
Well the original 2.4S engine didn't do it nor did happen when the existing 2.7RS spec. engine was first built and installed.

I had the heads redone 5K miles ago - no difference - new factory muffler (suspecting too much/not enough back pressure) no difference.
Throttle body spindles are NOT worn - rock stable idle with no increase in rpm if I spray 'Aerostart'/similar around spindles. Inj. pump re-checked by 911MFI pump specialist (local Porsche dealer uses the same company) all injectors likewise being checked. I've used all brands of 98 oct. pump fuel - no change.

There are no air leaks (before or after recent cyl. head 're-co') all butterflies are in sync. with balanced air flow through stacks - my Porsche trained mechanics (30+ years 911 experience) 'Bosch' tuning console recording the best 'CO' readings he says he has seen on a 911 MFI car.

I've deliberately kept the MICRO-SWITCH until LAST and you will soon see why. It has been/was checked and adjusted over and over and OVER - it works when and as it should - ie showing a drop in revs when manually activated at engine bay with thumb pressure etc. I've fitted a secondary throttle return spring to ensure the stop screw comes down positively.
.
I now 'cut to the chase' -

In total frustration when my mech. & I were test driving it the other day trying to figure WTF is 'wrong' he (for some reason) DISCONNECTED the wire/s from the micro-switch (NO feed/signal now to inj. pump solenoid/rack) after which the popping, cracking (on decel.) INSTANTLY disappeared!!
Isn't that the exact OPPOSITE to what should happen? My mechanic doesn't seem to understand the outcome/reason of what he did and yes - I am grateful the problem has 'gone but I just can't figure out why.
I am totally confused (and annoyed after years of 'suffering'/expense!) therefore any input to help explain this 'mystery' would be greatly appreciated.

My apologies for such a long post - but I thought I needed to/should explain 'all'.

Many thanks in advance!

[Flieger]

Not sure what to tell you other than mine pops on over-run also. Sometimes it is a big enough backfire that I even jump. Normally, though, its just some loud pops, not gunshots, and not too many people look.

I get 'ticks' through the intake stacks, but I have my plenum removed. The ticks are hard to hear except at low speed and the 'pops' take over when engine braking. I just live with them.

Try posting on pelicanparts.com and asking for Grady Clay or John Cramer. They are VERY knowledgable and there is great information and many pictures on the 911 technical forum for MFI users.

Cheers,

[JD911]

Thank you - I've been away for a week hence my late response.
I will contact Pelican as suggested.

Meanwhile, it would be interesting to see if your 'popping' symptom disappears if you disconnect the wire/s at the micro-switch thereby de-activating the injection pump solenoid which moves the control 'rack' inside the pump.

In any event, thank you for your reply!

[Flieger]

I discovered a couple days ago that my RPM transducer went missing during the long process of my project car. The box I looked at was the 2 stage rear defroster, which does me no good with my Lexan windows.

So, my problem was a missing (!) RPM transducer, not a faulty one.

I have a thread on this site and on Pelican about this.

I am looking for a core RPM transducer (or a working one) if you may know of one.

Anyone need a defroster electrical box?

[mrbeverlyhills]

My car does exactly the same thing. I have a 2.7 rs spec in a 73T and you just described my car.
My mechanic(30+years with these cars) and I are baffled, the popping stops when the micro switch is disconnected but I can tell it doesn't run quite as well, little stutter at idle but VERY hard to tell the difference. Pete Zimmerman is an old friend and owned Redline Service here in LA, had me reset the micro switch which helped for a while but it is back to it's old self.
I just live with it but it isn't fun. Would you mind posting a link to the Pelican thread so I can follow?
Other than that the car runs fantastically with only a minor congestive burp when applying partial throttle in a gentle acceleration, under full throttle it pulls like a train.
Thanks,

[Flieger]

Here is my thread: http://forums.pelicanparts.com/showthread.php?t=411623

I am looking for a core right now becuase I have nothing to repair. If you are looking for MFI info, try Grady Clay, John Cramer(sp?), Jeff Higgins and the rest of the Pelican MFI guys.

Here is the Ultimate MFI resources thread, including a schematic for the RPM transducer: http://forums.pelicanparts.com/showt...rpm+transducer

Some other threads with MFI info on the RPM transducer: http://forums.pelicanparts.com/showt...nsducer&page=5 (scroll down to Grady's troubleshooting procedure)

http://forums.pelicanparts.com/showt...rpm+transducer

http://forums.pelicanparts.com/showt...rpm+transducer (this one has contributions from the late Early S Man)