Backfiring with Black Smoke

[FL-Frank]

I brought my '72 911 to my mechanic for routine maintenance...oil and filter change, ignition tuning, tire rotation, valve adjustment, etc. I also asked them to adjust the MFI because although the car was running strong, I suspected it was set too rich since the tail pipe always had a lot of carbon buildup. Also, full acceleration would occasionally result in a belch of black crud out the tailpipe.

The shop is reputable, with lots of experience working on mostly Porsches of various vintages. In fact, they've been caring for my '72 since I acquired it couple of years ago.

In addition to the routine maintenance, they replaced the Thermo-Time Switch. After wrapping up all of the maintenance, the car sat in their shop for the next two days. When they went to start it, it cranked but wouldn't fire. After troubleshooting, they determined the switch had failed and replaced it.

Well, I picked it up today, and immediatley noticed a difference in the way it's running. Acceleration doesn't seem as strong and crisp as before, while a couple of full throttle bursts out on the freeway produced 2-3 loud backfires with a significant puff of black smoke each time. While driving in town, it also produces a sporatic "popping" sound. These are not as violent as the full throttle backfires, however, they're still audible.

So, my question is, what do you guys suspect is causing this? Mal-adjusted MFI, or ?????

Thanks,

Frank

[rs911t]

I had a similar problem: backfiring, bad idle, cutting out during "full throttle bursts". I bought an LM-1 and immediately discovered the MFI was set way too rich. Dialed it back and it run quite nicely. YMMV

[FL-Frank]

I had a similar problem: backfiring, bad idle, cutting out during "full throttle bursts". I bought an LM-1 and immediately discovered the MFI was set way too rich. Dialed it back and it run quite nicely. YMMV

Thanks Greg. What is an "LM-1"?

Frank

ps: Glad to see you've hung on to your '72. That is one beautiful 911!

[rs911t]

It's an wideband air/fuel ratio meter. See http://www.innovatemotorsports.com/products/lm1.php

[Jeff Higgins]

I would ask the shop specifically what they adjusted on the MFI. The high speed mixture is where most would turn to "adjust" it, and possibly the low speed. If they leaned this out significantly, that may very well be the problem.

Your big cloud of black smoke on acceleration is charictaristic of MFI, as is the black soot in the tailpipes. Especially if you have been cruising for an extended period at lower rpm's, or puttering around in traffic, they will all "clear their throat" with a pretty good black cloud the first time you open it up. It should clear up after that, and stay clear as long as you are running it hard. Any effort to "remedy" this may result in a lean condition.

One other oft overlooked area is the spark plugs. Because of the rich, sooty environment in which they run in an MFI motor, the fancy whiz-bang plugs don't seem to survive. Any of the new designs with the thin wire, or multiple wire electrodes seem to foul very quickly. If your shop "upgraded" to something like this, that might be the problem. MFI likes the old style, fat electrode, cheap plugs. I run NGK BP7ES or BP8ES's.

[FL-Frank]

Thanks for the tips Jeff.

I dropped the car off at a different shop (http://www.early911sregistry.org/for...ad.php?t=16187) yesterday. They reported today that after some minor MFI adjustments, it's running "quite nicely now". They did a bit more fine adjustments this am, and plan to do a final check first thing tomorrow am.

I can't wait to take it out for a nice long run this weekend!

[sithot]

I suspected it was set too rich since the tail pipe always had a lot of carbon buildup. Also, full acceleration would occasionally result in a belch of black crud out the tailpipe.

It's just about impossible to get a tailpipe that isn't black these days. The fuel we use doesn't do the "gray-brown" burn. I'll admit that if you were spewing black smoke your car was indeed running "fat".

If you want to experiment try some racing fuel. I use a C12 (which does contain lead). My car sits a lot. These types of fuels do not break down as easily as pump gas which is pretty much history after 2-3 months. MS101 is a very good no lead fuel.
All of these are available from VP Racing Fuels.

http://www.vpracingfuels.com/vp_10_techdata.html

Tom

[sithot]

I suspected it was set too rich since the tail pipe always had a lot of carbon buildup. Also, full acceleration would occasionally result in a belch of black crud out the tailpipe.

It's just about impossible to get a tailpipe that isn't black these days. The fuel we use doesn't do the "gray-brown" burn. I'll admit that if you were spewing black smoke your car was indeed running "fat".

If you want to experiment try some racing fuel. I've used C12 (which does contain lead). If a car, motorcycle or any engine for that matter sits a lot you will have issues with fuel break down. My car sits a lot. These types of fuels do not break down as easily as pump gas which is pretty much history after 2-3 months. I have never had any issues with a vehicle sitting up for extended periods with this "good stuff" in the tank.
MS101 is a very good no lead fuel.

All of these are available from VP Racing Fuels.

http://www.vpracingfuels.com/vp_10_techdata.html

Tom

[nickd]

I pop in about a gallon of leaded race fuel to 10 to 15 gallons of unleaded street fuel - them pipes go nice a grey after a run. I was told that the lead lubricates the mfi unit some while back and it sure does run better.

my 2 cents worth

[FL-Frank]

I dropped the car off at a different shop (http://www.early911sregistry.org/for...ad.php?t=16187) yesterday. They reported today that after some minor MFI adjustments, it's running "quite nicely now". They did a bit more fine adjustments this am, and plan to do a final check first thing tomorrow am.

Description of Service:

Checked timing, adjusted points, repaired electrical contact on distributor, adjusted injection pump (low speed), checked throttle body calibration, test drove, re-adjusted cold CO values, final test drive. 3.5 Hours.

I can't believe it's the same car! It's smooth as silk and pulls real strong now, without any backfires or pops or any black smoke at all!

Many thanks to Bruce and the guys at KSR!