What's causing the slight backfiring

[FL-Frank]

'72 911 T, 2.4 with MFI. Permatune CD Box with recently installed Permatune Coil. Petronix breakerless ignition. Chevron Supreme, 93 octane fuel.

Starts effortlessly and performs perfectly until completely warmed up. Once at operating temp, at approximatley 3K rpm, and under very light throttle, there are a number of slight backfires...nothing dramatic, just a pop, pop, popping sound. If I let completely off the throttle, no popping and when I accelerate, no popping.

Prior to installing the new Permatune Coil, there was a Brazilan silver coil in its place. Although the current popping symptoms were not evident prior to replacing the coil , it did occassionly put out a big loud single backfire along with some black smoke when running the Brazilian coil.

Any thoughts as to the cause of the slight popping along with recommendations to eliminate them?

Thanks,

Frank

[orange911]

My opinion based on the symptoms and conditions is that the fuel mixture is too lean either at part load. To really find out the devil in the details, you should run it with an LM-1 with an o2 sensor in the muffler inlet. Mine did this and it took the LM-1 for me to get it in sync.

Not knowing your skill level, I hesitate to give too many recommendations on CMA. I started on mine in December of 08 and it took 5 months to put it all back together and have all in tune. There were times I could drive it and some I couldn't. It doesn't take that long to do it, I just did not have time between family, work and weather to do it all in a week. There was also a lot of learning curves I had to account for. I am a lot better off doing it myself, because I am much wiser with MFI than I ever set out to be.

I can now make slight adjustments to an MFI pump on the side of the road in 2 minutes and take off to get more measurements. This was how I dialed mine in. It would be easier on a dyno, but you don't get to do it at your leisure and it's not as fun to drive on the dyno.

I will also add that the MFI's tend to thirst for a rich condition. I had always heard this, but it didn't come to light until I had an instrument that told me what the AFR was after each adjustment. When I had it at the 13.8-14 range (just on the rich side) it would still pop. When I got it to about the 13.4-13.8 range, it was virtually eliminated. It only took one click on the part load adjustment to make that much difference.

[Capote59]

I have a 1971 911T and also have had the same thing popping(or backfiring?)happen. But mine are obviously carbs and not MFI. Also happens about 3K RPM. Was going to have this checked out in the next month or so as is not a huge issue(I hope it isnt.)

Chris Johnson

[911quest]

I would suggest checking the Igntion system. More so the points.

[orange911]

I agree that you should also check the ignition as this is one of the first in CMA. I should also note that I replaced my distributer and replaced the points with pertronix with great results and the backfire was not entirely cured (but I was running really lean). I noticed you were looking to replace your dizzy and I wanted to give my feedback. I think you would be best off to send it to a reputable rebuilder and have him go through it vs spending money on a used one. Don't shy away from pertronix either. This was how I knew my dizzy was bad. It accentuated the worn condition and I trusted it wasn't the pertronix and sure enough it wasn't. I have used it with excellent results on my Lucas dizzy. I sometimes think that those that have bad feelings toward them are hoping for a cure all to an otherwise worn out dizzy. It won't take the slop out. Good luck and keep us updated on the status.

[sithot]

Buy the BOSCH MFI book or buy one of these.

[vicrola]

Had a similiar issue with my '72 E. Had the transducer rebuilt...problem went away.

Vic

[Flieger]

I think the issue is the micro-switch trigger.

The RPM transducer/ microswitch/ shut-off solenoid system is working properly because there is no backfire when he lets completely off the throttle.

When he is on the throttle, the engine runs smooth so his mixture is good there.

The problem is under light throttle. I experience this same gentle backfire when I rest my foot on the throttle while engine braking downhill.

I know my backfires are due to the microswitch not fully closing under this light throttle pressure. The stiction in all the linkages and the light pressure from the weight of my foot is enough to let the screw on the throttle arm rest on the microswitch but not activate it. When I gently activate the throttle by hand in the engine compartment, I can see this happening. When I release the throttle in a quick motion, like when driving spiritedly, the microswitch closes properly. I have no trouble eliminating the backfires if I do not rest my foot on the pedal when engine braking.

I suggest acuating the throttle by hand in the engine bay and seeing if the microswitch is actually closing. If not, you can turn the screw on the throttle arm very slightly, although the real fix is to lubricate all throttle linkages and to make sure the throttle return springs (throttle pedal and on the MFI pump arm) have not weakened over time.

Best of Luck

[Kjell]

HI Frank just a slight sidetrack. Our 72Ts have a whopping 7.5/1 compression ration 91 octane is completely unnecessary. I have run for years with the lowest octane with no problem.

[sithot]

Loren at Systems Consulting is good for a lot of these Bosch issues.

http://www.systemsc.com/

I also had good luck with a fellow I found through the Mercedes club who was fantastic with cruise control units. 1st class guy all the way.

General Developement Laboratories:

http://gdl-online.com/begin1.html

Tom