mechanical injection popping and sorrows

I have a 72' T with mechanical injection and have spent many long nights ajusting and ajusting, but I can't seem to get it to stop backfiring through the intakes. If it's not one it's another or both or speratic on all cylinders. Ive heard about worn out throttle plates and how hard that can be. I was also told by the gimp who sold the car to me that I needed new injectors. With the price of injectors and rebuilding throttle bodies I thought it might be better to find some carbs. I also have never seen a book on mechanical injection. anyone? anyone?

I don't have an answer to your question but am commenting because I am in the same boat somewhat. I have 1971 914 with a 1971 911E 2.2 motor with mechanical fuel injection. I am having some different problems with mine and have been looking for help myself. I don't really have the backfiring problem you mentioned but I do have a problem with starting the car, especially cold. The car usually takes three trys to get it started. After it starts, the motor barely idles until it warms up. After that, it runs great. What is your car like to start? If you have a chance, please e-mail me at dman007@ix.netcom.com and let me know how your car is to start. Thank you in advance and I will pass along any information that might help you.

Do both of you guys have L Jetronic? I have K Jet, so I might not be too helpful, but had lots of trouble getting my 82 931 set up.
I'll have to think about it, but I think the two most common causes are bad timing or a rich mixture. The problem with L Jet is that when almost anything starts wearing, you get overrich conditions. Are you guys getting any other symptoms, like black smoke at idle, or black smoke at load? Those injectors don't go bad often, but can clog. I don't think it would result in the symptoms you describe.

I have had this problem with my early cars as well. The fuel injection that you have up is consistant up to 1973 as is the described problem. First, how many miles are on the car. The throttle bodies will wear at a fast rate if driven around town. This, because of stop and go traffic allows the shafts to vibrate in their bores and they eventually start to pop. One of my cars went over 100,000 miles and they were finally in need of a rebuild. The one thing that is important, is that all 6 throttles are synchronized. If not, the car will run incorrectly through the range. Normally the early cars will have a flat spot in the 2800-3300 RPM range that can be worked with by adjusting the mixture through the fan housing. You could also have a problem with the pump. If you switch to carbs, make sure that your state will allow it to pass emmissions. California, New York, New Jersey, and I'm sure others, will not let them through. Please e-mail me (marcw@pipeline.com) if I can help. Good luck.

[Wayne 962]

The price on carbs nowadays can run from about $500 for a cheap set all the way up to $1-2K. I have people who are looking right now for good sets of triple barrel Webbers, willing to pay well over $1K.

Beware of certain carb rebuilders as well. Some tend to not repair some of the carb parts that are worn out and not available in the rebuild kits. For more details, you can drop me a line...

Thanks,

Wayne

Wayne:
I have a COMPLETE WEBER CONVERSION for a 911.
I had it in my 911SC 3.0L. I have 46IDL Webers, jetted according to PMO guidelines (same with Venturies), Mechanical advance distributor recurved by AI, intake manifolds, fuel pump, pressure regulator, K&N filters, chromed water shields. ALL you need to convert to Webers. They where rebuilt about six months ago but only have about 300 miles. These Webers where to much for me! All that power and quick response made me speed to much and too many tickets made me pull them off. I would advise anyone interested in them to use them for racing. They run good in street but they are too fast. These
46IDAs new $2,250.00. The whole set $3,500.00. I'll consider selling for $1,450.00 Serious inquiries only.

Sorry guys, typo on the Weber type. They are 46IDA Webers the rest is correct.

re: Weber carbs for sale. I've got a '75 911 with a fuel system that is giving me fits. If you still have that set of 40IDA's for sale let me know. Suiner@aol.com

Chris: I've got a similar intake backfire on mine...usually cylinder #3, sometimes #2. I just had my MFI pump rebuilt, so I'm sticking with MFI for the long haul. I was told, however, that I need to have my throttle bodies rebuilt and all the ball joints replaced.

Your symptoms sound similar. Have you ordered the MFI book from Pacific Fuel Injection??

dman007: My MFI system has a cold start mode (hand throttle plus MFI bypass when cranking the starter). I don't know about 914's though...

I'll have to add my 2 cents here. I just rebuilt a 2.2L with E pistons, S cams and had the worst running webers (about 110K miles). I had them rebuilt by a very fine fellow in San Jose, CA and changed the jetting from the stock 914/6 (which is what I have) to the E specs. The carbs run quite well now but I also get popping as discribed. The problem is minor (it always did it when cold when brand new !) but I believe the answer is that the carbs run just a little lean (good milage though). The most important jets are the idle jets. The E specs call for 55's and I could probably illiminate the popping with larger ones. The nice thing about the carbs is the ease in adjusting for engine changes!
Bryan Wilson

I have a 1973 T with Bosch K-Jetronic (CIS) that was having similar problems; Backfiring and running rough - uneven idle, and the problem was very bad at around 2000 r.p.m, although it was occurring throughout the rev range.

Remember not to jump to conclusions; Porsche (Bosch) fuel injection systems are reliable and very high quality; they seldom go wong. The first place to start with any problem like this is to check the ignition system...

After much investigation, we discovered that the backfiring was caused by a malfunctioning fuel injector on No 2 cylinder. This was leading to the engine firing on 5 cylinders from time to time, and so causing the backfiring.

The solution was to replace the fuel injector.

This is one of those cases where an official Porsche garage might actually save you money. This is because they have the correct diagnostic equipment to locate the cause of the trouble first time around. I wasted a lot of time and money on trying to locate the cause of the problem; for example I bought 6 unecessary new spark-plugs, a new rotor arm, a cold-start valve and so on.

Hope this helps.