DJet Frustration, Ever play that Whack-A-Mole game at the arcade? Options

[ConeDodger]

Remember that arcade game Whack-A-Mole? You take a hammer and whomp the moles as they come up through the random holes? McMark and jcd914 and me had that kind of day playing Whack-A-Mole with the DJet on Jim's engine (jcd914) in my car...

Recently the engine started running bad. It had a low speed miss and then would go like stink when you got into the advance curve. I have a bunch of new 914 DJet components so I started throwing parts at it. This is a method used by people who aren't mechanics to fix something they don't want to pay a mechanic to figure out and fix. Take a note if you aren't a mechanic - this method is often more expensive then just paying the damned mechanic. So, I replaced the MPS with one that I bought from George at AA a few years back and never really used, I replaced the harness with a Bowlsby harness, I put a resistor in the CHT circuit... Each thing I did made it run better in some ways but often worse in others.
So, I decided that since I was having the Sacramento Porsche Family Reunion today and McMark was coming up, how about having him take a look at it and paying him for his time. Mark was game. He brought his LM 1 Wideband AFR and we methodically went through the tuning process. Here is where the Whack-A-Mole game began. Everything we did would improve it but unmask some other issue. Some things did nothing when they logically should have. We replaced the distributor with a rebuilt one from Rich Bontempi, it ran worse. We discovered that two injectors were not firing. So the distributor was good but the trigger points, which were new by the way, were bad. Swapped in the trigger points from the original distributor and it ran on all cylinders again but still ran bad. Lean, very lean... Then Jim Dupree pulls in the driveway and steps out of his car with an MPS that he pulled off of one of his spares. 30 seconds to plug in the vacuum line and the harness plug and the problem is solved. Turns out the new MPS from AA is bad or badly tuned for the motor. I suspect the later as it will hold vacuum.
So after three hours of Mark's time and lots of parts. The engine - Jim's engine runs very nicely again... Ahhh... the joys of a 30+ year old car (IMG:style_emoticons/default/headbang.gif)

[dr914@autoatlanta.com]

When the new MPS units were discontinued by Bosch we had to solely depend on Brett Instruments now Fuel Injection Corp, to rebuild the old ones. Without going into the intricacies of the units like some of the guys now have done (and praise to their efforts) we never broke loose the glue from a rebuilt unit except in the case of a dissimilar engine, to adjust the units. Usually when the rebuilders set them to the so called factory specification, they work just fine on an engine without any other problem, and yes the first installation usually means they go from rich because of the old broken diaphragm sensor, to lean with the new one. We have been able to correct this by returning a bunch of other problems to factory specs, like curing vacuum leaks, replacing lean running injectors, replacing temp sensors adjusting fuel pressure etc etc., working around the mps and using it as a constant. We of course have the luxury of substituting components and found that very few times if the rebuilt pressure sensor did not make the engine run properly, that any other substituted would make any difference either. It was always another problem.
We have found in the past that way too many so called "mechanics"; would take apart the MPS., dig out the glue, and adjust it until they made the engine run better. This operation would do nothing but compromise the customers car by covering up another problem and was NOT a solution. Again I stress that when dealing with a factory d fuel injection problem, it is imperitive to work with constants to cure that problem and not wing it adjusting one to compensate for the other.
Please note that I agree that there is not substitute for brand new factory components and that many rebuilds are compromised by inferior replacement parts, but that is really not the route of the problem. In our shop we NEVER depend on just one example of a replacement component upon which to base our diagnosis and repair. ( I realize that everyone does not have that luxury!)

Remember that arcade game Whack-A-Mole? You take a hammer and whomp the moles as they come up through the random holes? McMark and jcd914 and me had that kind of day playing Whack-A-Mole with the DJet on Jim's engine (jcd914) in my car...

Recently the engine started running bad. It had a low speed miss and then would go like stink when you got into the advance curve. I have a bunch of new 914 DJet components so I started throwing parts at it. This is a method used by people who aren't mechanics to fix something they don't want to pay a mechanic to figure out and fix. Take a note if you aren't a mechanic - this method is often more expensive then just paying the damned mechanic. So, I replaced the MPS with one that I bought from George at AA a few years back and never really used, I replaced the harness with a Bowlsby harness, I put a resistor in the CHT circuit... Each thing I did made it run better in some ways but often worse in others.
So, I decided that since I was having the Sacramento Porsche Family Reunion today and McMark was coming up, how about having him take a look at it and paying him for his time. Mark was game. He brought his LM 1 Wideband AFR and we methodically went through the tuning process. Here is where the Whack-A-Mole game began. Everything we did would improve it but unmask some other issue. Some things did nothing when they logically should have. We replaced the distributor with a rebuilt one from Rich Bontempi, it ran worse. We discovered that two injectors were not firing. So the distributor was good but the trigger points, which were new by the way, were bad. Swapped in the trigger points from the original distributor and it ran on all cylinders again but still ran bad. Lean, very lean... Then Jim Dupree pulls in the driveway and steps out of his car with an MPS that he pulled off of one of his spares. 30 seconds to plug in the vacuum line and the harness plug and the problem is solved. Turns out the new MPS from AA is bad or badly tuned for the motor. I suspect the later as it will hold vacuum.
So after three hours of Mark's time and lots of parts. The engine - Jim's engine runs very nicely again... Ahhh... the joys of a 30+ year old car (IMG:style_emoticons/default/headbang.gif)