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)

[pbanders]

Good comments from George. RE: "factory specification" - that's part of the problem here. Before Bret sold their business, I spent some time on the phone with Don Burdenhart (owner) talking about their procedures. Don built a D-Jet simulator, very much like the tester that I have, that let him measure the injection pulse width as a function of vacuum. He used a set of "new standards", NOS MPS's, to build calibration charts for each model, and that's part of the problem...

While nearly all of the MPS's (certainly those for the 914) have the same part-load adjustments, the full-load adjustment varies as per the displacement and motor type. That's the main difference between the 037, 043/044, and 049 MPS's. What I've found is that there is significant variation in the full-load adjustment across a set of NOS MPS's, as much as 0.4 ms. Additionally, I've found that even though the part-load setup on an MPS doesn't vary much, the exact setting required for optimal running DOES vary. In other words, you can install an MPS and the car will run fine, but that doesn't mean you're not running at 4.0% CO or greater under part-load conditions, and wasting fuel or failing emissions. The MPS is very sensitive to changes in part-load, as I mentioned before. Even a 0.1 ms variation is very significant, and variations of this magnitude are not uncommon. From data I've gotten from Jake Raby and from my own measurements, you want the part-load CO to be between 3.0 to 3.5%. I've gotten some good results even at 2.5%, but I haven't verified this isn't too lean under hot running conditions.

Like George says, if you don't have the gear and understanding to do this kind of testing and adjustment, then it's more likely than not than not that you'll either mask an existing problem or make things worse. I've outlined some of the procedures on my pages, but over time, I hope we can come up with simpified procedures and perhaps services that can help owners get the most out of their D-Jet cars.