MFI for idiots

[kecho]

Hey friends,
Long time Porsche guy here, and new MFI owner. Loads of 356 history and a few early 911's, some CIS injected cars, but my first MFI car.

I just picked up a '75 Euro MFI Carrera coupe recently, and I'm experiencing some (apparently common) issues with MFI system. The usual...spitting, popping, backfiring, either at cruising speed or de-acceleration. No real loss of power, seems to run fine on acceleration, idles like a Swiss sewing machine, but pops and spits while cruising at between 2200 and 3300 rpm's.

When I removed the air cleaner housing and jacked up the hand throttle to about 2800rpm's, I got lots of popping and spitting on cylinders #5 and #6.

Now, remember, I'm an old carburetor guy and all this talk about correlation, and AFR readings is WAY over my head.

SO...my question (besides some suggestions) is..... "Is there a wonderfully complete and straight forward repair manual or "simple to understand" guide to adjusting/tuning an MFI system?"

I feel I'm a pretty decent wrench and can pretty much handle anything around a Porsche (early that is), but I'm a bit confused about the MFI system.

Any and all advice is greatly appreciated.

Thank you,
Chris

[tharbert]

Short answer is "no." About the closest thing is, basically, this forum and some awesome MFI threads. The first thing you need to do is fine/download Check, Measure, Adjust, the Porsche manual for setting up car for the pump. The gist of it is, as the car goes out of tune, it will start to run more poorly. Only through a methodical re-tuning of the car will bring the most out of the MFI system. Do not start adjusting the pump. It's not the problem. Start reading here. This is a collection of MFI info: Ultimate MFI I don't have time to look... Search for CMA II. It's an updated version to account for our engines getting older.

Where are you at? There are a handful of MFI enthusiasts sprinkled around. You may be lucky enough to have one close by. It helps to put your general location in your signature or login info.

[Flieger]

If your speed switch (fuel shutoff above 1500 rpm or thereabouts on closed throttle) is either broken (due to a broken transistor lead) or disconnected, that can also cause pops on decel.

[kecho]

Thank you both for the feedback. I will check out the other MFI threads and proceed.

I'm in San Diego, and would greatly appreciate leads on a good/great person or company to contact.

[tharbert]

The speed switch is a nifty little 1970's electronic box and is almost certainly not functioning. You'll eventually find it. It's a cigarette box shaped aluminum, err, box plugged into the electronics left of the engine. Unplug it and give it a shake. You'll probably find it's like a baby rattle. Those are components that have corroded and come loose. I can probably help you with that. I've repaired my own with the help of this forum.

I don't think that's the problem here if you are holding the throttle at ~2800 RPM. The speed switch only engages with the throttle closed. It cuts the fuel to the engine so it won't backfire while slowing. Some people like it broken because of the "rumble" it causes.

I'm looking forward to hear what you find and how you go about fixing it. Good luck!

[NickP]

Chris

Have Glenn Roberts at Wayne Baker's shop help you with your MFI. Glenn will know how to diagnose and fix your issue. Sounds like a cool car, hope to see it around.

Nick