OK for those who have been following I was worried about compression. Nope: the top clip on the dizzy failed and the spring advance was stuck on. And the washer holding the battery ground strap broke in half, leaving a half circle contact.

So I actually have a car that drives and starts. But the idle will run at 1900 and then after a minute or two go south and then run at 400 (or stall) and then after a minute or two go up again. Heel and toe practice at stop lights I could do without.

I had to mess with the MPS to get this far and the A/F goes dramatically lean as the RPMs go down, but that could also be lack of spark. Goes a little too rich during moderate acceleration too, but this is the sweetest spot I have found. The normal A/F at idle and cruise is 12-13. It has a webcam 73 +2.5° and crane ignition (optical).

pbanders I seem to recall complained about idle stability with the D-Jet, but I doubt it was this bad. Ideas ladies and gentlemen? Thanks.

In my experience, and I attempted to use hotter cams back in the 70s (with consultation with some of the D-jet gurus of the era), you're never gonna get much in the way of low RPM (idle) and transition drivability with that cam. The vacuum signal the MPS sees isn't clean enough, nor sufficient, for the system to operate properly. Technology may have changed, and time has passed, but D-jet is still analog and still primitive. The sales people at WebCam live in a different reality from the rest of us .................

The Cap'n

Thanks Capn, you described the symptoms pretty much spot on. I do get more power when it comes on, but it is a bit finicky. Any one else have a cure?

The 914 MPS has an extra diaphragm that allows the base setting to float based upon air pressure and vacuum.

This makes it way more sensitive than the other available MPS units that are from the same Era, same Djet system.

Vw, Volvo, Mercedes (to name a few) all used Djet in the early 70's

Try this (not poking, just speaking from real live experience with a modern o2 sensor as a guide)
Find the MPS unit from a VW 411.

Slits had a few, ask if he still does.
He knows the part number also if nothing else...I cant remember it and the car is not here anymore.
I have a box of MPS units from different cars of the era...I was searching for one that would work in this situation and I found it.

That MPS works for larger motors and slightly more overlapped camshafts.
It also has a similar fuel ramp, so its will idle better.

That unit does not have the lower diaphragm like the 914 MPS does.
ITs also externally adjustable with an allen wrench, and you WILL need to set it to your motor and altitude.
Setting it can be done with an AFM setup.

I use one that stuff up into the tailpipe far enough to heat it up, then I run the wiring over the trunk and up into the passenger compartment.
From there, its a drive and tune run around town on different roads, speeds, loads etc.

Its not a huge amount of tuning and you are seeking to get it so it wont lean out...and not run so rich at idle that you have a sucky idle.

Takes about 30 minutes to get it perfect...or as perfect as you can get within the limitations of Djet.

Now, once you get it close to perfect, you may want to look into a multi spark system, or even a crank fire system for ignition.
You can run more advance on the larger motors with higher CR...but only a few degrees and you need to control it much more than what the factory distributor can do.


Rich

Is that the same grind as Rabies 9550? If it is I got mine dialed in after AFM installed. They want to idle rich

I have a copy of a cam card that indicates that it is. I have been very cautious about calling it a Raby cam because it certainly isn't. I have the idea that the profiles are at least similar. That is useful to know.

I want to autoX on Sunday (only racing against myself), so I think I will check everything along with adjusting the MPS. I find that assuming something works because I went over a year before doesn't seem to work on a car this old.

Thanks a lot Rich,

I drilled the plug and made a cute miniature set of tools so I can adjust it in the car. I seem to be robbing Peter to pay Paul though, moving flat spots around. I suspect my idle problem is not due to the MPS and I am going to hunt around for stray air tomorrow.

I am thinking the second diaphragm (provided new by one of our generous sponsors) is there to provide a second enrichment curve, but the way it is adjusted is hard for me to understand. You move the stop so it changes the point where that curve kicks in - sort of - actually it appears that you only really control how much of the pulse lengthening is going to happen on one curve vs the other. In practice I am getting a bit lost.

I can adjust through a hole in the cap that still allows the cap to serve as a stop. I have an easy time with the little screw basically adjusting rich/lean and the cap adjusting where WOT is. The part I am having trouble with is what in practice to do with the outer hex adjuster. Any hints would help. Is it normal to drive around, stopping to adjust as you go?

I have to say this has been a humbling experience and a very interesting challenge. I have enjoyed the support of the board.

if you have a hole in the cap that is under the epoxy seal, you now have a leak in the MPS.

If you suspect air leaks, you can run the motor at idle with NOTHING attached to the plenum except the MPS.

So, remove and cap all of the other lines.
See if the condition continues.

Rich

I did a lot of adjusting today. Leaned it back again now that the dizzy isn't in permanent advance mode. Turns out there are air vents that are on that side of the case, so my hole in the cap is not a problem. I took your comments and the Capn's to heart and asked myself hoow to make a damper for the vacuum signal. I was thinking an inductor capacitor citrcuit would be great, but this is air. In the end Apple computer company solved it. I took an old aluminum cooling radiator from a dead computer and hooked it between the plenum and the MPS. Preliminary tests are very promising. Pictures if it really works.

Thanks again

I want to autoX on Sunday (only racing against myself), so I think


Hey! What about me?

I need to see this one when you are done.

Trust me....the 411 mps will be sooooo much easier to set.

Do you have an 02 sensor in place to measure mixture, or are you doing this by ear?

I do have the O2 sensor - my best friend and the head temp gauge as another tool. At the autocross things weren't going so good, but afterwards a half turn of the little screw towards rich worked magic. I will need to post a thread about this I think. Drove to work today in splendor without hiccups. Probably jinxed it, but maybe not.

As usual, the other Warren (above) spanked me at the auto-X. Sadly I cannot in truth say he looks like this:

I want to find the 411 MPS, but things are gradually falling into place. The computer radiator turned out to provide only minimal improvement, so it went back to the heap of things I cannot throw away. Thanks Rich, Capn, Stu, Mark, and all the others.