I have been fighting the D jet slow idle on my 1971 1.7. Have checked and rechecked everything. Bought a smoke machine. Bought a new timing light with advance. All new ignition parts. Checked MPS. Checked all sensors with ohm meter. Disassembled and cleaned Distributor. Good compression. So started disconnecting things. When i disconnected and plugged the vacuum retard I was able to adjust a 900 rpm idle with the (finally effective after removing vacuum retard) idle air bleed. So I did a search for "914 vacuum retard" and this link came up. Read the last post. Exactly fits my symptoms. I'm going to try what he describes tomorrow and see what happens. But I'm pretty sure I'm on the right track finally. Thoughts?
https://shoptalkforums.com/viewtopic.php?t=2513

Lots of good stuff in there but also some odd bits. What I did, after all my vacuum leaks were fixed, was set the idle using the timing.

I am still not super happy but pretty close. Since you have a 71, pcv, I believe, is passive or non existent?

How do you feel about your TPS?

If those are not issues than warm your engine up real nice, close that idle screw and maybe turn it open a half turn or so. Then loosen your distributor and set the idle by your timing.

This only works if everything else is good. If you have vacuum leaks and messed up injectors etc, don't bother.

I did what you are saying yesterday. It worked and now I don't feel like the car is always going to die at idle. And no pinging under load.

usually a too low idle that cannot be adjusted even when the dwell is perfect and steady at 50 and the timing perfect and steady at 27 btdc with the engine rpm at 3500 warm, is caused by too rich a mixture. This could be caused by a leaking manifold pressure sensor or too high fuel pressure.

Both checked and fine. Even tried another MPS out of a good running car. The low idle persisted. Bumping the timing just a bit at idle makes all the difference.

Yes the crankcase vent is just into the air cleaner so no vacuum leaks there. TPS taken apart and cleaned. Looks like new with minor wear. Adjusted properly. One thing I'd like to know is what happens when you unplug the TPS on a properly idling car. I have tried different fuel pressures and none help the idling problem. Bumping up the timing works.
May be something in the relationship between FI trigger points and the ignition timing.
Going to mess with it more this winter but pretty happy with it right now. Would like to understand what is going on though.

Idle is controlled by the ECU when the two idle contacts in the TPS switch give idle control to the ECU. Without the TPS connected the combination of sensors and other settings such the air bleed screw, timing etc, control idle.

So it seems idle should change when the TPS is unplugged. Mine stays the same. I might be in the market for one of your FI wiring harnesses.
Probably next is to check continuity to the ecu.

I have found the idle "pad" on the TPS is pretty large. Does your ECU have the richness at idle adjustment? I am not sure 71 has it. That is a quick test to see if you are in the idle circuit. If the car doesn't think you are idling that adjustor doesn't affect the idle. I suspect it isn't on a 71. Sounds like it is working now though. Might be interesting to see where your timing ended up after the twist.

No adjuster on the ecu. I have a 74 with the adjuster and wish this car had one. I am going to try and order the FI trigger points adjusting tool and try that too. I cleaned them but that was all... I'll post when I get a number on the timing, haven't put the light on it yet just by ear. I don't know how your car reacted to changing the timing but on mine it is almost like a switch is thrown when I get enough advance. Like it suddenly stops struggling then the rpm rises smoothly with more advance from that point.

Yep, exactly what mine did. When I did figure that out I think I let out an audible sigh.

A 74 2.0L ECU has the idle mixture knob. Sounds like you have an earlier model year ECU without the knob. Don’t bother doing anything further without getting the correct ECU for your car.

He has a 74 and a 71. The 71 doesn't have it.

Yes this car is a 1971. I know the history of the car from day 1 and it has the original ECU. The 74 car has the knob, as it should. I'm going to check how much advance I'm getting in the 1971 at idle w/o the retard connected to try and narrow this down.

So with the retard disconnected and plugged-timing at idle of 850rpm right now is @tdc. At 3500 rpm it is @30 degrees BTDC. Connecting the retard pulls the timing back 3 fins (2 spaces don't know what that is in degrees) and results in barely running. Going to take it out tomorrow to see if it pings but at least the idle is resolved and I feel I'm on a good path with some numbers to work with. I'm assuming there is wear in the distributor somewhere allowing excessive centrifugal advance. Car has 105,000 miles.

You failed to mention of you tested the vaccum cannister on the dizzy for leaks. The cannister is of a single diaphram so plugging one side isn.t a valid test. To test it ....you should pull vaccum like your MPS with either side capped and pull vaccum and see if there is leakdown.

If this is leaking your car won.t idle right and timing won.t be correct either.

I have eliminated all vacuum leaks, tested both sides of advance/retard canister with vacuum pump and everything moves as it should and no leak down from either side. The only vacuum leak this car has is a tiny wisp of smoke from the shaft in the throttle body. The engine pulls 15in of vacuum at idle of 850rpm measured at the vacuum retard port on the throttle body. I am pretty sure my problem is a mechanical one in the distributor at this point since I'm getting at least 30 degrees of centrifugal advance.

So with it all hooked up you are at 850 but you are getting detonation while driving?

Retard only works when the throttle is closed, so that shouldn't be related. Advance only at partial throttle, so again probably not an issue unless it pings while cruising.

Plugs look good right? Not too lean? 93 octane?

Sorry wasn't clear. Haven't driven it yet. I need to check for detonation still but don't really expect any. With the vacuum retard attached I'm not surprised the idle sucks. I haven't figured out how to accurately measure retard with my timing light yet, but just by counting fins and comparing to the distance between the TDC and 27 degree advance marks I'd say my timing when vacuum retard is hooked up is about 15 degrees ATDC.

Yep, that is right. Now just plug the retard in, advance the timing until you like the idle speed. That's part of the cool bit with the retard. You can run a bunch of advance without ruining your idle. Just make sure your engine is warm before you play with this stuff (aux air and all that).

I just checked the timing on mine. Still not super happy with my tdc mark but l have 36/37 all in. 27 at 2700 with the advance plugged. Idles good at 850. I think my set the idle by the retard works ok.