-Update -

I removed the TPS, calibrated it as per instructions on the Bird. Here are some pictures.
I would like to be able to "RING" this out, not sure what I would be looking for. Though we know that one course is to install a new board and then calibrate. There is little else I could find to assure operation. Any experience with asserting these things are good.

Some pics
Click to view attachment
Click to view attachment

Any ideas would be great!

-End of Update-


I am a bit befuddled as to which way to go so I decided that the 914W team would like to hear about this puzzle.

Background
'74 2.0, garaged from '84 until 2018, 22K miles
Running since June.
Repaired or replaced:
Entire Fuel System - tank, lines, injectors, pump, pressure reg
Adjusted the valves
Removed and rang-out the engine harness,
Rebuilt the MPS
Rebuilt the Plenum and tested it for leaks - none observed
New gaskets at the plenum and manifold at the engine
Rebuilt Throttle Body, very minor leaks around the actuator (throttle), New circuit card
New Plugs, points, cap and inspected the distributor for all of the regular ailments - super tight.

Engine idles great, pulls like a monster - sounds like a German symphony

HOWEVER
Last night I drove it back from the paint shop on 93 heading south from NH to Boston (lots of beeps from newer Porsches ) and found that the car started to buck and the engine runs rough when just a small amount of throttle is applied.

More Detail

If you are doing 55 and want to maintain that speed, it is necessary to apply a small amount of gas to sustain the speed. This is when the cars starts to exhibit this "miss/run-rough" puzzle. If more throttle is applied the bucking stops, the engine is smooth and all is well. Conversely, while de-accelerating there is no bucking.

This problem is NOT velocity-dependent, sustaining at 30/40/50/60 will exhibit the same reaction.


WHERE WOULD YOU START TO LOOK IF YOU HAD THIS CHALLENGE?

Click to view attachment

kinda sounds like a TPS issue

the connectors become very brittle
did you calibrate the TPS during rebuild?
maybe check the wiring again to the TPS

Excellent idea!

Yes, I did calibrate and cleaned (rebuilt) with a 914Rubber circuit board. I will double check that it is a 2.0 and not a 1.7 and the other errors I have read about.

Thanks Brant!

Verify fuel pressure
Make sure MPS is calibrated for your engine. If it was rebuilt (even if it matches factory stock #s) it likely needs calibration.

Will do thanks!

I would start with thoroughly checking the ignition system, as a more likely suspect to eliminate first. Had a similar intermittant problem that was due to points wire pinched under cap...

As a quick check, isn't it possible to disconnect the TPS and run it like that just to see if the bucking stops?

Will be checking the throttle switch (TPS) today,
Completely slipped my mind that Pbanders was all over this,

https://members.rennlist.com/pbanders/DJetParts.htm

Also, there is an over analysis of same at:

https://www.sw-em.com/bosch_d-jetronic_TPS_investigation.htm

Where the writer refers to the symptom as MaC or Miss-at-Cruise

More to come
CTC911CTC

******UPDATE******

Started the car
Disconnected the TPS
Drove the car

The hesitation was MUCH worse, the bucking was about the same.

Disconnecting the TPS made the entire situation worse though the drive was just around the block.

As per PBanders:

Track Wear: Over time, the wiper track for the accelerator function will wear. Wear will be especially high at moderate to light throttle angles, corresponding to part-load cruising. Click here for a link to a 60X photo of accelerator track wear. While this TPS track is still good, the re-deposition of gold worn from the contact fingers by the wiper can be seen, and eventually will become sufficient to bridge the traces. This wear causes arcing and poor contact, resulting in the car "bucking" at a constant throttle angle. "Bucking" is a fairly common complaint and is almost always due to track wear. Check by disconnecting the harness plug to the throttle switch and driving at a constant throttle angle under part-load. If the bucking is gone, it's due to the switch. As I mentioned earlier, if you go to http://www.914world.com/ and search for user "davesprinkle", he's made a kit to replace the worn circuit board that restores your TPS to like-new condition.

I have a new card from 914Rubber in the car and I am pretty certain the card wipers are connecting to the card correctly. I will take a closer look at this tomorrow.

I will also retest tomorrow (too much rain today for a highway check) to make certain that the bucking is gone even though the acceleration is very poor. The barely accelerates until I hold the peddle about 1/2 way for a few seconds and then the engine starts to run strong after a prolonged response to the peddle.

Any ideas appreciated.

Thank you!

Looks like a lot of wear on those traces, to my eye.

Clean them with an eraser and spray the traces with DeOxit spray.

You can test the continuity between each circuit with an ohmmeter to verify that the TPS is doing its job.

I've experienced similar symptoms when the MPS was adjusted a bit too lean. At some areas in the fuel map it gets lean enough to cause slight misfire.

+2 on that board. I'd probably replace it. Also +1 on the MPS... or kinked fuel line.

Though the card is new, I might get the 45-year-old card back and put it in ..........curious how worn the new one looks, even though it only has 60+ miles on it - all of the wipers are scratching at the same rate - does not look too bad with the naked eye.

As to Fuel pressure - would this cause the bucking at maintaining speed but not when accelerating?

I will start the debug list - my next stop after verification of pressure will be ignition.

Thank you Teeners!

CTC911CTC

You did a test which seemingly exonerates the TPS:



A main function of the TPS is to give the injectors extra pulses for acceleration so hesitation would be expected.

But w/o the TPS in the circuit, the bucking was about the same. This would seem to eliminate the TPS from being the culprit.

I agree.. seems like something else is causing the bucking
I have also always understood that unplugging the TPS would test the trace circuits
and I have experienced this test working on my car.

I would follow Chris' advice.
testing the MPS, the CHT, and wiring loom would be my next move....

was there any work done to the car right before the problem started?
I always like to start sleuthing with the last thing touched

What changed right before detecting the bucking? Really nothing.

The car had not been driven since 1986. We got it working but could not drive it since I did not have a clear title - after 6 months of working this, I was able to get the gov here in Mass to provide a title. The first stop was the paint shop, on the way there I took side roads, all stop and go. Since I was not sure as to the car's operation I did the shakedown on side roads, no faster than 35-40.

On the way back I took the Interstate and cruised between 55 and 65. The car ran great until I was trying to maintain a constant speed and then the bucking would start.

Will check next:
grounds
Fuel Pressure
Wiring to the ECU from the TPS (have done this before)
Trigger points and operation
Ignition system (complete)

Last, I found the Tech Tips 700 book very helpful - thank you George!

Thank you Brant!

Over there we 5 years did you flush the tank and replace the lines???

Someone suggested restricted fuel lines or a plugged fuel filter
Those highway miles would require more fuel and hypothetically could highlight a fuel flow (lean) problem

Also check the CHT sender.

For overall operation of DJET, you should check the AFR although this requires installing an oxygen sensor in the exhaust. I did this on my 1.8L DJET when I first got it running. Turns out it was running very lean and I had to adjust the MPS.

I got a Jeff Bowsley FI harness. Solve a lot gremlins including bucking.

Good point Jerry, that CHT caused me a lot of headaches when I first started the engine - swapped it out (it was stone dead) and my richness trouble subsided.

Will check the AFR once I get the bucking sorted.

On to the bucking gremlin!

Thank You!

I wrote this precisely because of the bucking you are experiencing.
It is my contention that all you need to do is enrich the MPS setting a small amount to eliminate the misfire which results in bucking during light throttle cruise.
I bet you would also notice an increase in power at WOT.
Easy peasy.

If your MPS is not set up for in situ adjustability you can still open it up to make a change, and reassemble it in half an hour.

Chris!

The muffler you sold me works and sounds great!

Great advice, I rebuilt the MPS as per your instructions and developed a method of 'relative relationships' to put it back together. Using the H method I was within a few percents of calibration.

I am going to check all of the basics and then PRIOR to going deeper will make this adjustment.

Question - if the ECU adjustment is turned to the RIGHT a click or two would this accomplish the same thing?

Thank You, Tangerine Chris!

The ECU knob only affects the mixture at idle and not the mixture of the motor under load or at other rpm

Brant is correct. And because the injection pulse is so short at idle the range of the ECU knob is quite narrow. In my experience when the MPS setting is near optimal the effect of the ECU knob will be more noticeable, and idle will be clearly better away from either extreme position.
I suggest that you back out the center screw in the MPS diaphragm by a turn and a half. Record the inductance values for comparison then install it in the car for a test drive.

Great advice Chris - Will DO!!!!!

*************Update**********

The bucking has Stopped! And the car runs great!

Chris,

You were correct, the MPS that I rebuilt must not be calibrated correctly (I used the H version of measurement and adjustment) since I swapped the one I rebuilt and cleaned and looks like new with the one I bought through eBay which is old and ugly but holds vacuum.

The old one installed stops the bucking, so as soon as I get my bench rebuilt I will put the one that I repaired back on the bench and get it tuned correctly.

THANK YOU CHRIS!

Glad you got it running well again!

And no disrespect to Chris, but I suggested that the MPS needed calibration several posts earlier.

OOOooooopppppsss,

Sorry!

My mistake

UPDATE!


Bucking is back - very slight - I am going to adjust the MPS again as I had within this thread.