Hey everyone, please excuse my lack of knowledge on this subject, but I have a question regarding air / fuel mixture adjustment on my 71 1.7 with stock D-jet. I posted this question on Facebook but wanted to try here as well.

Back story: my car will only idle and drive when both vacuum lines are disconnected from the distributor. When connected (I used a diagram found on 914world so I know they aren't flipped) the car barely idles when cold or warm, and it is almost undriveable; it surges, bucks, and hesitates. If I remove both vacuum lines from the distributor it idles and runs great.

From what I understand from the replies on FaceBook, this could be an air/fuel mixture issue, but how is that adjusted with for the 71? Is there a certain procedure for this?

Thanks in Advance for any assistance you can provide!

For those going to PA for the Northeast gathering, I'm looking forward to meeting you!

you have to verify everything is correct before you adjust AFR

it almost sounds like a timing issue or the advance plate not working freely

after checking timing, fuel pressure, valves, etc.
then you will need to read the MPS articles on here and also the brad anders website

buy a wide band O2 sensor and hook it up to a display of some sort... either hand held, gauge, or lap top in the car.

you'll have to clean the epoxy sealant out of of the MPS cap
buy the tool for adjustment from CFR... and a new membrane if you don't pass the vacuum test of the MPS

then there are 3 adjustments in the MPS
this can not be accomplished with a wavetech (on the bench) or an AFR on the road

they adjust 3 ranges of RPM (WOT, etc)

brant

Agree that you should verify fuel pressure, timing and proper valve adjustment and your distributor first.

More info on you car would help. D it run ok at some point? If yes, did you make any changes to anythinl before the issues began?
Do you have the right MPS and ECU for your car? Has anyone tampered with the MPS already (evident by epoxy seal being gone or screws in place of the four rivers holding the two halves together)?

The car is 100% original. My father is the previous and original owner of the car. I got the car in 2014, and until last year (COVID project), it hadn't been started or driven in a decade or longer.

Since acquiring the car I've done basic maintenance: oil change and filter, SS fuel lines, new fuel hoses throughout, cleaned the fuel tank, had the fuel injectors serviced, adjusted the valves, set the timing and dwell. The ECU is original, as are pretty much all the other parts of the engine.

How do I check and set the fuel pressure?

to check fuel pressure you'll need to buy or rent a fuel pressure tester for FI. Your FLAPS will likely have one you can rent. Adjustment is made at the fuel pressure regulator. in the engine compartment. Since your car is 100% stock and was always in your family, I doubt you'd need to adjust the fuel pressure. More likely that you'r filter is clogged, the filter screen in the tank is clogged, or your fuel pump isn't pumping enough fuel.

How old are the fuel lines? If they haven't been changed in several years, I'd probably start there and replace the fuel filter. That may just solve your issues.

All the fuel lines are new, I changed them when I upgraded the tunnel lines to stainless.

After the basics like fuel pressure and timing the distributar needs to be checked internally for free movement.

I'd check the distributor plate is moving freely. Add a couple of drops of oil in the center shaft... sounds more like a retard/advance problem

check the timing for advancement also by watching the light as you rev it hooked up.
make sure you have advance plate movement.

fuel pressure tester at FLAPS is about 15$.... or buy a better gauge on line
number is 29psi running.

Unlikely, but I have to ask since others have made the mistake: did you put the fuel pressure regulator back in the right spot and right orientation after replacing your fuel lines?

So assuming a few things from this post.

1. You have checked the numbers on the distributor and it is the correct one for your 71 1.7.

2. You have a dual advance/retard diaphram on the distributor correct? So one vaccum line goes to manifold vaccum and the other goes to ported vaccum on the Throttle body. Correct? You've checked each side of the diaphram with a vaccum guage tester and both sides hold vaccum?

3. You've checked Jeff Bowlsby's vaccum diagram on his web site and it confirms the components and routing for the vaccum hoses for your car and engine.

4. I'd stay off Facebook for advice for your 914 and get help here.

Check this first and report back.

Have you checked your actual timing with a timing light yet?

I dont recall if 71s 1.7s were plumbed the same but 72-73 1.7s the dizzy vac lines should ultimately retard timing at idle (the advance line should be ported vacuum and have no effect) so pulling your vac lines to the dizzy does 2 thing. 1. Allows slightly more air into the intake, 2. advances your timing at idle (but shouldn't have much effect at WOT)

So while it is possible that the extra advance at idle is masking a fueling issue it seems unlikely. TBH probably 90% of the issues I have encountered with "d-jet" of the last 20 years eventually work out to being some issue with the distributor/timing.

So, as stated by other posters check the basics (timing, fuel pressure, part numbers match, etc) and report back.

What I wouldn't do is start out trying to "fix" the problem by "adjusting" d-jet components. for the most part they are not intended to be adjusted and are either good or bad.

Identify the failed component first.

There is so much truth in that simple one liner.

I did set the timing using a timing light and procedures explained on Pelican. I will double check all parts/part numbers to make sure everything aligns.

Thank you!

another thing that can make the car run rich is a failing Cylinder Head Temp sensor

you can check them with an OHM meter.

pulling if from the engine isn't hard.. involves a modified 13mm deep socket.
and allows you to put the sensor into the freezer or boiling water for the full range testing.

So per the book, timing is set/checked @3500 RPM with the vac lines disconnected.

Which of the 2 hoses kills the idle when it is attached?
Advance or retard?

Are you sure you are checking against the correct mark on the fan? Verify if the mark you are timing against on the fan is 27deg off the flywheel mark in the proper direction (this bit me more than once when i was new to 914s)

Edit: couple more questions
When the distributor hoses are connected and it is barely idling, could you describe the behavior?
Is the idle speed hunting?(indicating lean condition)
Is there a noticeable gas smell to the exhaust? (rich)
Does adjusting the air bypass on the throttle body make any difference?

This would be the next thing I would look at after ruling out any timing/dizzy issues.

Only 2 things really have a significant impact on the mixture, the MPS and the CHT sensor.

Bad MPS generally causes an overly rich condition. (unless there is a parts missmatch)

CHTs can fail multiple ways, broken circuit will cause extreme richening, shorted to ground will cause things to go lean, changes to the resistance curve can cause either.

Which of the two hoses kill the idle? I'd say both of them, but I'll check again tomorrow just to make sure. In regards to timing, I'm pretty sure it's going the right direction. When setting it to the other direction it would barely start or run at all.

It idles extremely low, to the point that the tach sits on zero. With both lines pulled from the distributor it idles at 1200 RPM, but sometimes will "hunt". There is no noticeable gas smell. I have no idea how to adjust the air bypass on the throttle body.

When cold, how many OHMs should be showing? I thought it may be this that is causing the issue.

Thank you for the assistance everyone, I certainly appreciate it! I can repair and replace parts with no problem, but diagnosing problems is definitely my weakness.

Could setting the timing with the vac lines connected cause the poor running when hooked up?

I'm also going to the North East Gathering and trying to sort out a couple issues as well. Looks like I'm less than a hr away from you. I'm no expert but if I can lend a hand with anything I'd be happy to help.

If you can keep your car running long enough to get the engine warmed up you don't have to remove the CHT to test resistance. Just unplug it before measuring.
When the engine is cold it should read ~ 2.8-3 K ohms.
After engine is fully warmed up it should read ~ 191-199 ohms.

Just about all of the FI troubleshooting info you could want can be found here:
https://members.rennlist.com/pbanders/DJetParts.htm

I think if it were timing, wouldn't it be difficult to start? That's one thing, the car starts quicker than my Nissan Altima; as soon as I turn the key it fires up, both hot and cold.

I tested it cold and it was 2.3 OHMS. Still haven't tested it hot

Yes I believe so, It's on the drivers side and attaches to a holder, correct?

Not necessarily. Its pretty surprising just how far off these cars can be on timing and or fuel and still start/run.