I'm no Djet guru, but I not a newbie and this is an unknown heritage hydraulic cam 2.0 GA engine. Engine was not running on arrival, it initially had a valve train issue (no rocker spacers) that's been fixed. We have replaced all the fuel lines, many of the vacuum hoses, clutch, cables...lots of work, big list.

74 Djet not being nice, suspect vacuum leak.

-high idle, 1500 rpm
-it stumbles, cuts in/out under 3000 rpm,
-runs real good over 3000 rpm, in fact super good power!
-backfires on deceleration.
-has no decel valve

It was running worse, like running on 3 cylinders, Ethan re-plugged all the connections, plug wires, etc and now it's a champ over 3000. But under 3K is different like the whole ignition or FI stumbling not just dropping one cylinder.
We going through all the hose connections again looking for a vacuum leak
If important I can likely find a decel valve in my pile.
I'm going to recommend that new Bowlsby harnesses are in order, most of the boots are ratty.

Any other suggestions?

Check that MPS

check TPS board , i had this where contacts were dirty and did that like rally bad stumble, thought the grounds for fi were loose but tps was the issue

but recheck the mps , and that will cause lean running and high idle as well, OR vac leak

Cool I have a couple of MPS and TPS units to swap out.
I should have thought of that, thanks.

What are you using for a dizzy?


Edit:

You need to run the Decel valve...first order of business.

Dizzy would be my next quess.

In addition to the above, lube the dizzy weights, and then again annually. 1 drop of 30 weight in the center of the dizzy shaft.

Stock, looks like a pertronix on it.

Man, years of performance engines and building stock longblocks for others I dont have a huge amount of time on troùble shooting the stock FI engines. If anything I have more time on Ljet systems.

This year so far 3 restorations of 2.0 engines and we may even put a 914 2.0 turnkey engine in stock.

It's probably the D-cel valve with those symptoms Mark.

I'm not a big fan of the Pertronix dizzy for D-jet. I know its expensive but I'll tell the dip switch 123 are the ticket for dizzy's and D-jet. Set and forget.

You might want to check whether the screws holding the Pertronix to the dizzy plate are too long and hanging up the back and forth movement as the vacuum changes.

replace the pertronix with points and condenser and check the dwell and set ot 50 then reset the timing at 3500 to 27 btdc. Also make sure that you have proper valve clearance on the hydraulics (like just under none) and that the compression is even and that the intake runners are well sealed to the heads

Do what George said.
Then unplug everything from the intake plenum and block it off with tape.
Leave just the MPs connected to vacuum.
It should purr at idle.

That is an unusual combination of symptoms, I suspect you may be looking at more than one issue.

Given it sounds like you are dealing with a lot of unknowns I would first recommend checking all the part numbers to ensure you are not running a mismatched system

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

If you poke around on Brads site there he has testing information for the individual components as well which is also good to check.

In my experience MPS failures have always resulted in a rich condition and usually at a level where it is very obvious you are rich. MPS parts mismatch or an MPS that has been tampered with however can cause any number of issues.

Decel valve, sort term you can get by without it just fine, wont have any effect how the injection is running provided all the lines have been properly plugged.


Assuming you already checked the timing...

Check the part number on the dizzy. 1.7s dizzys are generally easier to come by than 2.0 dizzys however the 1.7 dizzy has a more aggressive advance curve and when used on the 2.0 tends to over advance at idle causing it to stay high. Last d-jet car I built I had so many issues finding a good dizzy I just gave up and went with the 123ignition dizzy.

The other thing often overlooked as part of the d-jet system but i have been finding to be more and more of a problem lately is the PCV on the oil tower that was used on the 72-74 cars. Pull the vacuum line to that and plug it off and see what impact it has on the idle speed.

Actually, re-reading all your symptoms....

Test that the MPS holds proper Vacuum before anything else.

While the missing decel valve wont affect how it runs the fact that it isnt there increases the chances that the MPS diaphragm has cracked.

Cracked MPS diaphram = small air leak to the engine = high idle
Cracked MPS diaphram = MPS not sensing load properly = rich running that is worse at lower RPMs/load.

backfiring on decel, provided this is popping out the exhaust, also points to an overly rich condition under light/no load situations.


A vacuum leak alone on a d-jet system wont affect the mixture as its essentially the same thing as just cracking the throttle.

https://jetronic.org/en/d-jetronic

As I said power for a 2.0 is great I don't realy expect any mechanical issue. The valve train we just set up, as it was missing the solid spacers on one side. I cut new spacers on my lathe, hydraulic cams reqire solid rocker spacers and steel pushrods.
I have most of a djet system including a pineapple rebuilt by tangerine, I'll swap out some parts and see if I get lucky.

The customer already has a huge bill, I'll try points and a different Djet dizzy first.
Looking for a decel valve.




Will do.




Thanks.

Watching!!!

Didn't work on today, too freaking cold even for test driving.

Agree with everything JamesM said and others who suggested the MPS. Test it first.

The TPS could be causing a stumble, but not the backfiring. Stumbling and backfire are likely tied to the same issue, which if you can analyze the AFR I bet you'd find it's running rich and likely because the MPS leaks, isn't adjusted properly, or there's an MPS/ECU mismatch.

Don’t forget the cheap one and clean and regrease the distributor advance plates....that can also cause perfect above 3k and weird below 3k depending upon where the plates get stuck each time.

Rich

No luck on the decel valve hunt so far, but I did find a good looking '74 2.0 ECU/harness.
Still have to some more sheds to look in, but it's -4F (-20C) outside.



I have a half dozen pineapples, one is a rebuilt by Chris, another is a rebuild from a known US outfit, I forget the name right now. I'll test them all.



Will do.