‘74 1.8L L-Jetronic idle hang… SOLVED!, Thanks everyone! Options

[wonkipop]

EDIT
was thinking a bit more about the test @Van B did.
even though the decel valve hose was clamped off it was probably still able to draw a bit of air given its hard to close off a hose completely when you clamp it (air can still pass through where the hose curves at either end of the clamp).
was probably enough to do the quick drop to 1200 rpm and then go down slowly from there as the valve wasn't closing quick enough. when i was testing my various valves etc a couple of years ago i was taking the hoses off and plugging them with a bolt. when Van had the hose clamped it was taking the improperly adjusted decel valve approx 75% out of the equation? why i was thinking there was something funny about it stopping at 1200 rpm in revs drop.

good thread, i got some info before i screw around and play with re-installing a decel valve. (IMG:style_emoticons/default/beerchug.gif)

Wonkipop, that observation was what unstuck my mind. After I made the post (sometimes just trying to explain it can help you understand it) I realized that the only explanation was that the valve is open at Idle. And something else it made me realize is that the new valve didn’t respond to a throttle blip like the old one did. On the old valve, a light blip would often bring the rpm down.

So, taking the old one and putting the mighty vac on it I started pulling vacuum while blowing into the side tube. And like I said, flow started at 18. Thus, I formed the hypothesis the new valve would be open at less vacuum. Tested, confirmed, 15inhg.

Putting the data together in my mind, I decided 20 was my target value. 15 was way too low. 18 still wasn’t working, but was closer to the edge. 19 seemed like too fine a margin.


Emery, Clay,
What I’m starting to understand about this old school fuel injection design is that it makes all its decisions on air pressure. So, the T in the small vacuum line informs both the fuel pressure regulator and the decel valve; but does the opposite action.
If I do a full throttle pull to 5000rpm and chop throttle or power shift, the rapid change in manifold pressure will simultaneously cut fuel and add air. That makes sense to me since the old school AFM has a barn door that must “swing” closed. During that swing fueling doesn’t immediately stop because the computer only understands to fuel based on how open the door is.

These two little devices add air to burn that uninvited fuel and shut the regulator to prevent a runaway fuel situation. Simple but still clever.

-Van

yep - how it works. manifold pressure affects fuel pressure with L jet.
part of the fuel pressure regulator test for L jet is to test with the small vac hose off at idle. should be approx 35ilbs fp.
then put the vac line back on, fuel pressure should drop to 28ilbs at idle.
rises again to full pressure with the throttle open and engine powering.

[914werke]

Resurrection. (IMG:style_emoticons/default/rolleyes.gif)
It seems as though much of this troubleshooting (& solution) was based on the use of a earlier adjustable Decel valve.
The later motors (depicted in the vac line diagram) use an larger non adjustable valve.
Aside from Clay's warning what are the detrimental effect of removing it from system?
One of the things I preferred about D-Jet was the snappier "feel" that system provided compared to the smoother more progressive response that you get from L-jet?

[914werke]

I'm not sure how much you are a stickler for originality, but I recently ditched my throttle body and replaced it with a Gowesty throttle body. Some modifications needed, but man what a difference it made.

@timothy_nd28
Im curious of the detail on how you plumbed the Gowesty unit. It comes with only one vac. port? I already created a TPS mounting plate

(IMG:style_emoticons/default/idea.gif)

[Van B]

Resurrection. (IMG:style_emoticons/default/rolleyes.gif)
It seems as though much of this troubleshooting (& solution) was based on the use of a earlier adjustable Decel valve.
The later motors (depicted in the vac line diagram) use an larger non adjustable valve.
Aside from Clay's warning what are the detrimental effect of removing it from system?
One of the things I preferred about D-Jet was the snappier "feel" that system provided compared to the smoother more progressive response that you get from L-jet?

75 uses the big can. The 74’s use the standard small one.
I think my car has a pretty snappy feel now.

The setting I’m using on the decel valve works pretty well for my taste. It makes the decel valve more subtle in its function. It will still catch the revs, but they drop more normal (quicker).

[porschetub]

Awesome. Is it necessary on an ljet?

Don't remove parts !!! ,they are part of an engineered system which was developed by Bosch for the type of injection used,Porsche never add stuff that is surplus and the supplied fuel management is what you need.
IMO the L-jet is a great system,pretty simple and stands upgrades like bigger B+P's ,bigger throttle body,good tuning exhaust system,the cam used has to remain fairly mild as the afm will give issues.
My next build is a 96mm bore FI block will be running this system as a customer wants an uprated motor for his bus.
My 2 go to guys for info on L-jet are mark henry and clay cause they exceed more experience on T4 application than I have learnt many years back on this system.
Cheers.