MK2 Jetta GL 4+E warm idle issues

[mr2jay]

Hey Guys, I hope you can help me with this as it is doing my nut 

Got a 1990 VW Jetta 1.8 GL 4+E which has the Pierburg carb fitted. Before anyone says "Fit a Webber", I don't have the funds for that 

Right, the issue - fairly recently she has started getting idle issues. On start up, she sits just over 1100rpm but once she is warm, she drops right down to about 400rpm. Now, as you can imagine, this creates a problem with running.

So far I have replaced all of the vacuum hoses with new silicone ones (So we can rule out a vac leak) but it is still happening.

I have read so many different posts about these carbs and I am confused as to what the unit on the left of the carb that has the idle adjust bolt (13mm) actually does. I think it is the 3/4 point unit? Mine touches the threaded screw on the accelerator flap when off, and sucks right in when warm. Can't see how the adjustment bolt would help so haven't touched it.

Help guys, I am getting confused on this

[mr2jay]

Right - small update....

Could this be connected to an issue with the accelerator pedal?

Drove it this morning and she idled at about 600rpm - still too low but at least it wasn't stalling.

One thing I DID notice though is the the accelerator sometimes goes PAST the stop on the pedal when coming off the gas....

Could this be effecting things?

[Santiago]

The position of the accelerator pedal when your foot's not on it should be dependent only on the throttle return spring on the carb, plus anything that prevents the primary throttle closing fully onto its stop.  The only things that ought to be doing that are the waxstat mechanism during warm-up, and the 3-point unit plunger before start-up and after warm-up (after the waxstat mech lets go).

If your 3-point unit plunger really is (piccy would help) retracting back to much less than 8mm say, something is wrong, probably with the power to the overrun/cutoff solenoid valve on the back of the carb.  See if you can measure 12V between the pins of its connector with engine not running but ignition on.

I can offer you a full explanation of the 3-point unit's function but it's rather long and involved, and probably not required.   Suffice it to say, there should be a state (fully warmed-up engine at idle) where the plunger is 'halfway out' and the screw resting on it, which should give you a steady idle speed around 900, adjustable via the 13mm thing on the back of the 3-point unit.