Erratic Idle Issue

Bought a ’87 1.8 GTI cabriolet DX engine with K-jet, not started in eight yrs and is now experiencing erratic idle.

  https://www.youtube.com/watch?v=XhtMLkI97xU

Haynes manual suggests to me that the air metering disc is not seated in the correct position and requires adjustment, it mentions bending a spring but wary of attempting as these units are expensive to replace. Can anyone help? When pulling up on the disc (plunger feels smooth and not seized)  while engine runs she revs fine, but left to return back to its rested position engine revs up and down then for a min then stalls. Difficult to restart, but will start.
I have started a checklist of basics to check: Fuel is fresh/sparks changed/compression test on head gasket passed ok/cold start injector sprays fine/fuel injectors swapped and new o rings fitted…that is as far as i have got. As soon as i noticed the issue with metering disc i thought best i try and rectify that before going any further checking for manifold leaks as the problem could be right there. Any help greatly appreciated
see attached pics

I have limited experience and this is unlikely but i had a similar issue with bouncing revs when idling that stalled the engine on my 1.8DX GTI and that turned out to be the warm up regulator wire had come loose - but unlike yours it ran fine for 30-40 seconds then started bouncing until stalling.

Good luck tracking your issue down  

**Update**
Okay, so I widened the spring clip so the air metering disc sat in the perfect resting position.
I turned the dizzy fully anti-clockwise, as it seems to fire well at that point.
I removed the thermostat and flushed/reverse flushed the cooling system and have refilled with fresh.
Started her and she ran but it felt gutless and was backfiring back into the K-jet.
Using a 3mm allen key I adjusted the co2 clockwise a full 360 from where it was originally and then she ran fine but was idling at about 1800rpm.
Way to high and it sounded like the tappets were screaming.
Further fettling to the idle means she is now running at 1k and all seems well, no backfiring and power is responsive.Tappet noise still present.
There's old but clean oil in there and  I checked oil pressure and at 2k rpm it was 2.5 bar so that's fine.

My two concerns now are with the noisy tappets…would fresh oil help? or is it rocket cover off and check clearances? I thought these were hydraulic self adjusting?
And with the co2 adjuster on the K-Jet…What I have done seems to have worked but is there a default setting for the allen screw as I am concerned It took a full revolution to get the mix right..should I be concerned? As I understand it,it will be messed with when it goes to the garage for the emmissions test but I'd like to be close to passing!
Really appreciate any advice .

start again.

double check the timing belt is fitted right as per markers in this thread, ignore the digifant setup sections:
https://clubgti.com/forums/index.php?threads/how-to-diagnose-and-tune-your-mk2-digifant-gti-8v.124949/

start the engine and get it up to temperature, disconnect and plug the vacuum line to the distributor and set timing to 6 degrees BTDC. you can use a basic timing light with the diamond marker on the flywheel as per picture in my thread above.

you can check basic kjet system using the pointers in my 16v guide, obviously ignore all the fancy 16v bits:
https://www.clubgti.com/forums/index.php?threads/how-to-check-repair-and-tune-your-mk2-corrado-1-8-16v.195423/

theres a section in there on setting up the co, this also works on yours but its done with everything connected, no special wires to disconnect or anything on a DX. idle is set to around 800rpm when hot, you'll need to fiddle with this as you mess about with the co most likely. if you have pro co equipment then you disconnect the breather if the oil is really old and contaminated. if its fresh you should be alright, aim for 1.8-2% or wherever its happy.

once running nice you can start to dial in a bit more advance, 8 degrees is usually fine if not more if engine is healthy and you're using good fuel.

wow that is a lot to take in

Robjonny - Thank you I do appreciate your advice..I shall try that and post an update shorly