1983 mk1 running issues

Hi everyone, I'm in need of some help as I've spent so much time and money on my beloved mk1 and I keep hitting brick walls. The problem I have is to get the car to run perfect I have to loosen the main fuel feed on the metering unit all the time, if I leave it loose with fuel pumping everywhere it runs and revs up lovely, as soon as I tighten the nut it runs terrible and when u rev it up there's a gap before it picks up and when it's driven it fails to pick up or run anywhere near how it should. I've looked inside the tank and it looks clean, I do know I need a new filer neck as the breather has rusted off, I've changed the fuel pump, metering unit and filter but no different, can anybody point me in rite direction plz? 

ideally next step is get your fuel pressures checked, both system and control.

if it runs better with the system pressure artificially lowered (pipe cracked loose) it could be incorrectly adjusted/sticky air flap, pressure regulator fault (if same on 2 metering heads probably not) fuel return to tank blocked, fuel lines to WUR blocked, faulty fuel injector(s), mesh on WUR fuel inlet blocked, or a faulty WUR

have a look thru this thread, ignore the 16v specific bits just concentrate on the basic kjet tests. also with ignition timing this is done at idle with dizzy vac pipe disconnected and plugged:
http://www.vwgolfmk2.co.uk/forum/viewtopic.php?f=37&t=25972

Hi jonny, thanks for your message, I'll check out all that u advised and hopefully one of those is the fault, thanks again. Paul