I don’t think I can check fault codes on a mk1, no OBD2 reader?
I do have an AFR and it does seem to run ok although to be honest I’m not really sure what I should be looking for. It does hunt about a little but really not nearly as bad as some I’ve seen.
Fuel filter/fuel pump is an attractive target as it only happens when car is at full throttle.
How would I go about measuring fuel pressure in the system? Do I need some special kit?
Possibly this is a stupid question but I’m not sure where the MAF would be on a turbo setup. Would it be before or after the Intercooler? It seems like at least cleaning this out would be a cheap next step.
You’ll most likely have an IAT instead of a maf with the turbo kit, if you still had it it’d be on the intake pipe to the turbo after the filter just like the standard setup
You can get a cheap inline pressure guage off Amazon and stick it on somewhere
Also if your afr is jumping around at idle it may need a better earth, I had to move the earth on mines as the reading kept jumping up and down, then once it’s stable you need to adjust the values on the table so that it matches what it shows on the laptop if they’re different but stable
Fuel filter is a nice cheap and easy check and if it turns out not to be that at least you’ve got a new filter and it’s only cost you about a fiver and 10 minutes work
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You can use an led in the diagnostic port but I’ve never done it myself
Yes there’s an LED kit you can get we have used it on ours.
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Thank you, I will definitely invest in one of these and have a play about with it, I had no idea there was such a thing for the mk1. Just when I think I am starting the get to grips with something I realise I still have so much still to learn…
Thank you to everyone that said fuel filter!!!
Changed it over the weekend and then started the car up. Run rough for a few minutes, initially on three cylinders I think and then all smoothed out.
Went out for a run and after a quick thrash I can confirm no more misfire! So relived.
There is still much to do including sorting out the fact that the AFR is still showing very rich, hunting around 11.3-11.6. I need to get out with a mate of mine and a laptop to have a look at that.
Then the MOT is due at the end of January. 
I’m fairly confident that the offside rear arch will need some welding and I’m not convinced that the air vent indicators will pass either. That’s before I think about the fact it is decat…
So many worries, so little time.
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Great that its running smooth again.
Shouldn’t be any issues with the decat just make sure they do the correct emissions test, ours has been ‘decated’ and is the same age as yours and no issues last year. We also adjusted the AFM as we were getting some high readings, we have a K&N on the end of it which I think can affect it.
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You’re welcome, glad it worked 
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Thanks, so am I asking for an emissions test for a specific year or for a decat car? Should I do a pre-mot check, I do think that the car will need some welding on the near side as well, there is a hole through the wheel arch.
Someone has told me this is an MOT fail?
Youre welcome
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I can’t comment on the welding, perhaps post some pictures and someone on here will know? In terms of the test, yes the emissions test is different for older cars you just need to make sure that they do the correct test otherwise the car will fail.
“3
For all petrol engined vehicles first used before 1 August 1992, all gas engined
vehicles and vehicles other than passenger cars first used before 1 August 1994 the
test procedure consists of a metered check at the normal idle speed, the so- called
`non-catalyst’ test.”
from this doc
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FML.
The car is still misfiring.
The first run it wasn’t misfiring but could only do two proper pulls and I so desperately wanted this to be the fix.
The misfire is not as bad as it was but is still there. The fact that the misfire has improved lads me to think that the issue is likely to be fuel related. Maybe I need to change the fuel pump? Thoughts?
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Thank you very much for this, very interesting read, gives me a bit more hope!
Further update:
Over Christmas I changed both intake filters and relocated the breather away from the ‘hot side’.
Both filters really were in a very sorry state with the ram air turbo filter basically falling apart and the breather filter basically wasn’t filtering at all as I realised the existing filter had been burnt by heat from the turbo. Probably won’t make any difference to the performance at all but makes me worry less about crud going through into the engine.
Feels good to get the big K&N filter into the car finally, I’ve been hanging onto it for about 6 months, originally bought it to fit to my mk2 but never got around to it.
Before
After
However not done anything else about misfire other than pass the car on to a tuner last weekend. Waiting to hear back from them on feedback and see if the can map out the rough start up as well.
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I’m sorry that I am not able to contribute technically to this thread but I have to say I am really enjoying it and just want to say much kudos to you for doing it and posting about it.
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It’s back! Over a month later but my little money pit is back.
The misfire issue apparently was to do with how rich the car was running so the injectors were at capacity. Jake at Boost Labs has upped the psi on the turbo to about 10 which he assures me is comfortably within safe limits for stock internals and has leaned out the map slightly.
That’s the good news.
The bad news is that I also asked him to give the car a health check and an MOT. The MOT check was fine but the health check threw a few bits up.
Suspension is not original but lowered a long time ago and basically everything is a bit knackered, really needs new springs but might as well swap to coil overs.
New bushes etc all needed.
Starter motor on the way out.
One of the belt tensioners is on the way out. Now he has mentioned it I can’t unhear the faint squeak. 
So I’ve paid £445 for a new MOT, an engine that gets less fuel and more boost and a shopping list! 
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Your baby is clearly living up to its name! Good to hear it’s otherwise healthy.
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Yup, I’m looking at about another £1500 I think.
£500 for Meister R coil overs.
£160 for starter motor
£? For tensioner, not sure which one but think it is alternator belt. Probably £100-150
£580 for full bushes kit.
£100 for realignment once I’ve changed all that.
Thoughts on upgrading the anti roll bars at the same time as bushes? Not convinced how much difference it will make?