Thought it was about time I updated on the thread...Still no absolute solution, BUT the twin runs ok when not on pilot - still seems to tick over periodically on 1.5 cylinders 'till I pick it up on the throttle (have played with pilot air setting and throttle stop according to KTB). But what I HAVE found is that if I remove the pilot jet from time to time the hole in the jet seems to be blocked with a small,spongy blob the size of a pinhead - a bit like Evostik that has gone off in petrol. Now I had my tank fully refurb'd....tank sealant? I also took out a petrol tap and a larger blob of the stuff was wrapped around the top of the thread.This is my current thinking so I am hoping that the passage of time will eventually stop disolving off the inside of the tank which I know was coverd with seal during the refurb.!
I think you have probably solved the problem yourself,
I've had similar in the past and all down to minute bits of "gunge" blocking small fuel-ways, however I'm not sure about the "dissolving off", you refer to, as all the petrol sealing compounds I've ever used and there have some different makes, in different bikes, all needed to "cure" before adding petrol, the times varied, by ambient tmperature and actual application, I would not have expected any "bits falling off" afterwards, I wonder if there's something there worth looking at, as such small items can be major problems, untill removed.
Anyway, I've been running Mk1's on my outfit for fifteen years quite happily, without any major problem, although I did fit K&N filters at the original rebuild, I still think it was a wise choice, as there is very little wear on the slides, compared to the ones I replaced,
go carefully.