swap injector from cyl 3 to another and see if problem moves , possibly sticking open
and too much fuel for idle but would get burned off at high speed , worth a try anyway
also depending on how you stripped and rebuilt could have a damaged or missing injector O ring
thing to remember is if car ran well before then its a man made problem
so just go back over everything , timing always looks a bit out on yb as marks are piss poor
also timing would'nt cause missfire on single cyl so i would scrub that from list
Agreed
The timing sounded out from the way he described the bottom pulley. This need to be checked and sorted before any diagnostics were carried out. Needed a fighting chance e.g. not trying to find a misfire when the timing is out lol. He has said its now improved but with No. 3 cylinder still affected.
Anyway........
I would be checking cylinder specific things now also - plug, lead, injector, injector plug / wiring, No. 3 electrode in dizzy cap for burning / contamination etc.
It does not sound like phase sensor as this would be evident throughout and more pronounced.
Air leak from the inlet is possible but you have also mentioned that you possibly suspect valve contact. Nearly all YB's will have evidence of valve to piston contact. I don't think this is an issue with the compression readings you have.