Having that much old fuel in the tank (turned out to be about 4l) was a massive oversight given I only put a little fresh in - so I was convinced that would be the issue. But alas after dropping the tank, draining the fuel line and hooking it up to a fresh fuel source, it's made no difference. I do recall at least smelling the tank before I started and it was fine, plus the old fuel had a very red (not brown), tinge - so I think it must have had stabiliser in it.
So I'm back where I started, full revs with choke is no issue, but I can't past a quarter without. It feels like it's getting plenty of fuel without choke though, as it sprays unburnt fuel forward back out the carb as it starts to splutter and die? Fuel screw works at idle, too lean and it stumbles, then it stops having an effect too far the other way, but won't rev at all if too rich. It still doesn't start too well without choke though.
The float valve closes about parallel with the mating service, but upside down IIRC it rests beyond parallel, there's a little slack in float hinge. I will snip the plunger, but that would likely be affecting it with choke as well.
I don't have an ultrasonic cleaner but I think my last option is too boil the carb, maybe it will move something if that's the issue. As far as I can ascertain, it is the OEM carb for that engine, it's exactly as
@Forest#2 's diagram on the link above.
I did accidentally leave screw and washer 955 off the carb a couple days ago and started it up. Needless to say there was a massive vaccuum leak and it ran very badly - but it seemed like the same problem but worse. So am wondering if there somehow could be a vaccuum leak somewhere - more testing there. I should add, I'm doing this without the air filter fitted, but it would be a long shot to say that's leaning it out too much?