Just back from the Netherlands. Lifted the top off the carb. It looked like the float was stuck in the chamber at an awkward angle, so maybe it was jammed, or it was the small flap as Alistair suggested. Hard to tell. Anyhow, lifted the float out to find a whole lot of black sludge at the bottom of the float chamber! No idea as to how this has happened as all the lines are new, and there are two filters in the line.
Cleaned it all out, and it now runs fine. I’ll keep an eye on that float chamber.
