I finally got around to fitting the new float bowls with StayUp floats and aluminium needles today. Pressure of time meant that I didn't do any comparisons before putting them on, sorry. Anyway, fuel levels look to be around the bottom of the specified range. But in this instance the proof of the pudding is in the riding (?!), and the bike's very happy, so I'm very happy. For now, I think if anyone has an experience like mine, and finds themselves getting nowhere, they could do much worse than contact Burlen for a returns number, and go from there. As things stand, their customer service for me has been stellar. What I might have been doing wrong, and what Burlen have done right, I have no idea! But if I don't have to go in there again for a long time I shan't mind. Fingers crossed that things stay as good as they are now.
Also, while working on the carbs, I found that one of the carb/manifold nuts was slightly loose – you might recall I tested for air leaks due to minor popping on the overrun, but didn't find any. So I've taken that carb and manifold right off, cleaned up all the joints and re-assembled: blue LocTite on the carb/manifold studs, the nuts done up just tight enough to compress the O-ring and flatten the spring washers, I hope that meets with general approval. And the popping's gone! So I must've missed an air leak, hey-ho! At least I can leave the throttle needles alone! Pending the results of a plug chop, maybe ... Once warm, the tickover seems pretty steady at around 1,000rpm, which'll do fine for me, but since stopping the air leak I'll want to have another play with that. Cheers.