See also my two bits in MPH. The float level alters more than one might imagine if the carb isn't set at the correct angle. One degree of rotation means a 1/16" alteration in float level. The correct angle is not, as someone has suggested, vertical, but whatever angle is necessary (obtained by rotating the carb on the inlet stub) to get the float level JUST below the top edge of the bottom nut, the one that screws on to the mixing chamber.
What ought to work, but for me didn't, was to raise the float level until petrol seeped out of the hole the bottom nut nearly conceals, then to drop it until it didn't. That is however the float level the carb is designed to function at.
I'd like to claim that this insight came from sheer intellectual horsepower, but it came in fact from Sid Biberman, who gave me the clue about float level setting being significant, and Burlens (who make them) telling me what it ought to be. (They were also able to tell me what the threads were, when I needed to make a mount for an air filter on another bike.)
It needs also to be said that before I set them my bike ran OK, and the float levels were 1/2" apart. It did however run, although not as well as it does now they are 1) the same and 2) correct.
For information, my 289's are running 180 main jets (down, in stages, from 210's), #4 slides, and the needle in the middle slot. The plugs, NGK 6's, are sooty when I pull them after stopping normally (i.e. not a plug chop) but white underneath. Since race bikes are run so the plugs are "dark white", and I can ride the Vin 200 miles at 60-80 mph with no problems, I think they're about right.