John Muir recommends choosing main jet size by reference to the inside of the exhaust tailpipe. If it's sooty black, it's too rich. needs to be run for around 100 miles to check if a different size is appropriate
If its black already I think you're supposed to wipe the soot off first. I think it will always be black if you're running a bit rich to keep the engine cooler. Mine is black and sooty but the engines running well and I can get up to 28mpg so I'm loath to touch it. I used a petrol additive occasionally not sure if its a placebo or actaully keeps the carb cleaner.
i just fitted a new one, an empi from limebug and its running sweat, apparently a revised/improved design ???? I have to say I brought a separate gasket to go between the carb and the manifold and im so glad i did as the one that comes with the carb is poo.
