Re-read the "melt the PVC" thread. The purpose of melting the tube is so that you can taper it, thus enabling you to screw the tubing into the float bowl drain port (after removing the drain screw of course
). You then hold the other end of the tube up, above the carb (in a sort of J shape), and turn on the fuel. Fuel will rise in the tube and stop at the same level as the fuel inside the float bowl. The fuel level should be just below the float bowl mating surface.
Note that this is just a "spot check" procedure, if you need to adjust the level you'll need to pull the bowl and set it with a gauge or other measuring device. Actually adjusting the float level pretty much means you'd have to pull the carbs off the bike, but the spot check can be done without dismounting the rack.
Finally, if you have the sort of carbs that drain through the overflow spigots when you loosen the drain screw, no need to taper the tubing, just hook up to the overflow spigots and proceed as above.
cheers
mystic_1