How to set a Keihin PWK-type carb float
To begin, the float height is 19mm for both the 24mm and 28/30 PWK carb:
Measure the height from the bottom of the float to carburetor-body gasket surface. When checking the float height, the float should be resting, but not depressing, the spring-loaded float valve pin. This can be done by tilting the carburetor until the float tab just makes contact with the valve pin. If adjustment is needed, bend the metal tab on the float arm until correct height is obtained.
If you open the bottom of the carb, turn the carb over first. If you simply pull the bottom off straight down you will see that it catches because the float is hanging down. If you force it off, you can bend the tab on the float and then the shutoff valve won't work properly and the carb will overflow all the time. Turn the carb over so the float is at the top before opening the bottom and do it gently.
If the carb is overflowing all the time, check that the shutoff valve and float tab are correctly aligned. With the float installed in the carb, and the carb upside down, the shutoff valve should just sit on the tab, ready to shutoff the gas flow. If it doesn't touch, then the float will never rise enough to shut off the flow of gas.
To reassemble, with the carb upside down again, carefully put the cover back on. Before putting the carb back on the scooter, shake the carb and make sure you can hear the float rattling inside. If no rattle, the float is locked in place and you have not put it together right. Open and fix it.
Also, a customer writes in with a tip:
What caused the overflowing was the float binding on the overflow tube. There is not much clearance between the float and the overflow tube. If any difficulty is experienced removing the float bowl, there is a good chance that the float may be bent towards the tube, and, as a result, binding on the overflow tube. Having set the float level to 19mm, and squaring up the floats, the problem has been resolved. I hope this extra bit of insight is useful.
Another customer who was running out of fuel was using the plastic float bowl and he burned a few millimeters off the little stop that sits below the float so that it would open wider. Can't do any harm.