You'll have to forgive me if this tip is obvious to everyone but me - I thought it was useful. While putting together the carbs from my 1978 CB550K last night, I kept wondering if the fuel flow valves were seating correctly or whether they were leaking. It occurred to me that there was a simple way to test:
Tilt the carb so that the floats close the valves, and blow into the fuel line (I had just replaced this line, so no cancer for me, I hope). Its easy to tell if you have a leak: you will be able to blow air through. Then tip the carb so the floats open the valves - you should feel no resistance blowing through.
Turns out my valves were sealing very well. Yay!
