Motorcycle Repair: 72 cb750 carbs, float bowls, fuel drain

Question
I have a 1972 cb750 it has sat for a few years with fuel in the tank and carbs. I have cleaned the carbs and the bike runs good for about a mile or so.  Then one of the carbs does not get any fuel to it.  It is not always the same carb.  If I pull the bowl off then fuel will run into it.  Do I have a breather tube problem?

Answer
That's a little odd Ryan.  I've not seen this one before.  The float bowls are vented and if you take all of those hoses off you can make sure the hoses are not the problem. There are vents in the top of the float bowl chamber to vent the bowls and allow fuel in but there are also overflow vents that will do the same thing.  It's hard to picture both being plugged but if they were plugged it should be the same carb each time.  The needle valves are the only control on how the fuel gets into the bowl. Is it possible the tank is pulling a vacuum and not letting fuel into the carbs? Are the needle valves moving freely in the seats and the floats moving freely?  The only thing that make any sense is that the floats are sticking and when you pull the bowls off you bump the floats and they drop allowing fuel to start to flow again.  Try loosing the fuel drain screws to see which carb is acting up and once you figure out which is the offending carb, just tap lightly on the side of the bowl and see if fuel doesn't start to flow.  This would indicate a sticking float to me.

Regards
Rich