Here's some interesting info. After installing overflow drain tubes, indeed the evaporation rate of the fuel increases. I have seen bikes on the lift that are hot, when shut off you can see the gas vapor literally shooting out - downwards - from the bowl nipple. The fact the carb vent tubes are open makes a kind of backwards chimney effect... the air can come in from the vent to help speed the evaporation. But on the other hand... the overflow tubes alone act as carb vents, and at that point the actual vents become redundant and not needed.
To prove, take a set of carbs with no overflow tubes and while idling, block the stock vent tubes. the bike will immediately shut off because the air can't move in and out of the bowls to balance the fuel displacement. Now do the same test with a set of carbs that have overflow tubes. They are unaffected by the vents being blocked, gthe bike will continue to run, unaffected.
I have known this for a lot of years, but this is the first time (to my recollection) that I've written about it. I have found often giving information like this just sets some folks off, and the end result is more confusion than if I'd have left it alone. So now I'll guess we'll see what happens
Steve
