Those upper vent tubes on our carbs... why are they there?

[deltarider]

Never quite understood why those upper vent tubes are there. Yes, they vent, but so do the overflow tubes.

[K2-K6]

The overflow is specifically to allow excessive fuel, from leaking intake fail, to bleed out safely (well from not depositing into engine etc) away from other parts of the bike unimpeded.

You wouldn't want the carbs to suck air from that tube ideally, possibly water etc has access.

Vent tubes allow "safe" ingress of air from discreet source, in or out, to make sure there's no pressure plus or negative allowed to accumulate in float bowl, making them as near neutral as possible so that the vacuum through the carb venturi and pulled through the jets is absolutely as repeatable as possible to give consistency in jetting/mixture.

[McCabe-Thiele (Ted)]

Interesting explanation k2-K6 makes total sense when you put it so succinctly.

[deltarider]

Where should their open ends ideally be? Saw a video recently where the owner of a CB500 had lead them between the airfiltercase and the battery. See: 10:16 https://www.youtube.com/watch?v=lkxVVq_o8mY

[K2-K6]

Where should their open ends ideally be? Saw a video recently where the owner of a CB500 had lead them between the airfiltercase and the battery. See: 10:16 https://www.youtube.com/watch?v=lkxVVq_o8mY

Likely anywhere that gives a air supply without risk, something near the airbox intake itself mid bike and away from rain etc  looks most sensible.

You wouldn't want them to be influenced by air intake vacuum though, so not directly in the intake mouth for example.

[deltarider]

AFIK mine have been dangling in more or less the same spot where the exits of the four overflow tubes are. Hence my question.

[Erny]

mine goes back to top of the battery area