Topic: draining the tank...fuel line flow question

[johnspeck]

i'm having an issue with my left carb (as i sit on the bike) starving for fuel.
so tonight, i was draining my tank to check the petcock and clean the filter.


i noticed that both fuel lines don't flow at the same time with the petcock open.  if i pinch off one line, the other flows, and vice versa.  the lines themselves are clear, no debris or obstructions.  it does this whether i have the petcock on "on" or "reserve".  also with the gas cap open or closed.
is this normal?

[BlindJoe]

what kind of bike? have you tried this while it's running?

[johnspeck]

'72 cb350 twin.  haven't tried it with the bike running; it wouldn't run without the fuel lines connected, right?

i'm talking about draining the tank into a gas can so i can remove the in-tank filter.  when i open the petcock, fuel isn't flowing out of both lines.  it's either one or the other.  if i pinch off the line that's flowing gas, the other starts flowing, and vice versa, but not both at the same time.

[johnspeck]

to clarify, when i pinch off the line that's flowing (and the other begins to flow), i open the pinched line, it doesn't flow.  when i pinch off the flowing line, the other again flows, etc...just not both at the same time.

[BlindJoe]

          gotcha, thought you might have had clear lines, as long as the petcock is working properly it all has to do with the pressure inside the gas tank ( the force of gravity acting on the fuel) and the atmospheric pressure ( the force of the atmosphere acting on the ends of the fuel lines), when both lines are open the pressure of the fuel is equal to or lesser than the pressure at the ends of the lines, when one of the lines is pinched, the pressure of the fuel is greater than that of the atmosphere and the fuel flows; so in other words thats normal. The displacement of the pistons creates a vacuum which causes the fuel to flow through both lines and into the carbs. Hopefully that made sense 

[johnspeck]

nope, not clear...just standard black fuel lines.
thanks for the reply, appreciate it!

[mystic_1]

This is normal.  Hondaman talks about this in the "Thoughs of Hondaman" in the FAQ section.

mystic_1