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Sorry, this is a newbie question, but what is this pipe coming out from the petrol tank under the seat? The bike is a 1989 Kawasaki ZZR 250:

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2 Answers 2

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It's a gasoline vent tube, serving three purposes.

  • Gasoline vapors can be especially dangerous under pressure (pretty much why it is useful), and as the temperature fluctuates it is safer to relieve the vapor pressure than it is to potentially have it build up. The gasoline vapors should preferably not be ejected onto a potential spark source (battery) or hot running motor.
  • As fuel is drawn through the carburetors or fuel pump, that volume must be replaced in the tank with fresh air or the vacuum in the tank will resist further gasoline extraction. When drawn up a long tube with very little vacuum, there is little chance of it pulling in large amounts of water, dirt, dust, or other contaminants.
  • If the tank were nearly full cold and then heated, or somehow overfilled, this would allow the excess to drain through the tube toward the ground in a safer area than dripping on a hot engine or battery.
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  • If you want to see them in action... Expansion and dripping of fuel out the vent tubes can be readily observed in airplanes that are filled up cold and sit out all day in the Sun. Commented Feb 4, 2015 at 15:45
  • I've had it happen on hot days with my motorcycle as the fuel is kept cold underground, but it wasn't much and I had to deliberately overfill it by pulling the nozzle out while filling.
    – Ehryk
    Commented Feb 4, 2015 at 15:49
  • My bike is known to run poorly if that hose is blocked - the main purpose is to allow fuel to exit the tank easily. Commented Feb 4, 2015 at 20:08
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Its a petrol overflow drain pipe. In case if you fill fuel that overflows it ll be drained through that pipe. it may also have internal connection with a pipe with drains rain water or any water that tries to get into the tank.

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