Fuel Cap Venting - How's it work???

[Bogan]

Macca's bloody tracky is driving us insane, it's got like a fuel starvation problem. The tank has been replaced (dodgy peeling liner in the old one), the fuel tap has been blown out, the lines are clear, the carbies have been cleaned out, and we've got a fuel filter inline from the tank, but every now and then it's like it's not getting fuel to accelerate.

How does the fuel cap breather work? The cap pops down and seals, but is there a permanent breather hole in the mechanism somewhere or does it rely on air getting past the main seal? If this is blocked could it result in a vacuum in the tank and fuel starvation/empty bowls? Can we blow it out with an air gun and hopefully clear a blockage?

It's a 1985 GPz600R.

Anyone?

[MickLC]

There should be some sort of venting in the cap, and yeah if it's blocked it can cause problems. The simplest test mate is to go for a quick blat with the fuel cap open. If that fixes it then you know where the problem is and can clear it out.

The only one I've played with is the Surfers and you can just disassemble the cap to make sure the vent is clear, so you can probably do something similar.

[mike-s]

From experience the vent is either (on an older bike) a (sometimes spring retained) flap provides minimal seal and the excess fuel comes out the keyhole / air is sucked in via the keyhole
or (on a modern bike, usually where the cap is on a hinge) the flap vents into the inside of the lip of the fuel tank and the extra fuel (fuel expands on hot days) overflows out a vent tube that goes down the inside of the tank and comes out a vent tube down in the bottom of the frame. When fuel is consumed this works in reverse and air is drawn in.

This may not be 100% correct, but it is the general gist of it afaik.