I know this has been covered in many posts on many forums - that a clogged or restricted fuel tank vent is going to result in restricted fuel flow. My F3 sometimes seems either starved for fuel (or gets way too much air) around 115 120 km/h (I think that's getting up around 70 mph).

I'vetested the air vent solenoid to ensure that it's working right, and all the hoses are routed properly. THe fuel pump is pumping. It does seem that if I pull over when it happens and open and close the gas cap, it runs better - but that could also be because I slowed down, shut the bike off, and went out again. It also seems more likely to happen when I accelerate right up to that speed from a standing start, rather than riding along for a while at 60 or 70 kmh then accelerating up to speed.

To check the fuel tank vent, I tried putting a wire (guitar string - strong but flexible) through, but I hit what seemed to be a fairly sharp turn and couldn't get further. I tried to blow through - and couldn't - so I thought I was on to something. Then I grabbed an air hose blew through about 40 psi - and it seemed to be coming from below the level of gas in the tank ('cuz it sprayed gas upward!). Now I've come to my question: Has anyone looked inside the tank, or does anyone have a drawing of how that tube is routed in the tank? It doesn't make sense to me that the air vent would be covered by the gas in the tank - it should be up high where it may get splashed, but wouldn't be covered most of the time. I'm guessing if it's broken somewhere inside, it isn't likely to be something I can fix, is it?

UPDATE - Thanks for the tips - I had the right idea but the wrong problem. It was starving for fuel - but not from being locked in the tank. I have an F3 in an F2 frame- so there was no "stock" place to mount the fuel pump. Where the pump and(oversized, not oem) filter were mounted resulted in the filter resting in such a way that any time I had disconnected the fuel lines and air got in, this big filter held a huge pocket of air. When I used the fuel too quickly, the pump ended up pumping (a lot of) air, and having no gas in the filter to pump. I've now reoriented the (smaller, stock) filter and pump, and it runs like a charm. Ironic that each time I lifted the tank off to try and fix it, I created the problem all over again. The longer it ran without discounnecting the fuel lines, the better it was likely to get! thanks again guys.

 

There was probably a blockage in the tube keeping the gas from coming out and preventing venting. Also, when you put the tank on, you may have crimped the line, which would have the same effect. Otherwise, I have no idea why there would be gas in the line.

 

It's easy to crimp the breather vent pipe when you replace the tank as it runs through a bracket above the rear shock mounting. If it's bad enough it can build up a vacuum in the tank as gas gets used and not replaced with air and actually stop the fuel flow so the engine dies completely.

 

Thanks guys - I should have been more clear about the line itself - I pulled the hose off to do the testing. WHen I pushed the wire, and blew the air, it was directly into the stem that comes off the tank. I've been running it that way, so it's not a crimp in the hose. I just assumed that if the vent was working it shouldn't take any (significant)air pressure to blow through, so I stuck a clear hose on and tried to blow through it, and couldn't (gave me quite a headache). The compressed air worked, but I could see through the open lid thatthe air seemed to bubble upfrom down low in the tank, spraying gas upwardand that's what I'm worried about. Could my vent tube be broken inside the tank, so that most of the time it's covered by the level of fuel, and therefore not venting? There is tube that goes diagonally across the tank that's visible just below the filler neck...is that the vent tube or a brace of some sort? I know the overflow is just beside the filler neck, and seems to run down from there.

 

try running without an air filter for one run. See if you are not getting enough air. on my old bike, the filter was soaked, then dried, but it still restricted air flow too much...

 

if you are creating a vacum then you should hear a hisswhen you open your tank cap when the bike plays up, have you checked the connections on your fuel pump coz it may be an intermittent fault.