GB400 TT assistance

[derfel]

Hey Gang.
1987 GB400TT. Thanks for all your responses to my seat question. Due to that I'm asking for ideas on a leaking petrol filler cap on aforementioned bike. The rubber seal is intact (not perished), it appears to be on a spring loaded setup and looks like everything is in working order. Yet I still have petrol leaking out across the top of the tank, obviously this is worse the higher the fuel level is. There is no OEM part number for the seal . Any ideas?

[Bonez]

Are you filling it to the bottem of the filler tube or right to the top? The tube is desighened to leave an air gap.

A seal specialist like SKF may have a suitable seal if you can carefully take out the existing item.

[xwhatsit]

Hey I've got the same problem since my tank was repaired and painted. Would be interested to see what the story is here.

[derfel]

Are you filling it to the bottem of the filler tube or right to the top? The tube is desighened to leave an air gap.

A seal specialist like SKF may have a suitable seal if you can carefully take out the existing item.

Hey Bonez.
Filling to the bottom of the neck, not to the top for obvious reasons. I haven't picked at the existing seal to much in case I can't find a suitable replacement. I don't wish to destroy what I have and end up with nothing, will continue to snoop around. May pay a visit to M/C Restorations here in Chch and see what they suggest.

Ta

[slimjim]

would it not be air-gas pressure that is blowen petrol out and across your tank..i wouldn think that been lower in tank gas ..doesn't cause spill out..as the air in tank is not so under pressure..while when filled or just under fill...push's gas out...would look hard at overflow return hose in tank neck..may be blocked by rust-crap..and then start by indueing a second seal ring...can buy rudder string at pay-less-plactics and glue to form a second seal.. as it sounds like cap top is letting overflow come out..

[FJRider]

I would be inclined to agree with slimjim here... thats what the overflow is supposed to stop...
May have to remove tank to check properly... rubber drain line may be kinked...

[xwhatsit]

Alright children, I put on another fuel tank cap from my other rooted bike; despite looking horrible and ugly (and the rubber sealing ring looking worse), it actually holds the fuel in, even when filled right up to the bottom of the filler neck (where you're supposed to fill it up to, I haven't been able to do that lately). Even when slamming on the front brakes, going downhill, it won't spill any fuel out. Good times.

Looking at the other gas cap, I think when the insurance job was done on my tank, they either removed the sealing ring (to clean/paint underneath perhaps?) then put it back on, or they chucked the old one in the bin and bought a new one. Either way, it doesn't do the job. So I have a tank that keeps fuel on the inside now, but the cap looks ugly (it's painted original black, my tank is silver). So need to do a little research to find how to make the correct cap work.

I'll post up pics, I suspect it's a very similar design to the GB400.

@slimjim, FJRider: I think you've got a bit mixed up. These are not fancy sprotsbikes with space-age gas caps, drain tubes, race-style overflow lines; just a simple steel box with a cap on it. There's a couple of holes in the base of the cap to let air in, and a crude rubber ring around the underside, between the cap and the tank. When you take the cap on and off, it turns against the rubber ring, which wears out eventually.

[derfel]

Alright children, I put on another fuel tank cap from my other rooted bike; despite looking horrible and ugly (and the rubber sealing ring looking worse), it actually holds the fuel in, even when filled right up to the bottom of the filler neck (where you're supposed to fill it up to, I haven't been able to do that lately). Even when slamming on the front brakes, going downhill, it won't spill any fuel out. Good times.

Looking at the other gas cap, I think when the insurance job was done on my tank, they either removed the sealing ring (to clean/paint underneath perhaps?) then put it back on, or they chucked the old one in the bin and bought a new one. Either way, it doesn't do the job. So I have a tank that keeps fuel on the inside now, but the cap looks ugly (it's painted original black, my tank is silver). So need to do a little research to find how to make the correct cap work.

I'll post up pics, I suspect it's a very similar design to the GB400.

@slimjim, FJRider: I think you've got a bit mixed up. These are not fancy sprotsbikes with space-age gas caps, drain tubes, race-style overflow lines; just a simple steel box with a cap on it. There's a couple of holes in the base of the cap to let air in, and a crude rubber ring around the underside, between the cap and the tank. When you take the cap on and off, it turns against the rubber ring, which wears out eventually.

Hey xwhatsit, your on to it regarding spaceage filler caps. There is no overflow hole. Slimjim, the idea of knocking up a second seal is not a bad idea. The previous owner had a piece of thin foam cut to take up the slack, it apparently worked but was of a bio-degradable foam which would've eventually disintergrated from the presence of petrol. Also it soaked up water big time which wasn't what I wanted around the filler neck at all. So I removed it and that's when the petrol started to come out. So shall source a suitable replacement that won't degrade with the petrol. Thnx gang!