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Leaking petrol filler cap

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The filler cap on my MK2A Interstate fuel tank leaks when it's full. Fortunately the seal fittings are held in by a screw and not riveted, so I've tried fixing it myself. The filler cap seal has been replaced, and I've tried putting a bit of rubber hose behind it, along the lines of the hose-like part included in AN's filler cap service kit, but it still leaks. Can anyone supply that part, or tell me exactly what type of material it is, and its dimensions? TIA!

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Some after market tanks have the petrol cap pivot in the wrong place. I had one some time ago that would not sit square as the hinge was too low and the front of the cap would stick slightly upwards. You ma need to check this out and possibly re drill the hole up a bit higher.

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Thanks for the responses. Yes, first thing I did was check that the mating edge of the filler opening is flat, but I can check again. And, it's not the original tank â sadly that was trashed in a traffic accident, when the bike was parked, believe it or not â? Fortunately the person responsible was traced and repairs paid for by their insurers. But the tank was NOS; and it takes firm pressure to close the filler, so I've been assuming that the seal is held squarely against the filler opening by the spring that's behind it. But I can check that too. If I find anything I've missed I'll report back.In the meantime, I'd still be very grateful for pointers as to the material and dimensions of the hose-like part in AN's filler cap service kit. Cheers.
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not on a commando, but my petrol cap leaked i found that the two breather holes ended up inline with one another so put them opposite to one another solved it .

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The breather holes in mine are 90? apart (and clear), and too small to explain the amount that comes out â it doesn't slop out exactly, but for example braking to stop at traffic lights with a full tank produces a thin stream over the paint.

It just occurred to me to try holding the seal over the filler mouth, so one finger blocked the central hole for the screw and applied pressure, and another blocked the breather hole â then opening one fuel tap and blowing in. The tank's off the bike and empty, you see. A jet of air came from just where the fuel does, about 10?â15? east of north, if I take the front of the tank to be north. Obviously I'll have to take a closer look â? Thanks again for contributions so far!

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Ok, well, there's a thing â? I smeared washing up liquid around the filler mouth and pressed the seal against it â the seal picked up the washing up liquid in a perfect ring, no gaps. Then I tried the same with a small plate of safety glass, with the same result â this time a complete ring of washing up liquid was clearly visible through the glass, squished between it and the filler opening. Which pretty much takes me back to square one, I think?

The only thing I've noticed is that the rubber disc that forms the seal isn't quite central in the steel pressing. Perhaps that's it.

I started off asking after the hose-like part of AN's kit because I assumed that it must be my attempt to replicate it that was inadequate â from the way the seal is attached to the cap, I assume that part acts as a secondary seal around the screw, behind the seal disc. Maybe I should just shell out for the AN kit and see what happens with a new seal-and-pressed-steel-disc part from CEANDESS of Wolverhampton! Poor old piggy bank â?

Thanks again for the responses! It might take a while for me to get hold of that kit and try it out, but when I do, and if I remember, I'll report back. Cheers.

Sorry to come back to this thread but did the leak get resolved? I have a similar problem with a cheap alloy Roadster tank sourced from India. It fitted a treat to my '72 Combat but the paint was not petrol proof and flaked off because the fuel cap it came with leaked. I bought a better quality cap from Norvil but it didn't solve the problem either. I have levered the mounting part of the neck up as much as I dare to compensate for the cap being possibly at the wrong angle but the leak, albeit less, still exists despite the rubber seal looking like a good fit all the way round.

I have cleaned off the paint from the neck to expose raw aluminium but this also has had little effect. I'm hoping to solve this before paying for a proper respray so any definitive help would be greatly welcomed. I have read about cutting cork neoprene sheet to fit instead or as well as the rubber seal - worth a go?

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My Roadster tank cap leaks as well, it never used too and I've had the bike since new.

Everything looks ok with it.

I've given up trying to sort it, I just don't fill it to the top any more.

I wondered if that stuff we call petrol nowadays has an adverse effect on the cap rubber seal? 

 

Would be nice to know if anyone finds a cure for this problem.  

 


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