750 GT fuel slopping out of petrol tank

general_piffle

general_piffle

Senior member
Location
London/Hampshire
I'm talking about the screw on/down fuel caps, specifically on a 1969 750 GT. When the tank is full or even nearly full, fuel will often slop out past the cap and onto the tank. Inside the cap is a thin rubber gasket that I guess is meant to stop this from happening.

I'm thinking about getting some rubber sheeting, maybe 4mm thick or so, and cutting an identical one out to replace the original, which is probably about 2mm thick.

Has anyone with a screw on cap come across this before, and found a good fix? Thanks.
 
general_piffle said:
@Exmoor beast is what Mr Jelly is saying about these Vitax rubber seals true?!
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I did post on here a source of replacement seal that is the right size/ thickness from eBay but mine will still leak if brim filled and I brake hard .
The inside of the pie crust cap has sealing rings cast in a labyrinth to the air vent, supposedly the rubber gasket forms a seal across the rings.
My feeling is that the rubber doesn’t deform enough to provide a seal due to the sprung loaded locking mechanism not being powerful enough. Vent hole in these caps can be central or offset/ mine is offset. My tank also does not have the internal breather pipe either- I actually screw the cap on with the vent hole pointing backwards, which helps slightly. I imagine if you used a very soft gasket it might seal better - I’ve also got a NOS cap that has a thin gasket but have never tried it. Whether the filler neck distorts with use or not I don’t know ….
Don’t overfill or use the front brake is the answer!
 
OneThinBloke said:
Send me an SAE and I will post some cork. Period appropriate and works.

Accept that you will still get a VERY minor escape via the small cap vent hole, no getting round that!
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Thanks for the offer. That’s the thing I wouldn’t bother about a small amount. But it’s a fair bit. I think the original gasket/seal that’s in there now might just have stiffened over time and is no longer working at all.
 
general_piffle said:
I’ve had a look back through your content from your profile, but can’t find the link.

If it’s not too much bother would you mind posting it again?
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If anyone’s interested EBay part 372632387997 is a 3mm thick Viton Diesel/petrol resistant washer 85mm OD 54mm ID £9.49.
Bit expensive but fits & works perfectly. Woad Supplies, 3 DeBurton Close, Billingborough, Sleaford,Lincs, NG34 OPY
 
Exmoor beast said:
If anyone’s interested EBay part 372632387997 is a 3mm thick Viton Diesel/petrol resistant washer 85mm OD 54mm ID £9.49.
Bit expensive but fits & works perfectly. Woad Supplies, 3 DeBurton Close, Billingborough, Sleaford,Lincs, NG34 OPY
Click to expand...
Sleaford Mods and your scooter. :giggle:
 
I have made new seals for the early fuel caps out of leather. The leather will swell up a little bit when the fuel gets it wet and it will seal even better. And leather does not crack with age like rubber.
 
general_piffle said:
I'm talking about the screw on/down fuel caps, specifically on a 1969 750 GT. When the tank is full or even nearly full, fuel will often slop out past the cap and onto the tank. Inside the cap is a thin rubber gasket that I guess is meant to stop this from happening.

I'm thinking about getting some rubber sheeting, maybe 4mm thick or so, and cutting an identical one out to replace the original, which is probably about 2mm thick.

Has anyone with a screw on cap come across this before, and found a good fix? Thanks.
Click to expand...
May be good to dismantle and clean up the split pin, spring, and crossbar bits that hold the cap tight on the rim and make sure those bits are not all stuck, rusted, corroded, gummed-up, etc so that there is good pressure from the spring holding the cap tightly down on the seal - originally the seals were a cork ring so they expand when wet with fuel sealing better than thin replacement rubber seals.

And don't fill it so full!;)
 
Exmoor beast said:
If anyone’s interested EBay part 372632387997 is a 3mm thick Viton Diesel/petrol resistant washer 85mm OD 54mm ID £9.49.
Bit expensive but fits & works perfectly. Woad Supplies, 3 DeBurton Close, Billingborough, Sleaford,Lincs, NG34 OPY
Click to expand...
Hmmm, I tried fitting mine the other day when I was prepping for Willersey but would it fit, would it bugger!

So, I've ordered a 2mm thick one instead to try. I did try the two different heights offered by the split pin x2 holes in the rod that holds the crossbar in, took it all apart and cleaned it. But the 3mm washer/seal just seemed too thick to allow the cap to press down enough to twist and close.

I got around this by turning the old, tired, washer/seal upside down, which seemed to help a bit!
 
Mine had same leaking problem, so I used 1,5 mm thick viton rubber sheet on my S tank. Why 1,5 mm? Because I had it..🙂
Didn’t cure the leaking completely, until I noticed that the edge of the filler neck was slightly dented. I carefully removed the dents in the ,quite thin, steel, now it doesn’t leak anymore.

Marnix
 

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