Leaking gas cap - 1964 cb77

[02z06dave]

Hi everyone,
I've been having an issue with my gas cap leaking gas out all over my tank. It leaked with the original cap and old seal, worse with the original cap new rubber seal, and now with a new cap and cork seal. With the old cap, it was basically like not having a cap on. With the new one, it's better, but it leaks whenever the tank is more than half full. The new paint job on the tank is now ruined, so I'm hoping to find a fix before I get it painted again. Has anyone experienced this problem before?
Thanks,
Dave

[02z06dave]

Here are some pictures, maybe something is abnormal that I'm not seeing?

[OldStan]

Never had good luck with cork gaskets. have rubber (honda part # 17631-329-003) on both my bikes and no problems. One has the original cap with the rubber, the other a newer replacement cap. The rubber seems more forgiving of surface flaws.

[02z06dave]

I'll give the rubber a try on the new cap. It was terribly on the old cap

[Merz64cb77]

I've heard of caps leaking before. Where is the leak coming from? I know there is a post on here about anatomy of a gas cap and it's 6 pieces. I assume being new that the cap was built correctly but you could take it apart. I would also use the rubber replacement gasket.

[02z06dave]

I read that thread before. I took apart and cleaned the old one, no the new one. It's hard to say where the leak is from. With the cap on all the way, gas seaps out and runs down the side of the tank. When I take the cap off, everything is wet with gas. The seal is wet, the tank is wet, gas pours out of the cap.

[02z06dave]

I've also got a cb450 that shares the same gas cap design. That cap has never leaked even with the tank full to the top. If I swap caps, the cb77 cap does not leak on the cb450, the cb450 cap leaks badly on the cb77. This leads me to believe that it's not really an issue with the cap but the tank. Perhaps something is wrong with mine or its just a bad design?

This is the 450 tank opening