I have just put a "Fat Boy" twin tank on my Sportster & when i fill it gas comes pouring out of the l/h side cap, I have tried filling slow from the right hand tank allowing it to fill the l/h tank but when i fill the r/h side the same thing happens.
I usually end up losing about 1/4 of a tank of gas (5 litres), I have fitted a vented cap to the r/h side but does not seem to make any difference.
Any ideas - help???.

 

What year tanks did you use? Which petcocks did you use? Are they vacuum, gravity feed, or injected. Some pics would help.

 

I believe the tank is early 80's, bayonet style caps, Original fuel tap l/h side

 

The bayonet style filler necks do wear after awhile from the bottom so they don't seal as tightly. Go down to an auto parts store and buy two more gas caps, I think they are part #6702. Then take the rubber gasket seals from the new ones and put them on your existing caps doubling them up. If that doesn't work get yourself a thicker piece of rubber and some razor blades and cut yourself out two new thicker gaskets.

Also how are you filling your tanks? Don't do it like a cage. Pull the rubber boot back on the pump nozzle (if they have those where you live) and eyeball the fuel level as you fill the tank and just don't fill them to the brim.

 

Your left side cap needs to seal. Simple as that! NickD has it covered.