afriend of mine with a 84 brat a/t ,ea81 motor , carb, he bought a different gas tank [ used], but good shape, his problem is it is vapor locking ,,new fuel filters, different canister , ,he can only drive it with the gas cap real loose, , if the cap is tight the truck will hardly start, he switched the lines still the same thing, any help ??

Vent line from tank to cannister must be plugged or 2 way valve is stuck.

 

Should be able to blow easily thru the line from the cannister to the tank w/fuel cap loose.

 

Proper cap should have a vacuum relief valve to prevent this.

also new fuel cap, ok, thanks