Gas spilling out of filler neck. What can cause pressure in the tank?

[Thinkjoejoe]

I have a 1973 Chevy dually camper special 454 with saddle tanks.
Every thing is stock
Regardless of which tank shortly after starting gas will start pouring out of the filler at the cap on whichever tank I'm using.
What could possibly make my tank build pressure like that ?  Is my vapor canister clogged ?  What should I check first?  I have not found this problem posted any where. Thoughts?  Thanks -- Joe

[454Man]

Heat will do this. Check your exhaust to awe how close it is.. one of mine did the same over the summer.

What's under the hood??? If you have to ask maybe we shouldn't race...

[Irish_Alley]

did you just get this truck? does your truck have return lines?

[Thinkjoejoe]

@454Man. Yep it's got the 454 big block

I'll check the exhaust. Weather is not very warm out here.

@irish_alley
Yes I did just get the truck. It does have return lines. It's a rusty hunk but halls all my junk :-)

I was wondering if a bad charcoal canister would cause this?
But It does not flood or sputter. Does not dump gas into the carb.

[Irish_Alley]

i would check the fuel lines on the selector valve make sure someone didnt cross the feed side up (i.e. you select the drivers side buts its pulling from the passengers side and returning to the drivers side) so all your doing is putting more fuel into a tank your not using but you think you're. and this would increase the tank pressure and force gas up and out.

another test to see if its in the valve would be to bypass the valve  connect the engine side 3/8" to a tank's 3/8" and then do the same for the 5/16" return line.

[VileZambonie]

The tank vent needs to be functional or this will happen. Follow the vent line on top of the sending unit and see where it goes. Usually it's because someone stuck a bolt and a clamp on the line somewhere and eliminated the vent.