Hot weather pressure buildup in gas tank- is venting needed?

['41 Fat Bottom Girl]

Ran the tank down to a couple gallons left in the '41 tank today and before filling up, I noticed when I removed the filler cap a lot of vapor/air pressure had built up. Just hit our first several 100 degree F days here in the desert. The tank or cap have no venting. Would there be any concern about pressurized gasoline going into the fuel pump or maybe affect the shutoff of the carb float?

Should there be some venting to equalize pressure in the tank to atmosphere? 

If I have this positive pressure with hot weather and such a closed system, in cold weather could I create a vacuum (negative pressure) in the tank which might fight the fuel pump operation?

This is a new fuel cap, original is long gone... If I should vent it, it would be simple to create a tiny vent.

Thanx! 

 

[plymouthcranbrook]

There is a thread from April 11 that discusses replacement tanks and how the cap is vented. Going to see if this is it as the Ipad is funny sometimes copying addresses.

 

 

[Booger]

I think its a good question. Stock caps have no venting. As a gas jockey 50 years ago you could hear the pressure release as you cracked the cap for a dollar of gas

[Sniper]

The stock cap should be vented.  The cap on my 1 when I bought it did the same thing, so I bought a "vented" cap.  Except if it vented the pressure threshold was so high that I could hear the tank oilcanning as it heated up/cooled down.  So I bought another cap, same deal.  Ended up having to take the cap apart and adjust the vent release spring.

[Sam Buchanan]

It is very easy to vent the tank (it must be vented). Drill a small hole in the filler neck (please excuse the upside-down photo, you get the idea).

 

Edited May 15, 2021 by Sam Buchanan

[Plymouthy Adams]

I am not so sure of the replacement caps design what with the push from the enviro folks not to vent to the air 100% of the time as the old caps with open vents did and thus the push to design the modern old style replacement cap based on a light pressure spring.  

[YukonJack]

I put a locking cap on my 47 years ago. It was supposed to be vented but wasn't, burned out my electric fuel pump trying to suck gas out of the tank.