Interesting discovery about fuel filler necks

[Glenn]

My M37 fuel filler neck has the standard single position where you put the cap on and turn it till it's on all the way. My M43 has a fuel filler neck that is configured like a radiator neck, you can put the cap on part way and it won't come off but it's not tight, and you can tighten it further just like you would a radiator cap. Thing is, a venting gas cap won't work when tightened down on the second position, even with the vent valve in vent position. I know it can't be the cap because I tried 2 brand new caps and it still builds pressure. I guess I'll just put the non tabbed cap back on the M43 and leave it in the loose position. I thought it would be cool to have the cap with the tabs on it. Anyone else experienced this, or aware of this?

[Sal]

lm a little confused here Glen, Isn't the gas tank vented through the vent line that goes to the carburetor neck and not the gas tank cap ?

[Glenn]

I thought the tank vented out through that line myself till I discovered that the vent valve on the gas tank only allows inward flow to keep from pulling a vacuum on the tank.

[Rick C]

Both trucks should have similar caps. There are caps with a fording vent built into them that you can close manually