Gas cap...how do they vent?

[dart451]

Hello again,

I accidentally found my fuel problem on a Craftsman high wheel trimmer. I was about to throw on a new carburetor when I noticed that...when I disconnected the fuel line, NO GAS came out of it! I hadn't noticed this the first time I removed and cleaned the carburetor because I pinched off the fuel line prior to disconnecting it. To my surprise, when I started to unscrew the fuel cap, the fuel started flowing! I knew it has a full tank! So, my question is this...how do these caps vent the fuel? I did push the little aluminum inner cap up and down but still it wouldn't allow fuel to flow until I unscrewed the cap a little. I found another cap on my junk lawnmowers and it worked just fine. I just can't figure out how one fixes this issue with the original cap!?

[creia]

Great question dart and I have wondered the same thing! We have a fuel cap very similar to yours on the fuel tank of our 1974 Heald Super Bronc off-road bike. Ours is working/venting fine as we have never had a problem with it. On the vintage engines (1960s-1970s) we typically work on they always have the visible holes in the fuel cap. I could not find any holes on this cap, however.
Michael

[KE4AVB]

And here is the answer. Two 1/16" - 3/32" bleed grooves allows air bleeding thru the cap threading. Seal goes bad when overtighten and closes these vents due being crushed. Probably why Briggs has switch to a clutching cap.

And you know the rest of the story as Paul Harvey used to say.

[dart451]

Thank you KE4AVB! I knew you'd come to the rescue! lol