What function does the metal disc and spring inside Suzuki outboard fuel cap perform?

[Dipper]

I hadn't used my Suzuki DT2.2 for at least 5 years so thought that I ought to start it up in a dustbin and give it a bit of a run. Before storing it I had run the carburettor dry and drained the fuel tank. On opening the cap to put some fuel in the tank I found that the metal components inside the cap had severely rusted and the tank was full of rust dust which needed cleaning out. Why aren't these metal components made from stainless steel or brass? The new cap I ordered turned out to be the wrong one as the thread didn't fit my tank but I removed the metal components and fitted them to my old cap. As I was doing this, I wondered what function the metal disc and the spring beneath performed. It must have something to do with the small screw air vent in the cap but I can't work out what unless it provides a bit of resistance to stop the vent screw spinning closed. Does anybody know?

Incidentally, despite using 2 stroke fuel that had been stored for at least 6 years it started and ran on the third pull.

 

[penfold]

I suspect it's just a baffle to reduce fuel squirting out the vent in a heavy sea.

 

[Dipper]

I suspect it's just a baffle to reduce fuel squirting out the vent in a heavy sea.

Sounds plausible.

 

[Alex_Blackwood]

I suspect it's just a baffle to reduce fuel squirting out the vent in a heavy sea.

Or pressure relief for tank, Thinking hot weather increase in pressure

 

[Dipper]

The disc winds up against the spring as the vent is opened.

 

[Alex_Blackwood]

The disc winds up against the spring as the vent is opened.

If you put a little pressure on the disc, does the vent open??

 

[Dipper]

If you put a little pressure on the disc, does the vent open??

Apologies but I should have mentioned that on the new cap there was a cylinder of foam beneath the disc. I'm thinking that Penfold is probably right about the baffle. The disc holds the foam in place which acts as the baffle. My old cap didn't have this foam unless at some stage it fell apart. If I assume that the foam was there when it was new, that would make more sense.

Apologies but