vented tank question on 92 johnson 50hp

[MASTERBrian]

I picked up a bass buggy pontoon last year and it's sort of been a project boat. I've only had it out twice and once was a trip just to see what needed done to the engine. I rebuilt the carbs, had the shop tune them and all now seems good. The gas cap is a Kelch vented gas cap, but the vent hole had a wood screw in it. I finally believe I tracked down a replacement and it showed up today. I say believe, because even though it looks like the correct threads, it seems tight to put on.

In any case, it seems the vent on this can be opened and closed, by turning the vent screw, this is on new cap with factory vent screw. How much should this be opened or closed when running the boat? I'm guessing the vent screw should be closed when not using the boat, but what about when running the boat?