I have a 35 hp Merck with a 6.6 plastic gas tank with a cap that has a screw for venting.

I was told that some engines like to run with the pressure in the tank and that during operating the boat the screw vale in the gas cap might have to be closed. I know storing the boat to leave the cap valve open to be vented so that moisture does not build up in the tank and delete the gas.

 

Any advice on this matter would be greatly appreciated 

My current boat has some fancy self adjusting pressure cap, but on my previous one, I’m pretty sure I recall it being just the opposite of what you stated; I always stored it with the screw closed tight to not allow moist air into the tank for extended periods, but always ran with it open (unscrewed), otherwise the engine would eventually die out from lack of fuel because of the air restriction.

My old 1974 Mercury 20hp was the opposite. The little 3 gallon tank would build pressure...and flood out the carb...motor would not start. 

 

For the life of me I could not figure out what was the problem. 

 

Took the carb out, replace float...installed carb...and then hooked up gas while boat was in the garage...fuel started spilling out of the carburetor in two places. Ahaaaaaa....that's my problem!

 

So now, when I get to the lake...I loosen the cap on the fuel cell. Take away the pressure. Problem solved. 

 

Most outboards have a fuel pump to suck fuel out of the tank, a vent is needed so the tank doesn't collapse. I always kept my vent open so the tank wouldn't expand in heat, bever had a moisture issue, but also use a few oz of stabil in my fuel.  There are contraptions that are supposed to eliminate venting for the tree huggar crowds, but I understand they don't work well and most folks convert them.  In my experience, most vents are designed to not leak and I have never had one leak fuel.

Thanks for the information