My primer bulb is 3/8 too. So is the fuel line from the tank, and the fuel-water separator. I used the reducer from Amazon to drop down to 5/16 for the connection to the outboard.

I was thinking 3/8 should be better since I have a 150 hp but since you have a 200 hp and have a 5/16 line than I will be good, I think I'm good to go Hey Duke thanks for the advice and help