I have a late 50s early 60 Mercury thunderbolt 300, it has a fuel issue. I bought a new fuel pump kit, fuel line and primer bulb and cleaned the carbs. I went to start it and it wouldn't start at all so I squirted some fuel in the carbs. It would run for a bit and then die instantly. I then proceeded to check to see if it was pumping fuel to the carbs. I loosened the cap on top of the bottom carb. (Tiltson carbs) it was pumping fuel to the carbs so I proceeded to leave the cap slightly loose to see how much it was pumping out. As soon as I pulled it over it started an ran fine and was pumping fuel fine, the moment I snugged the cap back down again it would die again. So is it possible the screen type material inside the cap is plugged or do I have another issue present? Thank you for any help