I have a Japanese Mercury 15hp 2 stroke 2003 that has around 20 hrs on it. It starts and idles fine but dies on the throttle unless the choke is pulled out half way. I have stripped and cleaned the carby and blown out the jets but these all looked ok anyway. I tightened the carb onto the manifold (there wasn't a gasket so I assume it doesn't need one) checked all of the hoses for cracks and cleaned the fuel filter and the problem is still there. I notice on the port side of the carb where the fuel line enters there is a plate screwed into a black plastic block. Behind this on the carb side there is a short metal tube that sticks straight up. On my motor it is open. I was wondering what it is and whether this needs to have a cap on it as it may be sucking air. I haven't tested this theory as I need to be in the water as it run's fine on the hose and i'm sick of towing it down there, launching it and having it run rough.
