Ok I just decided to use an OS 120FS Surpass motor for my P51 and I will say right up fron I am not tto familiar with 4 strokes so please bear that in mind in any posts. I understand that there is usually one line for fuel to the engine and then another going from the exhaust system to the fuel tank for pressurisation of the rank now this is the normal setup for a two stroke. But on this motor it has an additional line going from the bottom of the crank case in addition to the one on the exhaust and the intake on the carb system. What does this extra line do and how is it setup. Is it an additional pressure line that goes to the tank or what? As I said not a great deal of experience with four strokes so let me have it.