Since you didn't say, I will assume that you have YS 4-cycle engines. The line on the same side of the engine as the needle valve is the pressure line to the fuel tank. It MUST have a check valve in it,the enginewon't run without one. The other line is the fuel supply line from the fuel tank and should have a filter in it. If you have neverrun a YSengine before, look here for a primer on tuning them. They don't tune the same as other engines you may be used to.
http://www.patternflyer.com/troy/?page_id=13
Paul