Since the problem goes away when the tail is raised, it sounds as if the engine isn't drawing fuel up that short distance between the fuel feed line and the center of the carb. The tank appears to be about 1/2 inch below the needle valve. The pressure from the muffler produces a bit of boost for the fuel draw, but the primary force is vacuum produced by the engine. When the piston comes down (the downstroke), it produces a vacuum between the top of the piston and the head, That vacuum draws fuel through the intake ports of the sleeve much like drawing soda through a straw. If this is a ringed version of the .61FSR, the ring could be gummed up from sitting, reducing the vacuum produced on the downstroke of the engine. If that's the case, it will clear up by simply running the engine.