C_Watkins,

I would say the engine runs for around 3 to 4 seconds before it quits. You might be right regarding the plane pointing straight up at idle the engine just won't run. It's hard to think of any situations, both in full scale and in R/C, where the plane is pointing straight up with engine idling, even when doing 3D/aerobatic.

The fuel lines are good. As I said before there was no fuel starvation; I only saw a few air bubbles in the fuel line when plane is straight up at idle. However I'll make sure the muffler is tight.
Thanks all!