If you search for keywords like half tank issue you will find many threads about this problem. it seems as though there isn't consistently enough gas volume/pressure going to the fuel tank to push the fuel into the carb/cyllinder. The pressure comes from the exhaust in pulses and the check valve basically traps the peak pulse pressure in the tank allowing for consistent pressure to the fuel line. I assume that when the tank is full there is obviously the hydrostatic pressure of the fuel plus less air in the tank so it can't compress as much, but when its empty, there is lots of gas to be compressed.

Anyway, Once you get this thing running, It's a beast! I like it much better than the Tmaxx, Savage, or LST2 that I have ran. Cen's customer service seems... inconsistent, but the truck is good.