Hola Douglas,

Is ti possible your engine is running a little hot? try adjusting the needle valve before you do your first flight, get the peak and open until it drops around 200 or 300RPM, if your engine runs fine the first time the pump is primed, you might be having a vapor lock, as once the engine is off if it is too hot the fuel will create bubbles inside the pump preventing from working so you will have to prime it again.

Boa sorte