the same thing happened with my friends engine on his rustler. the only time it would run is if i would put drops in the carb, so finally i messed with the neddles and i had to richen it up a lot and then it ran crappy. then it wouldnt run, and i would mess withe the needles for a while again and it would run like crap for a bit and then i would have ot mess with the neddles again till it wouldnt start at all. but we didnt do anything to the engine or anything.