The tank can also empty to "half" though a syphon in to the carb. The tank will empty until the fuel level in the carb matches the spray bar.
Not that I doubt you, but I've never seen that unless the engine was inverted. (But I never let my palnes sit overnight with fuel in the tank either.
)
it's a nitro airstrke and there is no way the tank is coming out without a saw !
That's too bad because I doubt you'll solve your problem without removing it.
Dennis-