When you had it running on the stand, did you let it idle for 30 second to a minute and then push the throttle open quickly? It sounds like the low needle valve might be rich and when you get ready to put it from the stand to the ground, It might take a few moments and the engine might get loaded up and then when you go to increase the thottle, the engine sputters and floods out. However, if you can let it idle on the stand for a minute and then transition to full throttle quickly without a problem but then the when on the ground it has a problem, then this is not the case.