i bought a used nexus..also i am new to helis...sometimes when i run up the engine i have no tail rotor control...tail rotor spins but no control..heli just spins around..pulled engine..clutch and shoes looks fine..gears look to be in aligment..i can lower the throttle and rerun up the throttle..everything works fine...this doesn't happen everytime.....ok what do i try next???