xcelorin motor and controller issues...?

[Dr_UpGrade]

Hi Guys,
Am running a sensored xcelorin losi system. Its been working fine with the 13.5 motor I've had in it. Recently bought a 10.5 motor and been having a few issues. It won't run with the 10.5 motor unless I spin the wheels manually (with motor in the car) or spin the rotor (when out of the car). This will happen on any throttle level full or minimal, but as soon as I put the 13.5 back in its fine. Batteries are fully charged lipo 30C 5000mAh.
Any thoughts? Cheers

Mark