Motor whizzing

[Ande]

The motor in my m4 started whizzing so i took it apart, there was some wear on the piston and the gear on the motor looked a bit rounded so i just fitted a new piston, new motor, shimmed it up, put it back together and it still whizzes, i put the motor in and tightened the cap back down on the grip, when i pulled the trigger nothing happened except for the wires getting hot, so i unscrewed the cap on the grip and it started to move, but only whizzing, if i try and press the motor onto the other gear it screeches as it makes contact then as i apply more pressure it just stops or locks up or whatever. I don't know what to try next, is it the gear that makes contact with the motor gear?

[straffham]

Could be a wiring issue, either a short circuit due to damaged wire insulation, or damaged wire inside the insulation, or possibly a worn trigger switch contact where not enough current gets to the motor to turn it under load, any of which could cause the problems you encountered. Only other possibility that springs to mind is the shimming is too tight, preventing the gears from turning freely? My hunch would be a wiring issue though.