k1 keeps overheating my motor

[jamesscott32]

Hi this is my first proper 4wd buggie.
I got it yesterday secondhand with a hobbywing 120a speed controller and a 10.5t motor an it cut due t over heating.today I put in my speed passion controller an 7.5t motor and it just keep cutting out/overheating.Seems OK just going round half throttle but when you start to open it up etc it just overheats/speed controller cuts it off.
I tried a thew different pinions an non made a difference.
The belts feel tight but I don't if they are over tight