Problem with ez 2350kv motor/gearing

Hi I'm having problem with 150 esc and 2350kv motor on 4s on mugen buggy and kyosho 777.
I'm using this kit with 13T pinion, esc is cold but motor get so hot that loose magnetic energy.
I'm lucky I can repair the rotor making it get its magnetic force as new.
My problem is why is this happening?
Is gearing too high or too low? do you suggest triyng a 11t or 16t pinion?
On 3s everything is fine, on 4s I hoverheat the motor in 5 minutes.
Anybody else is using this kit on buggy on 4s? what gearing u use?
thanks
cheers
gab