motor burnout

[mattf77]

I have recently put a 14t brushed motor in my Tamiya avante 2001 and it burnt out on the first run. I sent it back and got another, this too burnt out on its first run. I don't know why, the drive train is running as free as I can get it.

both times I was driving on hard pack sand/mud.

I run an Mtronics trident 12 esc, apparently able to go down to 12t brushed and use lipo 2S 4000mah core rc stick pack

any ideas please