Burning Brushed Motors

[extrememacko]

Hello electrical experts, help!

I burned two 27t brushed stock motors today after just 2 minutes of running:
I am using them with nano-tech 5800mah lipos.

I used the first one in my b4 and the arm started to melt at the windings, horrible smell.

An similar motor was then used in my XXX and same thing happened but quicker.

The B4 has an MRT MX speedo and XXX has an LRP QC3 speedo.

Can any one guess whats going on?