Motor advice needed
 

I have a Hirobo Zerda that I've been running with a 10t BL system but it was too much for the all belt transmition so I thought I'd down grade and use a brushed motor instead .
I settled on a Goolrc 17t brushed motor as I still wanted a reasonable performance from a modern motor as I like to drive my cars not just shelve them , I was running a 20 tooth pinion/pulley on the BL system with a slightly longer belt so that's where I started but the motor was running nearly 180C after 5 min and it desoldered the negative wire from the motor ! so I dropped it down to an 18 tooth pinion/pulley , the larger of the 2 stock pinions , after 5 min it was up at 170C , down to the 16 tooth pinion and shorter belt but I still had over 155C after only 5 min .
My belts are not over tightened , in fact the first belt was slipping and without the motor connected it rolled very freely.
So the question is , do I have a duff motor ? Or is a 17t brushed motor just overloaded by a 4x4 belt driven car ?
I find it hard to believe that a 17t motor should be so overloaded , BITD people were running 14t motors .
Or is it the 2s lipo delivering more usable current that's the problem ?