Did you check a gear box level (back end, toward to rear wheel) I face it b4!
Because a screw is broken and jam when i make the gear box. So now only 1 & 2 gear is workable~~~ may be changing a new gear is the only way. But i ask the model shop in KWS b4 and it takes me $600 to re-buy the tamiya parts, so i get it up......
And thats Y i wanna buy a faster motor, eg. 23T or sth else