If the gear mesh feels rough then it's more then likely to close to the spur gear and will strip the spur gear pretty quickly. Here's a quick way to set up gear mesh loosen the screws that hold the engine tight to the chassis. Then take a piece of note book paper and tear a small piece off the paper. Then take that piece of paper and place it in between the bell gear and the spur gear. Then push the engine up against the spur gear, but dont push to hard then tighten the screws up and then grab the paper and pull it out. That sets the gear mesh almost perfectly. It takes all but a minute to do this and it sets the gear mesh up all most perfectly.