Is the engine tightened down???? If not then the engine will move back and will tear the teeth up, also if the spur gear and bell gear are to tight this will also cause it to strip. The paper idea works well, just make sure that there is no binding between the two gears. Make sure that the two gears are able to move freely with eachother. Hope this helps, nd stay with it. There a ton of fun, your just having a bad experience so far but hang in there.