I have a 216 that is about 6mths old. It has recently developed a problem with the drive system slipping. when i engage it, it just shudders and I need to push it to get it to engage properly. The right rear tyre is bald, the left tyre has minimal wear. i have adjusted the drive cable and there has been an improvement but I still need to give it a "bit of a push' to engage the drive. I've now only got a few turns left on the cable before i run out of adjustment. Is there anything I can do to fix this problem. Is there something on the gearbox that I can adjust. will replacing the wheels assist.