I have an old Mercury 20hp, Model 200, Serial # 3432679. My best quess is that it may be a year 1978 model. If you increase the speed much past say 1/3 rd throttle, it slips out of gear as you are in progress. However, the direction lever on the outside of the motor will still be in the forward position. By stopping and reducing the amount of throttle it will reengage. Do you know what the problem is and whether it is something I can attempt to repair without any specialized tools and specifically wart parts would I need to order. Thanks for your help.