My guess would be a magnetization issue. Here is a suggestion: if you have an old style computer drive, these contain some of the strongest magnets you'll find and should be strong enough for what you need. The key is taking them apart...you will need a small star driver. But lack of strong enough magnetization could be the answer. Also lack of lubrication could be part of the problem. Also...fyi...2500 rpm with a standard 4.11 rear end, 27 inch wheels, a 1:1 final gear should give you 49 mph almost on the nose.
