I just put a new motor in mine, it was doing the same thing. If i pushed down on the motor the blades would go, but not as fast as they should. The gears in the motor were too tight, and i couldn't fix it so I spent $40 and got a used motor from
www.stangparts.com
Scott
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1992 GT 5.0L Convertible
BBK Shorties
Flowmasters
All Power
Super Chip