Small Engines (Lawn Mowers, etc.): john deere stx 38 vibration, fly wheel, john deere stx 38

Question
Hello there, I have a 1991 John Deere STX 38, (yellow deck) and i replaced the motor with one i had off a parts tractor because the original motor was constantly blowing blue smoke, however i took apart the whole motor and everything looked perfect. but anyway. with the new motor installed. the whole tractor vibrates like crazy. the motor doesn't even feel like its vibrating nearly as much as the tractor is. This happens with the deck disengaged. This happened before with the old motor and i believe it was a motor mount. As i increase the rpm it gets worse. it sounds more like a rattling once it stays at high rpm for a period of time. I mowed my lawn and couldn't feel my feet through my boots when i got off it. However the motor runs great starts right up and has much more power than the other one did. by the way its a 12.5 horsepower kholer engine thanks

Answer
Dustin
Somewhere in this mower things are not lining up, you said it does this when the mower blades are not engaged. The problem can be the flywheel may be unbalanced they have small steel plates that are used to balance a fly wheel.Try to remove any belts that are hooked up to the mower then start it if it still vibrates then you know for sure it is a engine problem rather than the equipment. If it quits start looking at the pulleys on the equipment to make sure they are in line with the engine especially the main engine pulley.