Small Engines (Lawn Mowers, etc.): vibrating kohler engine, belt pulley, balance weight

Question
I replaced a bad 16hp kohler engine with a used 15.5hp kohler engine.  Everything bolted up right but the engine is vibrating at all speeds.  Disconected the belt pulley and the shaft seems perfectly straight but, it still vibrates.  Any ideas would be helpful.  Thanks.

Answer
 Hello Rob:

 The Counter Balance Weight Inside the Engine is Chipped or the Pulley on the Crankshaft is Out of Balance. The Flywheel May be the Wrong One on the Engine. Other than these Possible Causes, I would be at a Loss to Explain. Hope this Helps. Let me know what Happens, Please. I enjoy knowing the Outcome. Thanks.

 Good Luck

 Respectfully

 John