Small Engines (Lawn Mowers, etc.): Cutting Deck-too much vibration, lawn tractor, 7 seconds

Question
I have a sears Lawn Tractor 18 HP that is about 2 yrs old. Problem started when I changed the blades. I bought both new ones from sears parts. Problem does not occur until I engage the cutting deck. 1-7 seconds, no vibration. 7-15 seconds vibration begins and at about 12 seconds is at its worst. Then the vibrtion decreases gradually until only normal vibration at 15 seconds.This cycle continues as long as the mower is running. I have lossened and tightened the blades several times to make sure star on blace center is centered. Any suggestions appreciated. Thanks

Answer
 Hello Ken:

 Check the New Blades for Balance. New Blades Sometimes get Through the Inspection Process and Are Not Balanced Properly. Also Check the Quill(Towers the Blades Bolt to) for Bad Bearings. Since you had this Happen After the Blades were Changed, I Think the Blade Balance would be the Problem. Hope this Helps. Let me know what Happens, Please.

 Good Luck

 Respectfully

 John