at 85000 miles, I noticed there was more vibes from the engine. I was puzzled as the fuel consumption & performance were the same.

Tried two different tankful of fuel with no change.

I thought it was my imagination but as a last resort, I had the plug changed (CR7). The one that came out looked very good so I looked like a waste of time but...

The engine has become smooth again (more like sewing machine than a tractor) and I am a happy boy.

How often do you guys change your spark plug. I also read some found CR8 (colder plug) make the engine run better. Any ideas as to why??