Hello Mark,
It may not apply to you but could be worth looking at.
I as well was having the same symptoms and posted it "Unable to lean for max RPM, why?"
End result The mixture lever was hitting the clevis before it could hit the stop on the carb. In short, there was about 1/8"+ gap between the lever and the carb so I was never getting full rich like I thought. We ground out a little on the lever so it would have clearance to swing into the clevis and give me full stroke. The plugs were very clean and almost too lean.
It appears that this plane has always been like this for 500 hrs and no one else (2 previous owners & builder) realized there was a problem. Good thing most of the flying has been around here at the higher elevations and not down at sea level on a colder day.
Good luck
Tim