What would cause one cylinder to run rich?

[Sasquatch]

Was doing the annual with my mechanic today. C-85, compression 78+ on all cylinders, all spark plugs looked good except the #4 cylinder plugs were a little sooty.

Been cold here. Last flight was about 1/2 hour at 1 degree F, with longer than normal idling to allow the engine to warm up before take off. (Yes I did preheat)

It was not bad enough to concern my mechanic, but clearly different from the other 3, just curious what you guys thought.

 

[reileyr]

 

[43-29080]

 

[Sasquatch]

 

[bob turner]

 

[cubdriver2]

 

[JimC]

 

[Sasquatch]