BIZARRE Spark Plug Color?

[beamn7]

I know you said it doesn't get hot but I would certainly put a t-stat in it and 50/50 coolant mix. The main function of the t-stat is to slow the coolant flow down enough to allow the proper amount of heat transfer from the block to the coolant as well as keep it at the calibrated temp. Straight water has a lower boiling point than 50/50 mix. Straight coolant or straight water can cause overheating.

As for the plug, I would say that cylinder is running hot/lean just as others have mentioned.