I have my MVVS 58 Prosport mounted inverted in a 30% YAK. I have not run the motor in over a year. I could not get the motor to fire this morning so I took the ignition cap off and found that it was full of black oil. I cleaned the oil out of the cap and replaced it on the stock spark plug (BRISK UR17). Two flips and the motor fired right up. I ran 24oz of fuel through it ( Motul synthetic- 40:1 -91 octane) . The motor ran very well I was pulling 6500 rpm on a NX 24x8 prop w/pitts muff. I pulled the ignition cap off and it was full of black oil. I pulled the spark plug out and it was completely black, seems like the motor is running too rich. The motor transitions well from idle to 6500 rpm no hick-ups its smooth but not instantaneous. It seems like the plug does not have a good seal. Is this common? Do you think a new plug might seal better? Do you think the motor is set too rich? I have not noticed any oil in the ignition cap previous to this incident.

replace the plug. sometimes they leak between the ceramic and the center electrode, this would fill the plug cap up as you said

I ordered two new plugs. Any operating difference between a NGK CM-6 and the Brisk UR17? Is one hotter?