With the needle clip in the lowest position, you should be running rich and the plug insulator could be dark grey to black. White or brown is good. very white could mean a too lean condition. Without an EGT it is difficult to say whats happening in the cylinder. The plug in your photo looks OK. Black around the edge is normal in a two cycle. If you are operating at sea level the third slot down on the needle should work fine. Have you made any changes in the carbs such as jet sizes? are you getting good coolant circulation?
You should first do what roger suggested and check the open throttle RPM. If the engine is running with too much pitch in the prop, you won't reach max RPM and it will quickly overheat even with a proper engine setup. Also get the EGT gauge working. This is a necessary gauge in a two cycle engine.
I don't know if it's just the camera angle, but the plug gap looks awful narrow? What rpm did you get that color plug at? If it was at idle for any time it may be more black than in cruise. Run the engine up to operating temp then up to operating rpm for a minute or two and shut it down. Then look at the plug. That will be more telling.
You need that EGT to work. It will tell you a lot and may keep you out of a maint. issue later.
Roger Lee LSRM-A & Rotax Instructor & Rotax IRC Tucson, AZ Ryan Airfield (KRYN) 520-349-7056 Cell
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