I've been running a BR7ES plug in my '97 YZ 125 and the bike runs fantastic with this plug... I hear that this plug may be too hot for a 125... is this true? Can running this plug actually damage my engine? I ride only trails.
The hotter plug is compensating for rich jetting by raising piston crown temperatures. In effect, it's "fooling" the bike into thinking it's jetted leaner than it really is. Put the stock plug back in and fix the jetting, and you'll have a crisp running bike that won't melt a hole in the piston during a long WOT straightaway.
The insulator on the plug may be tan when you pull it, but the color of it really depends on how you had been riding right before you pulled it. If you pull it at the end of a wide open run, it will probably be more white. After a really technical trail section, it might be black.