A single light sparkplug can result from things like:
-the exhaust valve is leaking slightly (or has a guide that is more worn than the others, by a bunch).
-the carb's float is sticking (dimples on the tangs is this one's most common culprit), so it runs lower bowl than the others.
-the other cylinder on that coil is not getting as much spark (this one is 'hogging' the power, so to speak). Plug cap resistance, wire condition, or even (rare) a bad coil can do this one. My own 50 did this in 2007, with #2 always white and poor performance below 2000 and above 6000 RPM. A new coil fixed it up.
-likewise: jittering timing, most often seen on the 2-3 points from a bent spark advancer shaft, will make one of the 2 center ones lighter. Really bad ones that jitter time on 1-4 and 2-3 both will cause 2 lighter plugs.
-on hi-mileage engines/carbs (40k minimum, here), the needle jet may be oval, with the wide end toward the back of the carb: the NEXT cylinder in the firing order will show a lighter plug. This can happen if the carbs were cleaned and the needle jet installed backward from its previous position, and it was worn enough to become egg-shaped (wide toward the engine). While this is pretty esoteric, we are seeing it happens just because these bikes will GO that far...mine included. All 4 of mine were in poor shape at 128k miles, and cleaning them made it MUCH worse when I didn't align them all the same: I ended up taking them all out and turning them all around, then installed a smaller mainjet to fix it. Gotta replace them one day...
