TTT

 

You well never get a dark plug running c12, that stuff burns super clean and the bike will blubber before that. If the one on the left is with c12 that is a great shade and the jetting should be fine being that the c12 flows more with the same jets and not less so it's not going to make it leaner, instead running c12 makes you run richer and will force you to go down a jet or two from the straight 91 jetting.

 

Plug color deep down inside indicates top end jetting. Middle indicates midrange jetting & the top of plug porcelean indicates low end jetting. Many people beleive you need to do throttle chops at each position to indicate jetting which is true but is a hassle. You can get same results by running it constantly for 20-30 mins making sure you hit each throttle position for a few minutes each. In other words, taxi it around for a few minutes then run mid throttle on a trail for a few mins & then try some extended wot trail runs or hill climbs & then check the plug. I also think you'll find that you can jet the best by feel/seat of the pants. The top end is probably easiest to feel by wot runs. If it seems to take a while to reach top speed its slightly rich. If it seems to reach top speed before you reach full throttle or if it seems to accelerate or surge when you back off from full throttle then you are slightly lean on top end.

I also agree that race fuel would make it run richer & doesn't make sense that the left is race gas & right is pump gas, sounds like the opposite!?
The left pic is tan on top but whiter down inside so I would say the bottom end is good but the mid is lean.
The right pic is slightly rich all around & probably the safeset to ride until your seat of the pants spider sense takes over, LOL....

 

More than you may want to know about plugs right here

 

This is why I was asking about the plugs. The left is mix and right is not. I took the plug on the left out and left the little screw on nobb so I would not confuse them. It runs great both ways but I do think it is a little better with the mix. Now the guy that did the work on my motor, did say I could go 2 gal c12 and 3 gal 91 but no more than that or the jetting will need to be changed.