I think you'll find the problem with the clutch is because of the leverage on the clutch itself, the armature is VERY small and struggles to give ample leverage to make the clutch light.
Granted, I've got a good grip and don't really notice how difficult the clutch it, but I'd imagine given a few more thousand miles I'll begind to notice it.

Ideally, I think a good place to start is by listing what bike you've got (X, R Z) year, what mods are on it currently etc.

Otherwise it could just start a guessing game.

How do you know that the bike is running rich?

Also, im pretty certain that changing the spark plug won't make a lot of difference.

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09 XT660X:

CCC Stainless Rounds, Stage 1 DNA, Stage 2 DNA, PCV With Custom Autotune Map, PCV AutoTune, Home made O2 eliminator, Self machined Fork Spacers, DIY Autotune Switch.