Conner, for once I have to disagree. NOx only forms at very high combustion temps, usually leanout conditions, where there is the excess oxy to combine with the atmo nitrogen. This will only happen at very high temps, which is why they dilute the intake charge with the egr to prevent it (which hurts max power potential)

HC and CO are due to insufficient oxy, or a rich condition (two ways of saying the same thing). The hydrocarbons are left over from unburnt fuel. The carbon monoxide only forms when you burn hydrocarbons and there is inadequate oxygen to fully oxidize the carbons to carbon dioxide (CO2).

The bowl vent valve is a common failure point in the Mikuni.

Everybody seems afraid to rebuild the carb. It is not hard, or expensive to do this. Certainly easier than doing the mods to install a Weber, and cheaper to boot. The Mik is a pretty good carb...