I posted this "cure" before, but it seems appropriate again.
First disconnect the fuel line at the petcock, then drain the float bowls by loosening the drain screws a turn or two and let all the gas drain.
Tighten the drain screws, and pour Chevron Techron Fuel System Cleaner, available at most auto parts stores, until the carbs are full. Let it sit overnight and drain the float bowls again. Reconnect the fuel line and use the PRIME position to refill the carbs with gas.
You should find that the crud in the idle jets has been dissolved and the bike should run better.
I've used this trick on many of my own and friend's bikes with very good results. Yamaha sells a carb cleaner that has similar directions on the label, but it costs over $10 vs. about $6 for the Techron. I figured that if put in without gas as Yamaha suggests it would work more quickly. That seems to be the case and NO damage has yet occured to carb parts.