I've got a 1996 918cc motor that is running rich. I replaced the first set of plugs and the bike ran great for a couple weeks. Now the new plugs have fouled on me. From what I can see looking at the plugs the bike is getting too much fuel and not enough air - hence it is running rich and fouling the plugs.

How do I adjust the fuel mixture on the CBR900RR motor?

 

If all the plugs are black, it must be something that affects all the carbs. It could be a dirty air filter or the choke could be stuck on. The jetting could have been changed to bigger main jets. The floats could be set to the wrong level. The needle/pistons could be sticking open and if yours uses a fuel pump, it could be forcing fuel past leaky float needles. Or maybe you live near Leadville, Colorado. All could cause rich running. If it was mine, I would start with cleaning the carbs, especially the piston/diaphrams.

 

Don't forget to synch those carbs, too.

Cheers