Check the adjustment on your choke cable, Yamaha actually uses a fuel enrichener, so when you apply the choke what you´re really doing is allowing more fuel to flow. If the enrichener isn´t closing and seating, then extra fuel will flow constantly. Also, with the YICS you need to keep the carbs up, they´re simple to tear down and clean but while they´re apart, replace the float valves and seats. If they´re bad or worn, they´ll also allow excess fuel to flow. The coils could be a possibility but what will usually happen is one coil will go bad so only one or two plugs won´t fire right and foul out. But that´s easy to check, pull the plugs and compae the spark. Good luck and ride safe.