1.  Check the carb to see if the main jet fell out it.  Do you have a new air filter or using a different type of air filter oil.  It sounds rich or could be a weak source coil on the stator.  if the source coil is weak setting the spark plug gap to around .018" might improve things temporarily. 

2.  Pull the fuel tank and check all of the wiring connections where the stator plugs into the wiring harness.  Make sure the female plugs grips the male plugs tightly  (Dirty or loose connections in this area are very common.  While you have the tank off check your stator values with an ohm meter.

3.  I have had 2 stators in the last month that had a broken wire going into the pickup on the stator.  Everything ohmed out to spec. until started moving the wires on the back side of the stator  causing the readings to fluctuate