I had this issue on my old 250 Rebel, and tried Chemtool with no change. Then I tried 4oz of SeaFoam and the issue was resolved after around 55 miles of riding. I'm no having the same problem with my 500 Shadow, but SeaFoam doesnt seam to clear it up. Still you may want to try it.I brought it to someone who has been doing this for years. I'll let you guys know because it's sure been bugging the HELL out of me!