I've got a 01 660R, just bought about 4 months ago and was just fine till around 2wks ago. Starts good, idles good, everythings good except 3/4 rpm and up in all gears sputters/ cuts out/ backfires somewhat, and sometimes dies if I dont let off within 5 seconds or so. Have been surfing the web for 2 wks now and read 100 posts and tried 20 different things. The parking brake has been blocked and unplugged, reverse has been bypassed, plug check and cleaning, coil wire check, new spark plug, ran seafoam through it, few stator test, unplugged single cdi wire on bottom, gonna clean the air filter good tonight, but its not too dirty. 5th gear will sputter back and forth for about 10-20 seconds and then it will have full power and start to top out fine, then cuts out again, sometimes dieing. I changed the oil to synthetic about the time this started happening dont think its connected in anyway. I got it with a stage one cam, jet kit, exhaust, and a k&n filter, all was fine until 2wks ago, kind of hinting towards carb problems, ethanol maybe, but it sounds so much like a rev limiter kickin in. Thanks for all comments