I have a 2005 660 that I can’t figure out. Quad came to me after sitting for a year. Needed the radiator replaced ( due to a crack in it) and a tuneup. Replaced the radiator with an aftermarket one ( no issues) cleaned the carbs( I think 7times after reading all the forums) grounded the reverse limiter at the CDI box to the negative of the battery. Cut the black/ yellow wire and put heat shrink on it at the CDI box. Put the 2 wires together for the clutch switch and even cut the red wire coming off the stator. None of them have fixed the problem. Quad idles just fine. Give it a small amount ( less then 1/4) and it’s all good. More than 1/4 and it pops and dies. Not like a revlimiter sputter, just dies. Valves are in spec the only thing I haven’t done is pull the valve cover and check timing. Oh I also tried a new CDI box with no luck. Battery is charging at 12.5 and when I give it a little throttle it will go to 13.8 or so. All stock jetting cleaned stock carbs and put back on. Also did clear tub method on float setting.