Throttle shafts have wear and air leaking past. ***INTAKE LEAK*** KEIHEN used felt as a seal!!!!. Replace with o-rings. Something resistant to ethanol. REQUIRES COMPLETE TEARDOWN AND BUTTERFLIES REMOVED. Not too difficult if you´re handy with screwdrivers and regularly oiling your chain. If you can´t do either, then get a carburetor friend. Take pictures before removing complex assemblies. Have a large table available and give it a day or two to complete. Don´t rush you´ll get it.