Too much oil and a highly polished starter shaft makes for a one way clutch that slips. Thoroughly rinse the one way clutch bearing with acetone. Now remove the backplate and starter shaft , and with some 600-800 grit sandpaper, sand the starter shaft the one way clutch bearing rides on down the length of the shaft taking care to not scratch the surface that rides in the bronze bushing in the backplate. Clean super super well, and reinstall in the backplate/reinstall backplate on engine. Install clean and DRY one way clutch bearing and rest of starter system. This should work well for a long time if you do it right. Ways to prevent this from happening in the future is not to run high oil fuel and super rich. 12% oil and 20% nitro is perfect for regular running. Don’t use “basher” fuel because it’s got too much oil in it - usually around 16%. Way too much.