Try opening the high-speed needle 1/2 turn -- if it still quits (splutters to a stop) it is too rich. If it cuts abruptly, or just dies away without blubbering, it is too lean. If it is lean, & assuming that it is open more than would be normal -- you may have dirt in the needle valve assembly -- remove the needle, clean it off & flush fresh fuel through the needle valve body. Check to ensure that the air-bleed hole is not obstructed with dirt & that you can see the end of the needle ~ 1/2 way across the hole.