My '74 cb360 (4700 miles) runs fine to about 5000 rpm & then the left cyl stops firing. I checked the plug & it looks rich. The right side is fine. The timing's set properly, and the plugs are gapped properly as well. The air idler screw on this carb is turned out almost as far as I can get it. I can nurse it up to about 9000 rpm in first gear, but whenever I try to open the throttle, the left cyl dies. I have tried to clean the carbs with Gumout spray carb cleaner, but haven't taken them off the bike yet (I'm lazy & trying to avoid doing it if I don't have to) Any suggestions?