My 1990 Exciter runs great for 10 minutes or so and then boggs down at lower RPM's or climbing hills after warm up. Runs great at high RPM's. Shut it down for 20 minutes to cool and it runs great again for several minutes. Sometimes when bogging down, if you go easy on the accelorator, you can crawl it into higher RPM's. Once higher RPM's are reached it runs smooth until you slow down and try to start again or try to climb a hill. If you don't let off the accelorator it stalls out. I'm new to the sport so I have been reading all the threads I can for info. Seems like several owners of older Yamaha's have similar problems after warm up. I plan to clean the carbs but I'm not sure why, if dirty, they would only fail after it warms up. Anyone have any suggestions?