I have the same issue.
Last year, my bike wouldn't start at the beginning of the riding season. A new battery later as well as a complete pull apart of the bike to clean the carbs n such, it was running fine. My issue last year was I didn't run it enough between the seasons and add the proper chemicals to the tank.
This year, I didn't add chemicals but I sure to run it for 10 min every 2 weeks. Battery works fine, and on choke it runs ok but if I push on the throttle it instantly drops RPM or dies completely. Any idea what to do?