1st off I would take the needles out of the carb and the carb itself off and clean it all out to make sure there isnt gunk built up restricting fuel flow to the carb. Thats where i would start. There are also alot of other things i would go through such as making sure there are no air leaks in the tank and fuel lines. Also make sure u are using good fresh fuel. And check the compression on the engine.... Turn the flywheel over and it should give u resistance... if it bleeds off that resistance pretty fast then the Piston and sleeve set is prolly shot. And if its shot once the engine warms up its not going to want to stay running. There is alot that could be going on, and trying to diagnose problems like this over the net is very difficult. Its going to be a trial and error type thing im sure. Good luck and if u have any question post up. Everyone is usually pretty nice about trying to help eachother out around here.