something could of build a nest inside the exhaust plugging it. Are you getting a good fuel flow to the carb, line may be gumed up? fouled plug or plugged vent lines on the carb.
First thing is to make sure the front end is very clean, you dont want any trash falling into the engine while its open. Next turn fuel off and run the bike out of fuel or drain the carb. This way the fuel will not run into the airbox and damage the air filter while on end. Chock the rear tires and using a floor jack and maybe a block. Jack it up as far as you can. It will lift up very easy with two people on the front. On a fresh morning I can do it by myself but to be safer use two. another thing to look at is the level in the fuel tank, if its full it will come out the gas cap vent line. maybe drain or siphon some out.Whats the best way stand on end?