If the bike has been sitting for a few years, you may just have a gunked up carb. The volatile components of the gas evaporate leaving behind a varnish-like residue which can clog up the carb jets. One of the easiest things to try before taking off the carb and cleaning it, is to put some Seafoam or, even better, Berryman's B12 Chemtool, 1-2oz/gal (no more) in the gas tank and try and run it for a while and see if it gets cleaned out. You can get those at any auto parts store or Walmart, Seafoam is about $10 a can. Berryman's is about $3 a can. If that fails, you may have to remove the carb and get it cleaned more thoroughly (disassembling it and cleaning out thoroughly every orifice).
And dump the gas too if that has been sitting for years and that's what you are burning now -- put fresh gas in the tank.