The bike probably needs a good clean and tune. Oil and filter, air filter, spark plug, all that stuff. Got a manual yet?
Most likely youve got some water or goo in the carb.
Drain the carb, then with the float bowl drain open, turn on the gas and make sure gas flows out the drain freely. On the right hand carb there is no float bowl but you can take the brass bolt/plug off the bottom and make sure there is no goo in there. Also good to turn on the gas briefly with that plug off too, make sure plenty of gas flows to that side.
Pull the gas line off the carb, hold a cup or something under it and make sure the gas flows free there, if not the tank has to be emptied so you can remove the valve and clean it.
Riding isn't the only fun with older bikes.