May just have a blocked jet. You can try purging the carb by taking the air filter off, getting someone to hold the throttle while you cup a hand over the carb throats to starve it of air. Sometimes it's enough to pull any blockages out of the jets. Use a mirror to see if you have a decent fuel spray while the engine is running too (don't put your face over the carb in case it feels like coughing a fireball up at you). Either the primary jet is clogged up or someone messed with the air/fuel mixture screw and the idle stop screw (option #3 is carb needs rebuilding)