You probably have the regulator to lean. Start with the regulator screw set flush. Watch the short fuel tube between the regulator and the carb while turning over the engine. You should see some fuel movement in that tube. If not, the regulator is probably set too lean. Back out the set screw an 1/8 turn at a time until you see fuel flowing there. Sometimes priming the engine will help get the regulator working. I would try that.