YOu need to start with the basics. Clean the carb, well. Air filter too. Eye the reed valves while you have the carb off. Pay special attention to the air bleed passages in the carb(opening/s at the carb throat that mix air with fuel in the pilot and main jet wells). Write the numbers of the main and pilot jet down while you have them out of the carb. Remove the needle jet to clean the carb(they aint clean clean unless you do). Check where the float level is before you remove the float/s. Note where the needle clip is placed and the setting of the fuel/air mixture screw.
Armed with this info and the knowledge that the carb is clean, the reeds are good, etc. you can run it and see if it is any better. If not, you can then make a decision on re-jetting it properly.