There may be a leak between the carburetor and the engine mix intake. Make sure the carb bolts are tight.

The other possibility is that various passages inside the carburetor are somewhat blocked. The best fix for it is to get a can of carb cleaning fluid from Auto shop (about 1 gallon), and submerge the carb in it for a couple of hours, and take it out and air-blow the cleaning fluid out. You need to take out the diaphragms, and the needle valve first though before the cleaning and assemble them back. It happen to my DA50 and the the cleaning process fixed it. I fix the carb of my lawn mower the same way.