The best carb to use on a four stroke is the Walbro WY series using intake pulse.
If it doesn't run right there are two things to look at one is pop off pressure and venturi size, you are wasting you time with butterfly carbs on four strokes but if you insist then the venturi size is even more critical than the WY.
A stock 31 cc four stroke should have a venturi no bigger than 9mm