It sounds like you need to adjust your carb.

Peak the RPM's (leaner) with the high needle and back off about 200 rpm (richer). Lean the low needle until it dies when you open the throttle, then open (richen) the low an 1/8 turn at a time until the transition is good. Repeat the steps for the high again. When your low side is correct the engine should be able to idle for a long time without changing RPM'S.

If your engine is new the high end should be a little rich. Make sure the motor is making a slight gurgling sound at full throttle.

Good Luck