Sounds to me like it could be a sign of being too lean. Open needle about a full turn more than you have it now so it will not lean out when you open it all the way when you start it, then start the motor open throttle all the way. Open needle more till it starts to 4 stroke then back it off (in) 1/4 to 1/2 turn ( it will start to smooth out ) I do my 4 strokes this way and I know for sure they will not lean out when in the air running wide open. To many guys try to lean out 4 strokes and the shutter and groaning of the motor shows it is too lean then it stops or back fires then off comes your prop nuts, washer and prop.

Just my 2 cents worth
Doug