If the regulator sticks, it's nearly always in the open position. When you throttle back the regulator spring pushes the plunger down harder on the seat, letting less fuel in. My hunch is that your reg is set a bit too rich. Try going in 1/4 to 1/2 turn and see what happens. YS engines do not like to be overly rich on any of the settings except idle.

I disagree with not adjusting the regulator. The factory setting is just a starting point, and sometimes it's right. It will vary with type of fuel and variations in weather. Don't be afraid to adjust it, that's why it has a screwdriver slot.

They usually stay set once they are adjusted, unless there's a big change in fuel or temperature.