The throttle on the echo carb works on a lifting mechanism. The throttle cable attaches to the top of the throttle and as the hand pulls on the trigger the cable pulls on the throttle. As the throttle turns, it is lifted and due to its design, the further it turns the higher the lift. The amount of air/fuel is regulated this way.
Sometimes a carb change requires a special look at the throttle to make sure it lifts enough for engine to run at proper speed. Pay special attention to the screw that sits under the throttle at the max opening. The throttle needs to lift up to pass over the screw head. Sometimes it hits the screw top and fails to open all the way thus less RPM.