[crash12888]

The throttle butterfly is being held open slightly by the throttle stop screw or your accelorator linkage.
The butterfly needs to close 100% inside the carb, then screw the throttle stop in just until it touches.
Make sure your linkage is not holding it open when you let off the pedal.
Adjusting the governor will pick up some speed for you.

Edit: Might as well explain whats happening.
When you let off the pedal it grounds the ignition and kills the spark.
When you do that and coast down an incline with the throttle held open even a little, the motor continues to spin and draw fuel but with no spark, then push to go you fire off all that raw fuel.