"the height of the nozzle is important and should be just below the center of the venturi." That's the nozzle tip just below center in the hole of the venturi. At the point where the nozzle and venturi is in contact, the surfaces gets worn. When that happens the nozzle eventually rise taller in the venturi. Wear is caused by a weak spring that makes the nozzle rattle and vibrate. The highest air speed is obtained in the center of the venturi and that is where the best atomizing of the petrol is made. So with a nozzle tip just a tad under the center, the droplets is lifted into the strongest air flow. With a too tall nozzle the mixing is disturbed by the airflow around the pin that the nozzle is. In the pic. is an example where the nozzle and venturi is so worn that the tip is some 2mm's above center. Height repaired back to specification with a narrow ring of gasket material.