1984 Other Volkswagen Models 84 vanagon GL running too lean

It failed the bi-anual Smog Check.
HC 63ppm passed. CO .56% passed.
NOX 1505ppm failed. Max allowed 1076ppm at 2722rpm 15 mph.

I hooked up an AIR- FUEL Ratio GAS ANALYSER to the tail pipe.
Start the engine cold. Air-fuel ratio starts at 11.5.
Pointer rises steadily as engine warms up.
When fully warmed up ( after 10 minutes and the tempreture guage points at center ). Air-fuel ratio now points at 14.3
Increase the RPM to 2200 , Air-fuel ratio drops to 13.9, then rises to 14.5
Let go of the gas, Air-fuel ratio rises to 15.0 then drops back to 14.3.

I turned the mixture screw on the air-flow meter.
Clockwise to its bottom and counter clockwise 5 turns. there is no change in the Air-fuel ratio.
I left the screw at 2 1/2 turns from the bottom.

There is no EGR valve on water cool engines.

Any clue as to why the Air-fuel ratio is so lean ?

Any help will bw appreciated.