73=rich, 74=O2 sensor not switching because it stays high for extended time due to the over rich condition.
check vaporizer primary pressure, 2 - 3 psi.
check vacuum lines to and from trim valve for vacuum leaks, kinks, etc.
If mixer (carb) has been replaced as you state, the idle air has to be reset.
What color is the secondary diaphram in the vaporizer, if it is tan colored it has been known for heavy ends from fuel to bleed through diaphram then they get drawn into trim valve causing it to stick. Later diaphram is blue.
Also check the balance fitting at top of carb, should be an orfice not just a normal fitting.