Failed emissions then passed retest
 

Last year for emissions test on my 83SC, I took the cat out of retirement and installed it drove until hot and had it tested Low speed CO was .01 HC was 17, at 958 RPM. High speed HC was 18 but CO was 1.39 limit is 1.20 at 2625 RPM. FAILED! No way, I thought, so I turned the mixture screw 1/4 turn counter clockwise. took it on the interstate for a serious Italian tune up. 3d gear at 85 mph kept the rpm up I stayed there for 15 miles. On the return I pushed it to 6500 rpm and backed off several times then back to steady 6000 rpm in 3d. Pulled in to the free retest facility no wait. retest was HC 11, CO .01 at 2558 RPM idle was down a bit to 862 RPM. I guess you have to cook that CAT to make it work.

Based on my experience with my 81, 1/4 turn can yield a pretty decent mixture change. I went from 2.8 down to spec of 0.6 in about a half a turn (180 turn of Allen key). Your 3-way cat also needs a certain mixture to do its thing fully. Not discounting the heat factor. I would certainly get mine cooking to go but I suspect your mixture screw change did more than you think. In any case , a pass is a pass. Nice work.

You should always test with an analyzer to get it right. Too lean and you can burn a piston.

Take it to a shop that has one or find a buddy that has one, adjust as needed.

Did the smog tech not notice that that the cat was missing?

I installed the CAT prior to the test. I don't think an 83 SC can pass without a functioning CAT. I guess a hollowed out CAT would pass the visual test but would have difficulties passing the HC and CO levels.