67 dart emissions

I have easily passed emissions with many older cars. I set ignition timing to about 14 degrees BTDC with vacuum advance connected. With car off check the idle screw settings. This is done by counting turns of each to fully in, small screw driver. Do not over tighten, it will deform the idle passage. If the turns do not match then, split the difference and open each the same. An example, Left 1.5 turns, right 2, then open each 1.75 turns. This gives you a starting point.

When the engine is fully warm. This is 10 + miles of driving and choke fully open. Adjust idle mixture with tach, for maximum idle using the same small increase on both. I use 1/8 turns. Re-adjust curb idle (throttle stop) to normal specification plus 30 RPM. Repeat mixture adjust, it changes with respect to curb setting. Then screw in mixture screws, to reduce RPM by 30 RPM (the correct curb idle), with equal turns of mixture screws. It might not take much, perhaps 1/8 to 1/4 turn.

The curbing of idle with mixture results in a slightly lean mixture, decreasing CO. To lean excessively leads to misfire and increased HC. The emissions is a compromise between the two.

A good running older engine will run well at CO of 1%, if the limit is 3.75% there is good margin.

When going for the test, make sure the car is fully warmed up, do not shut off in the waiting line.