67 dart emissions

RLF Cuda said

...And I stand by my point that more advance improves HC/CO emissions. The reduced timing in the late '60's and early '70's was done to reduce NOx emission as NOx goes up quickly as combustion temperature rises above 2200°F. Reducing timing to 0° or so reduced NOx. But in increased HC/CO. Improved combustion efficiency reduces these emissions which helps you meet tailpipe sniffing emissions tests. Plus increased ignition advance (to the point where knock is not heard though) increases fuel efficiency tremendously. I got my '67 383 4sp to pass '78 emissions standards in California, but it was a new tight build with 10.5:1 compression and a 284/484 Isky cam. I ran 10° initial advance and 37° total advance with a fairly steep advance curve all in at 2200 RPM. This engine would idle down to 400 rpm smoothly, a sign of good ignition and correct A/F ratio at idle.Hope this helps, and good luck.

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I agree with most of what you say with the exception of the ignition timing.
Put it on an exhaust analyzer & watch what happens. I promise you adding advance to it will not improve the HC/CO if it is already set correctly. I've been doing Emissions testing & repair as a state ceritfied technician since 1984 & I can't begin to count the # of cars that came in too far advanced & failed for hi HC or CO, & yes NOx too, that as soon as I reduced the amount of advance in it they came into range. Even a lot of cars with the timing set to specs that probably had weak catalytic converters on them would fall into spec with less advance. I should state that in my comment I assumed the test is an idle speed test being performed & not a loaded test. But either way just throwing more advance at it isn't the answer regardless of the test. Ultimately it requires all aspects set right to run efficiently. A/F ratio, timing & mechanical condition. I agree the 360 heads may be too much for the 273. Again that's why I say take it to a shop & make the adjustments on a gas analyzer. $100 isn't much to solve a problem & know better where it stands.