"5280 feet above sea level" -- you must live near the Mile-high City?

12 degrees BTDC is too much for USA emissions standards. Factory spec is 5 degrees. It should run fine on either setting.

The basic rich/lean control is the idle mixture screw. It must be adjusted for "smoothest idle", then leaned out (turn screw in) until idle speed drop 50 rpm or so (check the manual for specifics or see Tune-up).

IMPORTANT: CO and CO2 output can be affected by *all* parts of the carburetor. So it is important to check everything, starting with the basics of fuel float level and ensuring their are no vacuum leaks. Check your repair manual for specific details.