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I assume the test is with the engine carrying no load, that is, the car is not on a dyno. Idle and higher revs at no load are less clean than when propelling the car at cruising speeds. Mixture at idle and off-idle has to be somewhat lean, to hold down HC and CO emissions but all cylinders still have to fire every time.
Valves: temporary larger clearances, to minimize cam overlap effects.
Spark: all components in prime condition, new plugs temporarily gapped extra wide, say .046", which helps fire the poorly mixed fuel air charge under no-load conditions. Make your Crane ignition work for you.
Retard spark timing and reset idle speed on the high side. If you can, temporarily use a stiffer spring or two in the centrifugal advance to produce less advance when the smog tester revs it up. (These inefficiencies help emissions but nothing else.) Then adjust carbs for best behavior, biased toward lean, with these alterations in effect. Float bowl needle valves have to control bowl level reliably. Choose a warm day's warm hours for the smog testing, and arrive with the engine hot.
Something that worked for me on a well-used, 8-cyl Brand F engine that was showing a little blue oil smoke in the exhaust after downhills and waiting at lights, and oiling a plug or two: a dose of Chevron's Techron-containing "Clean-Out" in the fuel, for a tank's worth of driving ahead of the test. No more oil smoke or oily plugs. Six years later, still good, too old now for smog checks.
Record your changes so you can undo them after you pass, except keep as much spark plug gap as will not misfire under full load and still gives cold starts in the winter.
C O Greenlaw, Sacramento Cal
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