John,
Remember the "trick" Nissan used was to set the timing
at 0 BTDC, but with air injecting into the exhaust
manifold.
This allowed for UNBURNT fuel to be post ignited after
combustion. At that time (1968) only idle rpm
was emissions tested, so this did work.
It also lead to some other issues. (like overheating).
NOx was not tested at the time, but your correctly by
retarding the timing it lowered peak combustion
tempratures, reducing Nox emissions.
Glad my California Emission days are behind me, but
retarding the timing, adding a couple of cans of "dry
gas", fresh oil change... and keeping the car up to
temp before the test always got me through.
Haven't kept up with stuff for a few years.
Tom
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