Douglas,
Hi NOx is almost always caused by higher than normal combustion temperature.
The higher than normal combustion temperature is what enables the reaction
to take place which forms NOx.  The most common causes would be over
advanced ignition timing or a mechanical problem that would cause the timing
to be too far advanced such as a seized mechanical advance unit.  Another
common problem is an inoperative EGR system since putting a metered amount
of exhaust gases in with the air fuel mix also keeps combustion temperatures
down.  An overly lean air fuel mix also causes elevated combustion
temperatures.
--
"They that can give up essential liberty to obtain a little temporary safety
deserve neither liberty nor safety"

Benjamin Franklin