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As mentioned before Nox is caused by lean condition How about carbon deposits on back of intake valve(s) and in combustion chambers, ususally on top of pistons. The carbon deposit aborb the fuel and lean out the a/f mixture. Make sure fuel pressure is up to spec that would ensure proper workings of fuel pump and fuel pressure regulator. Change fuel filter, cause of insufficient volume of fuel flow at higher RPM. Make sure coolant resevoir cap is working properly, cheap and overlook often. Have radiator rodded to ensure proper cooling capability. Make sure spark plugs, wire, cap, rotor, coil are good. Varify w/ test equipments. Make sure brake, trans, tire are not adding additional load on engine. This might be able to be varified by your gas milage before and after tune up. And always, always change your oil and filter before smog test. HC in engine oil and fuel evap (charcoal canister) contribute to HC reading. Factory service manual section 20 page 8 for active carbon tank for pollutant-free fuel tank venting(layout). Check the vacuum hoses related to this as well. If vacuum hose #11 had a leak the carbon canister control valve will not get the proper signal and would allow canister to purge as well as intro of unmetered air into intake and lean mixture.
I will smog my 951 in about a month or so. I am glad you posted your questions as it gave me an opportunity to look up the systems and review for my own preparation to smog. Good luck.
James
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