Failing emissions

[gbskunk2006]

My 86 GMC w\4.3 litre (carburated not EFI) keeps failing local emissions testing, specifically the Oxides of Nitrogen (NOx) reading. 

My last test produced a reading of over 2600 ppm (max is 1978 ppm), that after having installed a brand new EGR valve, taking her for a very good run on the highway to clear out any carbon build-up and make sure my catalytic converter was good and hot. Just for good measure, once my tank was empty I added a litre of methyl hydrate to about 15 litres of premium gasoline (91 octane) just before taking her into the testing facility.

The other two emissions tests for HyrdoCarbons (HC) and Carbon Monoxide (CO) passed with flying colors.

Any other ideas are much appreciated!

[Irish_Alley]

when it comes to cars not so sure but chemistry i loved never took advantage of it but heres what i can tell you. too much oxygen(O2) will turn nitric oxide (NO) into nitrogen dioxide (NO2) so that being said i think your too lean and burning too hot, i would try to turn the fuel up a little, run it with 87 and advance the timing a tad. is this at idle or 2k

[SUX2BU99]

It is entirely your option to change the GVWR rating of your truck. ICBC allows this and by putting it in the 5001-5500 kg GVW category, you are Air Care exempt.

But about your issue, I had a 90 Daytona with a 3.0 V6 and 5 speed which seemed to keep failing on NOx as well. As I recall, it was the timing was much too far advanced which was creating the high reading. My mechanic knocked it back to 0 and it read fine. Bumped it up a little bit afterwards for better throttle response.

[TexasRed]

How's your cooling system? NOx is caused by too much heat. You could try richening up the mixture, running the test in a lower gear (if possible) or bringing the truck when it's cooled down a good bit.

[gbskunk2006]

From the responses here, and from what I found on Google, I think my next step will be to retard the timing and richen my air/fuel (again), although SUX2BU99's other suggestion is an interesting one for me and any fellow BC'ers out there:)

It is entirely your option to change the GVWR rating of your truck. ICBC allows this and by putting it in the 5001-5500 kg GVW category, you are Air Care exempt.

I forgot to mention that my last test was my 4th attempt!  Will keep you posted ....

[Irish_Alley]

put your timing at 0 deg. and richen it up see what that does. do you have to pay to get it tested?