86 c20 4 bbl carb (failing smog high nox new cat)

[Cfrench192]

When I add more timing it starts easy when cold.  Right now it struggles a little at 10 degrees initial.

[Cfrench192]

This week il try to mess with the timing.  I'll put the stock spring back in the distributor (it currently has one stock spring and one light spring). It's supposed to have 2 medium springs.  I also found that the distributer gets 15 lbs of pressure at vacuum advance (1500 rpms).  I will bring the total down a little bit with the adjustable vacuum advance.  Il keep the initial around 6-8.  Thanks for the information everyone.

[Cfrench192]

I retested the truck today.  I have improved the nox results at 25 mph.  I went from 2565 nox down to 1996 @ 25 mph.  @ 15 mph my nox got worse.  It was at 2973 and is at 3292.  Co2 percentage at 15 mph is 11.1.  25 mph it's at 10.6.
Before the test I altered the timing curve.

The distributor has a heavy spring and a medium spring.   Base timing is at 4 degrees.  Total timing is around 32. 

I took some vacuum notes at the egr port on the carb and the vacuum advance canister. 

The egr port doesn't get vacuum until about 1500 rpms.  The smog speeds are 1378 rpms @ 15 mph and 1442 rpms at 25 mph.

The vacuum advance gets about 2 lbs at smogs speeds.

Is there anything I can modify in the carb to get more vacuum out of the egr port?  I have all the tooling need if so. 

The air fuel guage reads 17.3 at these speeds.  At idle in park if I add 4 lbs to the egr valve the motor gets close to stalling. 

The carb currently has 73 jets and I believe 54 m rods.  (The way it came from summit racing).

There is a guy in my town that tunes chevy carbuerators.  He has a sniffer machine.  I might just have to have him look at it.  His rate is around $90 an hour.  I would hate to spend this amount for something I could possibly fix myself. 

The timing sparks around 17 degrees at these speeds with little egr vacuum signal.  The plugs are gapped at .045

[Cfrench192]

Nox at 15 should be around 1591.  @ 25 it should be 1537.