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

[Cfrench192]

Ok so to start off.   I bought an 86 chevy c20 a couple years ago.  The existing block was cracked at the time so I swapped in a vortec from a 93 suburban (Tbi). It has a very healthy 5.7L.   I left the swirl port heads on and converted the intake to an edelbrock performer and bought a reman carb (from summit carb # 17085000). I also swapped the cam for a comp cam part # 12-249-4.  I have replaced the hei distributer and it seems to still have some problems.  It is knocking at part throttle when it is hot.

I replaced the exhaust system with a 3" single exhaust and added an a/f sensor right before the new catalytic converter.  After studying the guage for a couple weeks I have come to a conclusion that the cruise circuit in the carb is too lean.  I have tweaked the idle circuit to maximum vacume 17 -18 inches in gear.  It idles awesome.  However I have a lean condition at cruise speed.  The apt screw is fully functioning .

My reson for tuning this carb is to get it to pass smog (in California).  The a/f guage reads 17.2 at cruise. Around town and on the freeway.  Does anyone know of a smaller rod that I can use in 73 jets that will richer the mixture?  I have been in and out of this carbuerator and cannot think of anything else that would cause this. Fuel pressure is about 4.5 to 5 psi.  The motor is running an egr valve with an air pump.  It runs good...  But not good enough

[77cruiser]

If you back out the APT screw the cruise should get richer.

[429bbf]

what size rods do you have.? I've seen 50 52 and 54ms i see most with 52s. fwiw

[Cfrench192]

I don remember off the top of my head.  I think 51 double taper.  73 jets.  Smog results are as follow:


 15 mph hc.  Co.  Nox.
1st number is maximum second is measurement
Hc max 122 hc meas 32....co max 1.96 measure 0.00. ....nox max 1591 measure 2973 (FAIL HI noX)

25 mph
Hc 77 max meas 22...  Co max 2.27 measured 0.00. .... Nox 1537 measured 2565.


Nox lowered at 25 mph but still failed.  Pretty sure it's the rod size.  Hopefully it's not the idle channel restriction.  Any ideas?

[429bbf]

your going to have to wait for someone that messes with all them nox tests . we don't have any tests in wyoming . we just make them run good and call it a day. fwiw

[77cruiser]

If you back out the APT screw the cruise should get richer.

Have you tried adjusting it, or is it backed out already?

[Cfrench192]

I backed out the APT.  Its definitely running richer now.  I backed it out all the way and threaded it in 1/2 turn.  Big difference in the cruise circuit.  I'm going in for smog tomorrow.  I will post the results. The cruise circuit is running as low as 15.3 A/F on the freeway (80 mph).  Before i turned the apt it was running 17.3.  If this fails I have a later model calibration kit coming in this week.

[Cfrench192]

I would also like to add that the distributor is hooked up to full manifold.  It has a new distributor with a light spring and 1 medium spring.  The test showed 03 btdc and passed the timing test. The total timing is at 32.  should I change the advance curve?  I am running a functional egr.

[77cruiser]

I think advanced timing contributes to high NOX too, may have to change the curve. How much do they charge to smog it?

[DieselPower]

I would think that with a fully and correctly functioning EGR, NOx would not be a problem.  Have you gone through any procedures to make sure that the EGR valve, solenoid, and associated wiring and vacuum are working 100%

[Cfrench192]

It's a 20$ retest.  I have a new egr valve so I will be replacing it before hand.

[Cfrench192]

Ok so here's the results still failing. 

15mph
Hc:  19 max 122    Co 0.00 max 1.96   Nox:  2769 max  is 1591

25mph
Hc :  13 max is 77 co is o.oo.    NoX is still very high at 2322.  Max is 1537


The cruise did richen after the apt adjust.  It had little to no effect for noX.

I guess I should play with the timing.  I tested the egr and it still works.  It can hold 15 inches of vacume with no bleed off.  If I open the egr with a pump, it will stall the motor.

I'm going to aim for dropping the timing to 8 btdc and switching the vacume advance to ported instead of full manifold. 

Does anyone have any other ideas on this.

[DieselPower]

How is your EGR valve controlled?  The valve itself may be fine, but the control side has a problem.

[Cfrench192]

The egr is controlled from ported vacume.  No electronics.  The truck has no computer and no sensors.  I readjusted the timing. 

The motor is at 10 degrees initial.  The distributor is all in at 2800 rpm  (27 degrees)+ 10 initial= 32 total.  When I bring in the timing with the vacuum advance I get 50 degrees.

It picked up some performance but I have a low end pinging and high end.  The a/f guage is still lean (17.3).

Due to this I think the rod/ jets are wrong.  Does anyone think I should jet bigger with a rod with a smaller metering area?  It's obviously not getting enough fuel.

[Cliff Ruggles]

The factory used a special ported vacuum slot for EGR, with a bleed-off port so it doesn't fully open the EGR valve at light load/light throttle cruising. Make sure your carb has the correct port, it as in the baseplate, passengers side on most models.

The correct ported distributor port is on the drivers side, front of the main casting to the right of the fuel filter housing area.

Since you are dealing with a "remanufactured" carburetor and not a rebuilt original unit, many items inside the carb may be altered, or not factory sizes.  They also miss-match components, so it's not surprising the carb is out of spec.....Cliff