General Category > Quadrajet Carb Talk and Tips
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
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