General Category > Quadrajet Carb Talk and Tips
86 c20 4 bbl carb (failing smog high nox new cat)
77cruiser:
Jets don't change the idle. If the IABP is big it will have some affect.
73ss:
I tried to reply earlier but had PC problems.. Anyway something is amiss here. Your a/f gauge shows around 17? The CO2 reading from the test is 11.7? the two should be very closely matched. 11.7 would indicate quite rich. At 11.7 the NOx levels should be quite low. Can you see what your gauge says during the test? You mentioned a hissing from the heat riser? Is it open? Heat is the enemy with NOx. Here in Ohio a few tricks to pass are. Install a cooler thermostat, Take the test on a cool day when the truck is still somewhat cool. Take the test first thing in the morning, in the summer. These are band-aids, But they can get you a passing test. Hope this helps
Cliff Ruggles:
Are you using the APT system to fine tune part throttle A/F.
Changing jets changes part throttle and heavy/full throttle A/F at any APT setting. The "M" series metering rods are correct for the later SMAB carburetors, and provide a .010" taper on the upper section for fine tuning the APT system.
I suspect the troubles getting thru smog are more related to the carburetor being commercially "remanufactured" than anything else. The "remans" from Summit are among the worst I've seen.....Cliff
Schurkey:
--- Quote from: Cfrench192 on February 21, 2015, 03:50:51 PM ---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.
--- End quote ---
50 - 32 = 18
Heavy vehicle, big tires, I suspect too much vacuum advance.
Progressively restrict the VA to 15 degrees (10 if needed) and see what happens.
ALSO those swirl-port heads need less timing. Reduce your initial or restrict your centrifugal a couple of degrees.
HOW HOT DOES THE ENGINE RUN? NOx is heat-related; which is to say that it's combustion-temperature related. Overly-warm coolant contributes to NOx formation--but "overly warm" probably means 210+ degrees.
Cfrench192:
The thermostat guage stays way below 200 with a 190 thermostat. I might have stated that the rods were 51 m. They are actually 54 (so hard to read) m series. I'm going to leave these components alone.
I haven't resmogged yet. The a/f guage goes over 20 at smog speeds 15mph and 20. When I take the air cleaner off there is a loud hiss (Don't know why). When I look down into the carb into the power piston hole I noticed that it is all the way down at idle in park. When I punch the throttle it jumps up. Yes I was trying to tune with the apt screw due to the fact that I do not understand carbuerators.
Is this hissing noise normal?
I'm going to start playing with the timing a bit. For some reason I believe that something is leaking in the carb. It has new gaskets. I would also like to add that it has the strongest apt spring offered in the kit I bought from quadrajetparts.com
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