General Category > Quadrajet Carb Talk and Tips
86 c20 4 bbl carb (failing smog high nox new cat)
Cfrench192:
I went through the vacuum system today. Everything is routed based on manufacture spec. I replaced the evap canister because one of the diaphragms came right off. The egr gets about 8 inches around 1500 rpm to 2000 in park.
When I rev up the Rams the a/f guage reads 17.3.
When I pull the air cleaner off there is a loud hissing coming from the heat riser tube.
Can there be a possibility that the idle channel air bleed is too large?
I got a calibration kit for the carb this week. In it I have two sets of primary rods
Specs:
1 pair at .044" x .026" tip
2nd pair at .050 x .036" tip
Smaller pair is stamped 508.
Larger pair is not stamped.
It also came with jet sizes 70. 72. 74. 76. 78 and 80.
I believe I currently have .73 jets and . O51 rods.
Should I try to bring in more richness with a greater metering area? I'm thinking about using the second rod with a 74 jet.
(I hate california smog)
All rods are double taper.
Cfrench192:
I replaced a bunch of vacuum lines. I have fixed every possibible vacuum leak outside of carb. I still have a very lean part throttle/ cruise. I want to use a smaller rod and use the same jet. if I were to change jets it would be from a size 73 to a 74. From what I under stand jet size only contributes to wide open throttle. Which @ 25 mph the secondaries don't even open.
I tested both vacuum passage ways. One directly to the port and one that runs from that port (tees) one side goes to evap canister other goes to egr. Both get the same inches of vacuum 8 inches at 2000 rpm. I'm pretty sure that the rod size is to big in at the tip. I will try a smaller rod first. If there's no change il change the jets to 74s. Does this seem like the right procedure for this?
I do believe I'm running higher compression heads due to them being out of a 1993 suburban.
The truck weighs about 6000 lbs and has bigger than stock tires.
The reason I am convinced that it's not getting enough fuel s because of the realy low hydrocarbons and high nox.
Any ideas?
73ss:
You are on the right track as far as lean. As a side note, I repair/replace horiba 3 gas analyzers here in ohio. Probably what they are using to test your truck. Hi Nox is caused by hi combustion chamber temps. the egr "Cools" the temps down some. Did you happen to get a CO2 reading on your printout? CO2 is the a/f ratio, That's why there is no p/f for it. There for diagnostics only. Should read 14.7 if everything is right. Should pass also. Most new cars are hanging out around 15.5. I would be curious to see how close the CO2 readings are to your a/f gauge. At 17 that thing should be rattling bad. I would suggest richening it up some. Idle mixture screws can affect it at the low speeds it's being tested at. What will most likely happen is You'll richen it to pass Nox and then HC & CO will go up and or fail. It has a see-saw effect between the 2. When cars fail for high HC, Nox is very low. I would try going up a jet size, Take some APT out & see what happens. But at $20 a pop you want to be sure. Any shops in your area have a 3-gas? Also 50 degrees total with the vacuum seems a bit high. That can aggravate it as well. Did that truck happen to come with 2 smog pumps?
Good luck
Cfrench192:
That was a very informative replay 73ss. Thanks. Co2 percentage at 15 mph is 11.7 @ 1378 rpms
At 25 mph it is at 11.3 at 1442 rpms.
There is only one airpump. I believe the diverted valve is hooked up to manifold vacuum. It's routed based on the vacuum diagram.
The a/f gage shows way lean. It goes way above 20.0.
Il try one bigger size jet first.
Thanks again.
Cfrench192:
Today I swapped jets to a 74. No improvement to a/f.
The current rods are 51 m.
I have some bigger jets 76 78 an 80.
Should I try to shoot for a bigger jet?
I noticed that the main rods are all the way down at idle. When I punch the gas it shoots up. It has the strongest apt spring possible for the amount of vacuum.
I'm pretty sure the main rods are working correctly.
I could also try a smaller rod. I have been reading cliffs book for months now and I can't figure this problem out. Maybe too much idle bypass? I have no idea.
Il try the 80 jets tomorrow and see if I can get a richer idle.
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