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

[Cfrench192]

 I routed the vacuum lines based on the vacuum schematic for this make and year.  There is pretty much a spaghetti bowl of vacuum lines in this truck.  For some reason the egr port on the carb that you mentioned goes to a thermal vacume switch.  The Line that comes out of the other side of thermal switch then has a t.  One goes to the egr valve, the other goes to the evap canister.  If I were to bypass this, where would the evap canister line route to.  I guess I could just block it off. 

Also there is a spherical shaped component by the firewall closest to the driver side as possible.  This component has a vacuum line that is not connected to anything.  Is anyone familiar with what this thing is?

[Cfrench192]

I just played around with the evap canister and one of the diaphragms flew off.  It nearly hit me in the eye.  The hoses are very cracked and are easily being pulled off.  It looks like I found a vacuum leak. This definitely explains why it's running lean.

[DieselPower]

You probably want to keep the evap system functional if they do a visual inspection before testing.

The thermal switch is I believe to keep the EGR from functioning until the coolant is warmed up to a preset temp.

Could you put a tee on the hose to the EGR valve and route a hose to the cab and read actual vacuum to the EGR valve while driving?

[Cfrench192]

That sounds like a good idea reading egr vacuum while driving.  How many inches should I be looking for at 25 mph?  If I add rpm to the engine in park it gets up to about 8".  I will have to wait until the weekend unfortunately.  I am replacing the charcoal canister and hosing.  It should be in this week.  It think I found a major vacuum leak.  There are a couple of cracked hoses.  Thanks for the help so far!

[DieselPower]

I have no idea what the testing procedure would be for that engine.  Do you have a service manual?  Being a ported vacuum source, the vacuum should match manifold vacuum off idle unless there is some sort of bleed before the EGR valve.

[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.

[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]

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.

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