My 78 Trail Duster just failed the stringent CA smog test.
The truck is all factory correct with 94,000 miles. It is a Federal emissions truck with a GVWR of 6100 lbs. 440, thermoquad, dual exhaust and a 727. Because of the Full Time 4wd, the test consists of a standard sniffer test at idle and at 2500 rpms. The first test it ran well UNDER the allowable limits of HCs at 2500. 360 allowable with 36 showing. At idle it went waaaay over, 600 when 300 was allowable. The tech thought that there could be a vacuum leak or two, so I replaced 2 vac lines and the carb base gasket. I went back, and now it is actually worse! The idle #s went to 1003 and the 2500 rpm test was at 50. What the heck?
My experience with smog guys seems to be bad. Whenever I have had a problem, they say..."Uhh, it could be an oxygen sensor"...OR "Ahhh, maybe it is a vacuum leak, or a fouled plug, or a ficklesnort spring." EASY for them to just throw my money around with no regard for my wallet. Are these guys actually trained or do they just guess all of the time?
This engine runs smoothe and has no smoke. No unusual odors coming from the exhaust. It has new plugs and oil. It is no hot rod, but it runs well. Would aged plug wires cause a fail at idle but still allow an engine to run so clean at higher rpms? I'm interested in any pointers on what I can do to get this thing to pass. Would a pair of catalytic converters make any differnce in the HC numbers? Any other ideas that I can try?
Thanks, Greg

Aged plug wires? Unless they are shorting out, they are fine. And you can check that at night in a dark area.

Did that truck come with an EGR and cat. converters when new?? I would think a pair of cats would help.

What about your initial mech. advance timing? More may help combustion efficiency at idle. It would be interesting to hear what other techs might say..........

If you have an EGR, check to see if it works.

If you list all the test results/numbers it will help. High HC is unburned fuel. Lean miss, ign misfire, rich fuel mix, etc. All the numbers may help us help you.
List the limits/max allowable also.

I used to have to retard the initial timing on the 360 2bbls to get them through. The numbers I can't recall right now.

Add some methyl hydrate to the fuel.

Sheldon

When I used to have to smog my valiant and first challenger in CA I would retard the timing a couple degrees and install a hotter thermostat for the test. They would both fail with the 180 degree stat but pass with a 210. As mentioned above though, definitely start with all maintenance items.

-Jon

There used to be a guy in SF (since retired) who'd get my 383 RR to pass (with the MP 284/484 cam no less): if the tune-up was fresh, he'd raise the idle speed, pull timing out, lean the idle screws, and get the motor HOT (210-220). Motor was not happy with that but it would pass with room to spare, and I'd just reset everything later. IIRC, same deal on the Hemi car.

A couple gallons of E85 cleaned up my Ramcharger quite a bit, but not enough to get it to pass. Only missed by about 1/2%. Get to try again this coming week.

Greg, yes, it's all a guess. It's often a best guess, but it is still a guess.
The average guy sees 5-10 cars a day. They are anything from a 6 year old low miles creampuff to some old POS. Try to figure out EXACTLY what is causing a misfire on each and every one of them without any work! CO (Rich) is fairly easy in comparison, but any of a hundred things can cause a miss.
Could you be able to tell which houses had plumbing issues AND be able to tell the owner what they are and how much it will cost just by walking thru it for 10 minutes?

First test:

Test RPM %CO2 %02 HC PPM C0 %
Idle 674 12.0 3.6 300 max 651 measured 3.0 max .24 measured
2500 2585 13.76 1.3 350 max 36 measured 3.5 max .07 measured

What the heck do I know. It looked pretty good besides the idle HC numbers. After this test, I replaced the vacuum line from the base of the carb to the vacuum advance can. I removed and plugged the vacuum line to the dead cruise control. I replaced the carb base gasket and checked for additional leaks with a can of carb cleaner around the intake ports, all vacuum sources too. I bumped the idle slightly.NO EGR, NO cats, NO thermo air cleaner, NO spark retard, stock dual exhaust.
The second test, the truck WAS a little cooler. It was before 9:00 AM.

Test RPM % C02 %02 HC PPM CO2%
IDLE 862 11.36 4.4 300 max 1003 measured 3.00 max .16 measured
2500 2478 13.48 2.1 350 max 55 measured 3.50 max .05 measured

Maybe someone can decipher these numbers! Please excuse how close all the numbers appear to be jammed together. I tried to space them apart further but they cram back together! The reason I asked about catalytic converters is that I could weld a pair of high flow cats in CHEAPER than rebuilding or replacing this carburetor.

I don't mean to dump on smog techs, its just that it costs money EACH time they test the thing, and I figured that they were in a better position to diagnose a problem.

Did you change the metering rods on the TQ? If not, make sure the M-rod 'tree' is not hanging up after acceleration. Also, make sure theres enough vacuume to bring the 'tree' all the way down at idle.

The carburetor is as it was when I bought the truck last September. I've rebuilt Holleys, but never a T-quad. A buddy had one rebuilt locally several years ago. It ran fine, but I don't know if it was smog compliant since the car was exempt from testing.
There lies the problem: For me, I can't determine if a carb is compliant or not. Mine idles very smoothe and produces no smoke.

300 max 1003 measured HC
3.00 max .16 measured CO
If this is right(I copied pasted)It almost looks like a lean miss. With no cats I would expect CO a little higher(maybe 1-2%)1000 HC is a misfire almost for sure. Absolutely sure there's no vac leak? How about a cylinder power balance to make they're all firing? I know you said it runs smooth but 1000 HC is real high cats or no cats!! Just throwing ideas out.

Lean miss at idle. Highest vacuum at idle... Coincidence?
I guess I'll look closer for vacuum leaks.

My point about the smog techs was written from frustration. When they make incorrect suggestions that cost customers MORE time and money, it just sucks. If the man would've said... "For an hours labor I'll probably be able to determine what it needs." I would have been okay with it. I agree that nobody should work for free. NO arguement there. maybe it is a matter of ME being an eternal optimist. I hear the man say... vacuum leak and I think that is all there is to it. A pessimistic man would always expect the worst.

the only way to make it pass is with a 4 gas system & make idle speed,mixture screw adjustments while noting changes

In the late 90's the highest emissions failure rate in Colorado were Chrysler 440 engines,years 76-78.On a more cheerful note for the roller test it was 82-91 360 Grand cherokees.