Hi Guys,
Need a little help tuning for emissions test.

I got a 77 Vette with the 3 year old cat but the engine compartment emissions equipment is gone.
I dialed back the timing from 36 to zero before the test.
The carb was rebuilt by Lars 2-3 years ago..maybe put on 2000 miles since then.

Results
2500RPM test
........................Standard.....Tes ted
CO%..................<3.00..........6.39 Fail
HCppm................<700..........126 Pass
Dilution CO+CO2....6.00..........17.29
.
IDLE Test
........................Standard......Te sted
CO%..................<3.00..........7.24 Fail
HCppm................<700..........446 Pass
Dilution CO+CO2.....6.00..........17.34

What do I need to change to get it to standard?

 

Dont use premium for the test.

 

The general rule of thumb for emissions is:

High CO is rich carb
High HC is timing

Your HC is good - bringing your timing back to 0 is probably overkill, and may be raising your CO slightly, so set your timing to the initial spec number for the car (probably about 6 to 8 degrees) and run the vacuum advance off of a ported vacuum source on the carb to keep it retarded at idle.

Once done, bring your idle speed up against the maximum permitted for the test - this is usually about 950 is so. Once there, lean out your idle mixture screws evenly until you get a 50 rpm drop in idle speed - this is your "idle lean drop." This should get your CO down into an acceptable range.

Keep in mind that if you do not have the A.I.R. pump installed, it will take a while for the Cat to "light off" and work well - make sure you're going into the test with a hot engine and hot cat - a working cat has a HUGE effect on CO.

Lars

 

Your idle reading of 7.2% C.O. is about a 11.8 to 1 air/fuel mixture which is very rich and the 6.39 C.O. reading is a 12.1 to 1 air/fuel mixture. I would have the mixture at idle set to about 2% C.O. (13.7 to 1 a/f) by adjusting the idle mixture screws in and then check the mixture at 2500 RPM to see what it is. If the C.O. reading at 2500 rpm is still high you may need to adjust the APT (adjustable part throttle) screw down to lower the C.O. reading at the 2500 rpm range.

you may also want to pull the pcv valve out of the valve cover to see if the C.O. reading changes, if the reading goes down by more than 0.5% C.O. the oil may need to be changed. Also you may want to pinch off the vac line to the evap canister and watch if the C.O. readings change, if they do change you may have a problem with the evap canister system.

We use a portable exhaust gas analyzer to tune the air/fuel carburetor all the time, the target C.O. reading for a mild to stock v-8 we tune to is 1 to 3% C.O. at idle (lowest HC and highest co2) and we target 1 to 2% C.O. reading at 2500-3000 rpm. a 6% C.O. reading (12.2 to 1 a/f mixture) is a good power mixture for wide open throttle/maximum power. Note: all the readings should be taken with the smog pump disabled and in front of any catalitic convertor because they will lower the readings and thus make the readings less than accurate for tuning.

The good news is once you get the C.O. readings where they belong you should save a few $$$ because you gas mileage should go up.

I hope this helps Henry @ olescarb

 

Thank you for the good info Lars.
Always a pleasure to hear from you.

I bumped the timing back up to 8 to 10 range.
My tack is not accurate so I'm going by ear on the RPMs. I took the idle mixture screws down(clockwise) just over 1/2 turn before I noticed a drop. Is this in the range I'm looking at for screw turn..to much or to little? Those screws are pretty easy to turn, do they vibrate and get off much?

I only heated the Cat up about 15 minutes last time. This time I took a 30 minute drive to get it all heated up. I did notice a bit better throttle response. Pulled up to the test station..D'oh they are closed on Sunday. Oh well it is a nice day and the drive felt good.

Henry
Your info is good stuff too..I just don't own an analyzer.

 

U may need to reinstall the AIR system if removed. The AIR pump injects air into the exhaust ports and it completes the combustion of unburned gases - which should reduce the HC more than the CO but i truly can't recall which does which right now. But again the AIR system does work and actually not all that parasitic to power. My modern Z28 EFI coun't pass smog without the AIR sys.

Hope this helps,
cardo0

 

Which station do you go to for testing? I have always liked the one on Emerald just east of Orchard. They are open on Sunday (and knock a couple of bucks off for Sunday tests). They are sometimes helpful on the testing too like "Try keeping the RPMS at the high side of the range, but I didn't say that..."

I've passed there 3 years in a row with NO emissions equip & dual exhausts on my 81 (Thanks to Lars' input too!!). Fortunately, they don't do a visual inspection for pre 84 vehicles.

Anyway, I don't know about Qjets, but have a digital tach on my timing light if you want to check it. PM me with a phone number or email if you want. I'm near Maple Grove & Overland.


Dan

 

Guys,

I'm curious about one thing. The timing and carb recommendations above make (chemical and physics) sense, but I was wondering about an additional easy to do item. Adding a reasonable amount of alcohol (E10 plus some bottled alcohol additive) should provide additional oxygen in the fuel to reduce the CO levels in the event that carb modifications aren't easy to do. Has anybody done any back to back tests to see what the sniffer results would be by doing a slight fuel modification like this?

Just curious.

 

If the test is a ilde emissions test only you probably don't need to worry about EGR valve. You need to find a shop with a Ehaust gas analyser and tune your Vette up, there is a good reason why I have a HUGE Allen 4 gas test unit in my garage no matter how much my wife complains about it, I'm keeping it!