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Emissions & Catalytic Converter

Dec 27, 2001
Matthews, NC USA
1984 Coupe
Still trying to pass emissions with my carbureted 84.

Retested on Thursday an got the following.

HC - 316 Pass=220
CO - 5.89 Pass=1.2

This is after replacing all the plug wires, plugs, Dist. Cap, Rotor,
adjusting mixture on Holly 600 CFM, premium gas. Haven't checked the timing yet, but it can't be too far off because it's running very smooth with good throttle response.

The guy that replaced my front "Y" pipe said the cat's insides were burned out.

How much, if any would the cat contribute to these numbers?

Am I beating a dead horse here?

What kind of emission test do you have to go through? Your are fighting a losing battle if it's an IM240 unless you have a feed back carb(uses an O2 sensor) if all you have is an idle test you may have a chance but you will need a good cat and someone to set up the carb to pass at idle then when passed reset it to run better may not be real easy. Yes a bad cat can cause problems but you also have a carb that I bet is not feed back or even emission legal so you will have to find someone who can try their best to get it close enought to pass. Good Luck.

David Fulcher
So I'm assuming you didn't pass!

BEFORE you buy new cats.. get the TBI installed & running ( you might kill 2 birds with one stone)

If you use the new cat on the carb motor you might just torch the cats.....

Fortunately I have to buy only one cat. I believe the 85s and up had 3.

The emissions test here in NC is the idle type. They took the last measurements at 831 rpm.

I guess I'll have to try the new intake and see if I can get everything hooked up correctly.

Everyone else I've asked said the new cat was a must so I guess I'll drop another 150 on a new cat from Ecklers.

Thanks for the help,

My 85 only has one CAT. Stick a cat on it and you will be OK for the test. CATs eat up alot of excess emissions. I tried to pass an IM40 without a CAT and "FORGET ABOUT IT". With the cat the emissions guy crawled under my car to see if the exhaust was hooked up!!!:grinshot. He said that my 150,000 mile car should be alot dirter than it was. Then he asked me if I had the engine rebuilt.

It passed with flying colors but, I have ALL of my emisions in tact!
With a carb, you might be on the high side of the specs but pass. Like some one else said " with a carb you may over heat the cats" It would take a while but you should be OK for the test if your carb isnt pooring gas into the manafold. I would run the carb a tad bit lean for the test and consider removing the CATs after the test. Stick the CAT set up on a shelf for the next emissions test. :)

Bill C
Try this...

Forgive me for suggesting this, but.....get a couple of fresh one hundred dollar bills and at your next recheck flash the firsh one to the tech and ask if this will help fix it. If not, break out the other one and ask if that is enough, then just raise your eyebrows and shrug your shoulders. :eyerole

You are at no risk; miscommunication is your excuse. I've done this several times in California, the worst of all smog states. You can't fart here without a permit. : ( Seems to work best in the bad parts of town where the techs are hungry. (literally)
It is sad that it has resulted to this, since all of my cars run clean.

Good luck; Chris N
Your Idle is high...

600-800 I believe is the window for this.

And if you get the cat FLANGE mounted you can remove it.. have a bypass pipe made at the same time you install the cat.

Can the car idle lower?
The less input... the less output.

The more you adv the timing.. the more burn time you get.

And if you have a tiny vac leak that will help.

If you have a "rumpity"cam no matter what you do.. she won't pass.

Vigman is correct!!!;) Use the 85 set up and you will have a flange on both sides that will let you remove the CAT and install a straight pipe after you PASS the test.

With a higher initial timing number you will be able to idle her down easier with out it stalling. This is only for the test process. With the carb set lean, the timing set high and a cat with a carb you may do damage if you run it in that condition for a long time, like weeks or months.

Study hard and get ready to pass the test! :D

Bill C
check on this...

you might wanna check around but i'm sure it's safe and will make it run ALOT cleaner for your test. a bottle of rubbing alcohol... sounds screwy but it actually works pretty good. Also make sure that baby is nice and hot before you go in. Trick is to go in on an empty tank when you run the alcohol that way when you refuel you'll be good to go....

Alcohol sounds cheap enough to try, but I heard one of the reasons we are all not running 50% alcohol in our cars is that rubber doesn't like alcohol. Tends to make it desentigrate. I bought RXP which is suppose to do the same thing. Can't hurt.


Advanced timing and lower Idle sounds like good advice.
My old cat & the new one already has square flanges. Maybe I modify the old one and put a straight pipe through it.


Ok 600 to 800 rpm. I'll set it around 700.
Will advance the timing. How small of a vacuume leak? Explain what the leak does?

A couple of people have stated that I could damage the cat with a carburetor. How? I'd be burning the same fuel, just not so finely atomized? The cat doesn't know if the engine is carbureted or injected.

The cat I have is the flange type and have just purchased a new one from NAPA. Will get it tomorrow along with rebuild kits for the TBs. Figured since I'm this far into the project, I shouldn't compromise.


Pay offs? Don't think I have thought about it, but I'd be one of those people who would get caught.

Thanks for all the advice,

Here Kitty!

How small of a vacuume leak? Explain what the leak does?

Like a small 1/4 hose that has fell off the fitting (Oops, It must have just fell off sir.)

The leak will lean out the idle mix a bit more than just backin out the screws. ( less in..less out)!

About the CAT on a carbed car.

Since the carb is not computer controled ( and the Q jets for ECM controled C-3's are)

Your fuel is not atomized or burned as well as that.

( note the 75 CATS were HUGH! yours is about 1/4 that size)

The CAT is rated for a certian temp internally.

The more unburned fuel the higher the internal temp.

The CAT is a brick of porous ceramic material. ( picture side of milk crate)

As the temp & pressure increase it doesn't melt..it shatters.. then the little bits clog other paths etc, etc.

So it finally chokes to death ( Potatoe in tail pipe syndrome)

So in an 88 If I have 1 semi fouled plug in 1000 miles I could blow the pre cat.

Your carb can't control the amount of fuel that it dumps in the manafold as good as an emissions feed back system. The feed back constantly monitors the fuel and exhaust system and makes minute changes to the mixture to keep it in spec (happy cats!!)
Carbs are nortorious for running rich and at certin times,WOT and Idle to FULL Throttle you get a H#!! of a shot of fuel. The excess fuel in the CAT's causes them to run alot hotter then they were designed to. Carbs can't keep the fuel to air ratio where it needs to be to make happy cats!
You will be OK for the test and then back to the garage! ;)

If you run a gutted cat as a by-pass tube, sometimes it makes a gutteral sounding exhaust note. Lots of popping and a nasty sound. :puke
Putting a pipe inside the cat to HIDE the cheat is a better plan! ;)

Bill C
Good News

My 84 Money Pit is now back on the road and legal.

Yes it passed inspection, even with the carburetor.

Had the front end rebuilt by my local garage and in exchange for bringing them my business, they made sure my car would pass emissions. (wink wink nod nod)

Now I get to drive it for a short while before I replace the intake with a CF intake.

Thanks for all the help and advice.


That's some funny stuff!

NOW how does the car feel with

rebuilt tranny
rebuillt front end
new carb
no hole in muffler

What's next??



THAT is what's next. Either that or some dingle balls.:D
What's Next

Took the vette out for a shakedown cruise last night. It runs and handles now like I thought it should.

Didn't get it much over 80mph, but it got there in a hurry.

I discovered that you shouldn't sneeze at 75mph. I did last night and Vette changed lanes.


replace the edlebrock intake and carb w/stock CF intake and TBs
Replace the Top
Replace the weather striping

Replace Stock CF intake with ported and polished intake with Brand new Tubo City TBs.

Then I'll slow down for a while and just enjoy driving it for the rest of the year.


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