Idle Emissions Help

Hi all, hoping someone here will know where to point me. I am still pretty new to working on engines so I am at a bit of a loss. My tuck is a 79 C20 with a 350. It is doing great on the loaded readings (about 50% the limit), but both HC and CO are double the limit at idle. It was sitting for a couple months before these test attempts. Saw a couple things in other threads, but nothing seems to matching up exactly that I found.

So far I swapped on a freshly rebuilt quadrajet (same hot air choke), new spark plugs, evap canister, and belts. It is running noticable better with the new carb. On the carb I even tried turning the idle screws all the way and still way off on testing. It is a stock rebuild on the carb so I am not sure if I should be trying to pull it apart and see what the rebuilder did as far as internal pieces or if this would point to something else. Really throwing me off that it is only way too high on idle.

TruckTim said:

Hi all, hoping someone here will know where to point me. I am still pretty new to working on engines so I am at a bit of a loss. My tuck is a 79 C20 with a 350. It is doing great on the loaded readings (about 50% the limit), but both HC and CO are double the limit at idle. It was sitting for a couple months before these test attempts. Saw a couple things in other threads, but nothing seems to matching up exactly that I found.

So far I swapped on a freshly rebuilt quadrajet (same hot air choke), new spark plugs, evap canister, and belts. It is running noticable better with the new carb. On the carb I even tried turning the idle screws all the way and still way off on testing. It is a stock rebuild on the carb so I am not sure if I should be trying to pull it apart and see what the rebuilder did as far as internal pieces or if this would point to something else. Really throwing me off that it is only way too high on idle.

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If you have the idle screws all the way in and it still idles,something is wrong. You have the throttle stop screw in too far and are pulling fuel from the transition circuit or you have a vacuum leak are the first 2 things that come to mind. Check those,you're going to have to get it idling on the idle circuit and carb adjusted correctly. Make sure it's timed correctly,you can't properly adjust the carb if the timings off.

Dang you were fast. Completely did not even consider the throttle screw. Did have the timing adjusted when I first got it, but wont hurt to double check. I will try everything you said one by one.
Thanks a ton for the advice!

TruckTim said:

Dang you were fast. Completely did not even consider the throttle screw. Did have the timing adjusted when I first got it, but wont hurt to double check. I will try everything you said one by one.
Thanks a ton for the advice!

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Turn both idle screws all the way clockwise until they lightly bottom out. Then back them out 1-1/2 turn. You will need a tach to properly adjust the carb, I use a timing gun with a tach function on it.

You will need to adjust the idle screw to be at 700 RPM with the engine fully warm, choke off, in Park . Then slowly screw out one of the idle screws. The idle should rise as you are screwing it out. When the idle hits 800RPM, adjust the idle screw again to get it back down to 700RPM. Keep doing this until the idle starts to drop as you screw the idle mix screw out. Once that happens, screw it back in until it hits peak RPM again and then screw it in 1/2 turn more. Now you have one idle mixture screw set at just slightly lean of peak. Do the same procedure for the other mixture screw. Once you have both idle mixture screws adjusted properly, you can final set the idle stop screw to run at whatever the sticker on your air cleaner says(its usually somewhere between 650-750RPM for a 350).

What are the actual readings? Another idle suggestion. Many times if the idle is increased to just below the max emissions will clean up, sometimes quite considerably. Need to look at the max speed for testing requirement. Back when they did testing here, was in the area of 1100 RPM, higher than anyone would normally adjust it.

Also, make certain that your EGR Valve is operating properly and not plugged up or rusted shut.

Thanks all for the responses!

Readings are Loaded 110 HC 0.51 CO (220 HC and 1.5 CO is the limit), but idle 4.18 HC and 4.08 CO (220 HC and 2.00 CO limit).

Last night I was able to find/fix a small vacuum leak and get the idle screws estimated. Do not have a tachometer so I am going off guessing on rpm.

They never mentioned a RMP limit to me so might have to try to ramp it up at least long enough to pass testing so I can take my time getting everything tuned just right.

Did not check the EGR valve yet, that completely skipped my mind. Before I go back to emissions I will take a look at that too.

Rule of thumb, high CO is rich. high HC is anything other than fuel from engine condition to a misfire. Usually, HC will be low with high CO but not always. I know this sounds clear as mud.

If this vehicle was in my shop and a quick adjustment would try to lean out the idle screws and watch the HC. If HC is now within limits you're done. If it rises need to dig further into it, such as overall tune up, engine condition, how old is the oil, vacuum leaks, ect.

Does this engine have a performance cam and what is the overall engine condition? Readings aren't horrible just bad enough to fail. With an air pump or cat would probably pass.

You are right about it not sounding super clear yet, but hopefully this battle gets me closer to understanding.

The engine was replaced by a prior owner, but it is about as mild as from the factory. The fluids were done just a few months ago and the cat looks pretty new, but I dont know. Just fixed the only vacuum leak I could find.

Those numbers are with the idle screws as lean as possible so I think there has to be something else going on past the carb adjustments. From what this thread is sounding like I may need to check for a issue with the egr or air pump and keep my fingers crossed it is not something deeper.

As long as the egr valve isn't hung open it has no affect on idle emissions.

When performing diagnostics the air pump hoses should be clamped off. If those readings are with a functioning AP system, engine is much richer than I first thought.

When you say as lean as possible how is it being adjusted? Emissions are usually lowest when mixture screws are a 1/4 turn in from highest RPM, lean drop method.

Excessively rich, the oil has to be changed. Fuel will pass by the rings and contaminate the oil enough to change emissions. Oil or fuel fouled plugs will contribute to HC.

I had the air screws all the way in for that test. I followed the steps previously mentioned to get them back to where better adjustment since. Oil and plugs changed a couple months ago and only driving since then has been trying to pass emissions several times.

Did not know I should be clamping off the air pump hoses. That is really good to know. Do not know 100% if the pump is working, so I guess I may need to get that rebuilt.

If idle screws all the way in carb has a problem. Can check air pump by pulling hoses, see if air is flowing. Beginning to sound like the carb is gross rich.

Thanks all for the help! Found a second small vacuum leak at the base, readjusted the idle screws, then bumped the RPM up to 1050 and it passed great.