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Bad emission results, is there any hope?

My 77’ Chevy Pickup had been sitting for 3-4 months, and now, I’m trying to pass Arizona state emissions. I have never had many problems with emissions(last year I blew dirty, I just changed the oil and was fine). Yesterday, I failed emissions with these results;

Hydrocarbons Carbon monoxide

1st idle: 1709 0.05

Precondition 197 0.03


Precon 120 1.00


Final reading 719 0.02


Idle Standard 250 2.00

Fail Pass

It seems strange to me that I would pass the carbon monoxide so well and fail the hydrocarbons so badly. Is this evidence of a serious problem or is this motor just done?

I drove it 120 miles on the freeway at 65mph to clean it out but I dont think that is enough. I’m really looking for some shade tree mechanic tricks to pass. What if I run half ethanol, half gasoline? The motor still runs great, but I am looking to put a new motor in it within the next 2 months so I just need to pass this one time.

Your running too rich. That is why the HCs are too high. Tweak down the idle mixture screws until the engine just barely maintains idle. That should fix it. BTW, drive it for 10 mins or better before going into the emissions station to warm it all the way up. Your initial HCs were way too high.

If tweaking the idle mixture screws doesn’t fix it, you may need a carb overhaul.

Thank you, i’ll give it a try

Changing to fresh spark plugs, and making sure the air filter is clean, are the most beneficial things one can do prior to an emissions test (or, after, for a test FAIL). Of course, increase the air flow needle adjustment, if the carb has that.

I put in plugs last year but they are probably pretty fouled if it’s burning rich. The filter is filthy though.Thanks.

A Rich condition elevates the CO not the HC…Sounds more like a vacuum leak to me, causing a lean misfire at idle. High HC is almost always caused by a misfire…

I’ll check Vacuum lines also, dont have my vacuum gauge anymore.

You shouldn’t need to drive more than 20 minutes to warm up the engine fully. What does the flier that came with your emissions test initiation say?

Year, make, model, engine size would be helpful…

I would suggest a new air filter and plugs to start off. You could also consider bumping the ignition timing up a few degrees.
It’s also possible that since this is a 77 model truck with a likely 350-4 barrel the Quadrajet could be acting up; and that’s not unheard of.

If it’s burning any oil then that’s a bit more complicated.

You just found out that engines don’t produce a lot of carbon monoxide. Your hydrocarbon readings at idle are way up there. A new catalytic converter will clean up those emissions but it looks like a new carburetor or finding out what problem is preventing it from working properly are in order. See if the vacuum advance is working, not just if it holds vacuum. It has to actually move the pickup in the distributor. If you are smoking, try straight 30 weight oil for the emissions test.