1975 K10 Emissions Failed- Help please

[4xcrazy]

Looks like maybe a combo of a carb dumping too much fuel and possibly the cats not being able to burn off the excess fuel.

I've had issues like this in the past as well, carbs were always a main problem, either needed rebuilt or rejetted to a smaller size, they were always dumping too much fuel at both idle and loaded, cylinders weren't able to burn off all the fuel, raw fuel gets expelled out the exhaust, cats can't burn off the excess, comes out as a high reading on the hydrocarbons.

You say it runs great, so I seriously doubt it's a misfire or ignition problem.

Stay away from Autozone carbs, their rebuilt units suck...bad...

 

[Mcsjr454]

have you checked the condition of the EGR valve? and or cleaned the EGR passage. While I know the EGR is supposed to reduce NOx emissions, wouldn't it also change the mixture in the combustion chamber to a polluted lean mixture? Thus causing a high HC reading. I may be way off here, and I apologize if I am.

 

[Blue71Chevy]

found this tidbit in my notes. I don't remember where i copied it from, another forum maybe. hope this helps.

High CO= Rich condition, CO is partially burned fuel
Causes
-High fuel pressure
-Leaky injectors
-fualty temp sensor
-thermostat stuck open
-clogged air filter
-sticking choke
-high float level
-leaking accelerator pump or power valve
-charcoal canister full, fuel to topped off, faulty purge solinoid
-Fuel in the crankcase

High HC= incomplete combustion
Causes
-bad ignition, coil, plugs, wires, cap, rotor
-over advanced timing
-low compression
-stuck EGR

 

[COCHEV]

It took me multiple attempts to get my old 82 passed. I had a quadra jet carb on it and it ran great. Kept leaning it out with the idle mixtures screws till it passed. Pain in the arse.

Afterwards i set it back up how i had it

 

[bradley bryden]

thanks, i will try that

 

[kevin400ex]

a weak coil in the hei distributor will cause it as well. it may seem like its running fine, but its not burning all the fuel. an old school ignition anlyzer is what finally got me pointed in the right direction. i didnt think it could be coil since it was new from the parts store, but it put out half the voltage of the junkyard oem ac delco coil i replaced it with.

 

[4xcrazy]

If It's failing under load, leaning out the idle mixture screws won't cure it, you'll also need to adjust the screw inside the little airhorn in the middle, can't remember what It's called right off, my buddy Paul messes with his every time he runs through emissions and gets his to pass every time.

 

[K5ing in AZ]

Just wondering, Did Chevy use electric chokes on Quadrajunk carbs. I don't remember unhooking any wires as I threw them in the trash.

 

[4xcrazy]

Yes, there were carbs with electric chokes. Honestly, here in Arizona, you don't even need the choke to operate unless you live up in cold country. I've never had them working while living in Phoenix area, summer or winter, it really doesn't get cold enough to need it. Sure it might stumble a bit during winter, but It's not that cold that the motor wont heat up fairly quickly anyways.

Adjust the choke flapper to always open, let 'er run.