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Help with the C20

Cucv_M1008

Cucv_M1008

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So my uncle is almost ready to give me my grandpa's truck 1985 C20 350 four barrel th400 14sf 411 gears. he went to get it smogged and it would not pass, the Hydro carbons were to high. What would cause this? I'm not sure where to look, does that mean a tune up or something else?

Thanks.
 
My hydrocarbons were a little too high when I first had mine tested at the DEQ.

Then the 2nd time they were Cheech and Chong super high.

Third attempt they were within range.

All I did between attempts was basic tuning: set the timing, adjusted the carb with a vacuum gauge, eliminated all vacuum leaks and made sure the vacuum hoses were routed properly according to the diagram on the radiator support (and some helpful youtube videos). Oh and I put in a set of fresh spark plugs as well.


Dumb question, but does it still have the air pump, pipes, hoses, and catalytic converter? I imagine it'd be hard if not impossible to pass without those.
 
regal403 said:
My hydrocarbons were a little too high when I first had mine tested at the DEQ.

Then the 2nd time they were Cheech and Chong super high.

Third attempt they were within range.

All I did between attempts was basic tuning: set the timing, adjusted the carb with a vacuum gauge, eliminated all vacuum leaks and made sure the vacuum hoses were routed properly according to the diagram on the radiator support (and some helpful youtube videos). Oh and I put in a set of fresh spark plugs as well.


Dumb question, but does it still have the air pump, pipes, hoses, and catalytic converter? I imagine it'd be hard if not impossible to pass without those.
Click to expand...
It has the air pump, it's not hooked up but it's all there and has passed before with out it hooked up, as far as a cat I'm not sure I've haven't looked under it in a while I'll have to check. I can throw some new plugs in it.
 
My 84 Jimmy called for 4° of timing, might want to check yours.
 
High hc is unbrunt fuel. So you want to look for things that cause rich run. Miss fire etc.
If nothing obvious. Change oil, plugs, increase the gap slightly, but put them at spec after test.
Clean air filter. Makes sure engine and cat are hot when test is done.
Don't let the cat cool off before test.
Normal the air fuel adjustment screws are blocked, if yours are open, adjust idle air screws, 1 at time, for highest rpm, then turn in to drop 25-50 rpm. As long as idle is still smooth.
Bump the time 2*, most test allow +or-.
Make sure the pcv filter is clean and freash air is getting into crank case.
 
Wes Harden said:
High hc is unbrunt fuel. So you want to look for things that cause rich run. Miss fire etc.
If nothing obvious. Change oil, plugs, increase the gap slightly, but put them at spec after test.
Clean air filter. Makes sure engine and cat are hot when test is done.
Don't let the cat cool off before test.
Normal the air fuel adjustment screws are blocked, if yours are open, adjust idle air screws, 1 at time, for highest rpm, then turn in to drop 25-50 rpm. As long as idle is still smooth.
Bump the time 2*, most test allow +or-.
Make sure the pcv filter is clean and freash air is getting into crank case.
Click to expand...
I forgot to say it does back fire some times.
 
So the back fire might be caused by a bad check valve on air pump system. Easy way to check is pull them and see if air flows both ways.
You can cap the injection manifold with a pipe cap, for a test drive.
I know you said the air pump system wasn't hooked up, but I assume it is still there.

Edit air shouldn't flow both ways on either check valve
 
Last edited:
Wes Harden said:
So the back fire might be caused by a bad check valve on air pump system. Easy way to check is pull them and see if air flows both ways.
You can cap the injection manifold with a pipe cap, for a test drive.
I know you said the air pump system wasn't hooked up, but I assume it is still there.
Click to expand...
The system is intact, just no belt from the water pump to the air pump
 
It would be unusual for a bad check valve to cause a back fire, with out the belt. It could still be allowing fresh air into exhaust manifolds to create an occasional back fire.
 
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