1977 C20 hd emissions vacuum lines

[njones]

I can’t find where some of my vacuum lines are supposed to go. I just bought this truck and I am pretty sure something is a little off. I have a single vacuum line coming to the carb in the front bottom next to the ac solenoid. I believe that the airbox connects to the kickdown that is mechanical. I have the repair manual.

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I cannot figure out if I need a line out the back of the carburetor where the advance is. It has two ports that I can see but the diagram shows nothing.

Also, where does the actual vacuum come from?

I am pretty new at dealing with carburetors, so I probably called something wrong.

 

[AuroraGirl]

I can’t find where some of my vacuum lines are supposed to go. I just bought this truck and I am pretty sure something is a little off. I have a single vacuum line coming to the carb in the front bottom next to the ac solenoid. I believe that the airbox connects to the kickdown that is mechanical. I have the repair manual.

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I cannot figure out if I need a line out the back of the carburetor where the advance is. It has two ports that I can see but the diagram shows nothing.

Also, where does the actual vacuum come from?

I am pretty new at dealing with carburetors, so I probably called something wrong.

The one out of your rear of your carb goes to the black canister filter then goes to the vacuum booster ( you have vacuum booster, right?)
The vacuum is produced by the restriction of the intake system vs the flow out of the engine (small opening, exhaust gets pushed outward) in a very simple way.
Can you show us what your trucks engine bay looks like? where does that single vacuum line you have go to? Your vacuum advance?
Do you have a stock air cleaner housing with a THERMAC (the flapper in the snout that looks like a circle on the air cleaner housing near where air enters?

 

[njones]

I do not have a vacuum canister and I don’t have egr as well.

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This is the engine bay. As you can somewhat see I have a line that goes from the carb directly to the distributor. It has a mechanical choke that needs hot air. I believe that will come from the valve cover? It says in the service manual the v6 goes to the manifold. Nothing for the v8. There is a pipe out of the valve cover on the passenger side but that doesn’t seem right.

The other connectors in the front are plugged.

In the back

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I have this. No idea where it goes.

I also believe the “spark control switch” seems to be gone.

The air cleaner is the original with the flap. It seemed to be plugged into the thermal choke as that is the same size hose. However that doesn’t seem right.

The upper stove to the air cleaner pipe is also gone.

Also, thanks for answering so quick.

 

[fast 99]

Last picture is secondary choke pulloff, goes to manifold vacuum. Port in on back of carb currently plugged, make sure it holds vacuum.

Choke heat goes to intake, filtered air supply to carb rear upper port, pictured parts are required. Intake need to be clean for exhaust to warm the area and heat riser working.

Can't tell from picture if intake has provision for heat. Alternate solution, electric choke thermostat can be installed to running power supply. Will require additional oil pressure switch and related wiring.

 

[njones]

Ok, it’s starting to make sense. I get the secondary choke cutoff. For the hot air this is the air cleaner I have.

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It seems I don’t have the hot air parts then. I would probably have to plug this into the carb as well, right?

 

[fast 99]

Provide a picture of the intake area beneath carb on RHS. I suspect carb is newer than truck.

Air cleaner heat has nothing to do with choke heat.

 

[njones]

After doing some research you are correct about the carb being newer. It is a 17080206. So, a 1980 model.

Here is an ok picture of what I think you are asking for

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thanks for your help.

 

[fast 99]

That's what I thought, you have a mix master of parts. Easiest/least expensive thing to do is get an electric choke thermostat for that carb and plug the vacuum outlet. Existing intake is for a divorced choke. Either the intake needs to be swapped out to a later style or an earlier carb purchased. Those 2 won't play well with each other.

If you decide to go electric, a power supply with key on power interrupted by an oil pressure switch when engine isn't running. That is accomplished with a T fitting at oil galley near distributor. There really isn't a fast fix to what is there.

 

[njones]

Thanks again. I am learning fast.

 

[AuroraGirl]

That's what I thought, you have a mix master of parts. Easiest/least expensive thing to do is get an electric choke thermostat for that carb and plug the vacuum outlet. Existing intake is for a divorced choke. Either the intake needs to be swapped out to a later style or an earlier carb purchased. Those 2 won't play well with each other.

If you decide to go electric, a power supply with key on power interrupted by an oil pressure switch when engine isn't running. That is accomplished with a T fitting at oil galley near distributor. There really isn't a fast fix to what is there.

he also has a provision to use the THERMAC on the air cleaner, but its up to him if he wants to make it functional. OP, do you live where its cold?

 

[njones]

No, I’m in Arkansas so mostly warm

 

[75gmck25]

I have a similar carburetor on my truck right now - a 17080213, so maybe you will find these pictures useful.
- I use full manifold vacuum for the distributor, but you could use ported instead. Whichever you use, the other small port must be plugged.
- The large threaded port on the back is for the power brakes, and the large 3/8” vacuum nipple on the front is for PCV.
- There are vacuum lines running to primary and secondary choke pull-offs, and I have converted to an electric choke.

 

[Finkaire]

California, just had mine rebuilt, no other options!
Should be a diagram on the core support

 

[75gmck25]

GM didn’t put the emissions diagram and other info on the core support until sometime in the late 70’s. AFAIK, for my ‘75 the diagram was on the side of the air cleaner, and it’s long gone.

 

[bucket]

I'm assuming your '77 should be like my '78. There's a thermal switch that controls the vacuum advance, but nothing else. I can get pictures of the setup, if needed.