1981? 350 in a C20 - Vacuum/sensor questions

[Oldbear42]

I posted this on my build thread but thought it would be better here. I recently purchased a 1981 Chev C20 Scottsdale. It is the result of mashing a few trucks together (seller bought a few to restore their truck). So I'm starting from scratch and don't know what is original and what is not.

I've printed off some stuff I found online about the vacuum routing for these engines but I'm also asking the hive mind here. The three in the first picture are on the top front of the engine and two of them appear to be in the coolant system. The second is on the passenger side of the air cleaner. Are they all emissions stuff? Did my engine have an air pump (it doesn't now)?

The engine runs without these. It is a bit sluggish but that might be due to air and fuel filters (I just changed the air filter and plan to do a tune-up (plugs, cap & rotor with wires too) but I'm looking for advice & information. So most of the vacuum lines under the hood appear to be attached - just not these.

If the vacuum "things" are not needed I will remove them and hard-plug them so the +40-year-old plastic doesn't strand me on the road someday when it breaks. I've learnt my lesson the hard way with the plastic fitting for the brake booster vacuum on my '67 Olds 98.

Thanks in advance. I hope to repay the learning someday.

 

[Ricko1966]

Now get a vacuum schematic for your truck and start hooking things up like they should be. At minimum pull the air cleaner off and start shooting LOTS!!! of pics for us those sensors had hoses leading too and from them,we don't know where those hoses are hooked up now. Bare bones? Hook vacuum advance to manifold vacuum,adjust carb as necessary. If it's got a th350 or th400 run a steel line with short rubber couplers manifold vacuum to vacuum modulator. I say bare bones but technically pcv wouldn't be bare bones,but I wouldn't want to do with out it. Manifold vacuum to brake booster.

 

[Oldbear42]

Now get a vacuum schematic for your truck and start hooking things up like they should be. At minimum pull the air cleaner off and start shooting LOTS!!! of pics for us those sensors had hoses leading too and from them,we don't know where those hoses are hooked up now. Bare bones? Hook vacuum advance to manifold vacuum,adjust carb as necessary. If it's got a th350 or th400 run a steel line with short rubber couplers manifold vacuum to vacuum modulator. I say bare bones but technically pcv wouldn't be bare bones,but I wouldn't want to do with out it. Manifold vacuum to brake booster.

I'll get more pictures. All of the items you mentioned are currently hooked up to the best of my knowledge (vac advance, brake boost, etc) - I've only had the last few evenings to work on it. Trucks runs and drives - I drove it home 3.5 hours (all back roads...) but I'm looking to see what the vacuum sensors/pots/etc are.

I've got a few diagrams to look at on Saturday but not the original from this truck.

 

[85K304SPD]

Vacuum advance, heater, pcv, brake booster. That is about all you need. All that other stuff is basically ways to regulate the advance and warm air, I think. Lots of little parts to replace, if you have to have them.

 

[Oldbear42]

Vacuum advance, heater, pcv, brake booster. That is about all you need. All that other stuff is basically ways to regulate the advance and warm air, I think. Lots of little parts to replace, if you have to have them.

I'm in Alberta so we have zero air care - no need for emission control to keep it on the road. The plan for tomorrow is to identify what the vacuum devices are and make plans to remove anything else it doesn't need. I'd rather plug or cap the unused ports so I have fewer chances for aged plastic to fail.

 

[Rusty Nail]

Aged plastic...

Aged.

Plastic.

I got nothin??

You must be registered for see images attach

Stick a timing light on it. I run 9.5° advanced - all fixed up.

 

[Geoduck]

Rick nailed it. This is more or less how my 1985 305 is plumbed now and it runs well.
Important, run a new distributor vacuum advance hose directly to the intake manifold port behind your carburetor. OEM setup ran that hose through an number of sensors and branched out God only knows how many other places.
The gas tank vent hose to charcoal canister, charcoal canister to Quadrajet is important IMO.
Your Quadrajet likely has one or even two choke pull-offs. Their vacuum lines must be in good order.
I can try to walk you through the three ports shown in your first picture if needed.

 

[Oldbear42]

Rick nailed it. This is more or less how my 1985 305 is plumbed now and it runs well.
Important, run a new distributor vacuum advance hose directly to the intake manifold port behind your carburetor. OEM setup ran that hose through an number of sensors and branched out God only knows how many other places.
The gas tank vent hose to charcoal canister, charcoal canister to Quadrajet is important IMO.
Your Quadrajet likely has one or even two choke pull-offs. Their vacuum lines must be in good order.
I can try to walk you through the three ports shown in your first picture if needed.

Thank you to all who gave information and advice/guidance. I found it interesting that there are great vacuum diagrams up to 1980 and then again in from 1984 and on but the 1981-83 looks like a scan of a picture of someone's decal from under a hood and the picture was taken from space...

I think I've got it mostly sorted. The sensor/valves I showed plus a few others are not hooked up and the hoses that need to be hooked up for the carb and such are hooked up. The truck ran okay for the 3.5-hour drive home. A tune-up (next few days) and filter change (done) should help the performance - also found the broken wire for the transmission kickdown switch which should help with passing on the road.

A couple of the vacuum ports are poorly plugged so I'll fix them with proper caps. I will be replacing the vacuum hoses as I don't know the age of them and would rather replace in my shop than fix them on the road - a vacuum line is cheaper than a tow. I'm going to remove the not-used items and pipe-plug the holes so I know it won't break and leak. I'm flushing the coolant soon anyway so I change take these out of the water jacket.

There is also a valve on the RH exhaust that is disconnected and the plugs in my exhaust manifolds - so I will have to remove that when I change the manifolds to headers and build the dual exhaust. Someone already took the time to remove the emissions system - I had not dealt with these systems before so I didn't know what I was looking at or what was missing.

I don't seem to have the emission/evap canister but I will go look under the truck tonight - the benefits of having a lift.

 

[Oldbear42]

All the information and research and this is what I came up with. I miss having my old parts catalogues - I'm old enough that when I started as a partsman we had racks of books at each of our desks. Sometimes easier than on the computers. All are not needed and will be removed - the rest of the system is plumbed as it should be so we are good to go - thanks all.

 

[greyghost]

not needed tvvc, ported vacuum goes to dist., intake vacuum to brake booster, bottom carb vacuum to defrost ball if it has one, same to cruise control vacuum can, choke pull off, one into cab for idiot gauge to moniter engine performance n fuel efficiency. only wires to tranny are reverse light, kicked down uses vacuum connects same place booster does, if no kickdown check backend of tranny pass. side short hose from modulator to metal tubing, if squishy replace hose n ck modulator valve for fluid leaking out, tubing runs up to back of intake to short piece hose

 

[Ricko1966]

So we still don't have pics of how things are hooked up and what's plugged, etc. Yes we know what a tvs is and an idle compensater etc. We still don't know what's hooked up where. You've even gotten several advisement not all the same on hooking up the vacuum advance. I still stand by manifold vacuumnfor vacuum advance. Ported vacuum advance was primarily to light the charge late at idle,so the charge would continue to burn in the manifold,cleaning up emissions at idle. Ported Vacuum and Manifold vacuum match once the throttle is open. If it is currently ported,and you move it to manifold ,if your vacu7m advance can is working your carbureator will need adjustment. Which is another good thing you can close the throttle plate further,increasing vacuum at idle and keeping you further out of the transition circuit. Some people don't realize being in the transition circuit at idle cause a low speed stumble. And when tell people about the stumble 97 people are going to jump right up and say replace the accelerator pump,10 are going to tell you to bump timing and 11 will tell you to disable all the emisdions crap whether you have any or not.

 

[Oldbear42]

So we still don't have pics of how things are hooked up and what's plugged, etc. Yes we know what a tvs is

I didn't know what the sensors were but now I do - I included the picture to help the next person that comes looking for some extra knowledge.

I took a bunch of pictures when I had the air cleaner housing off - I hope they help show what I have.

 

[Oldbear42]

The second batch. I think everything is hooked up correctly. I realize the speedo cables are not hooked up and the cruise control is not in the right position - I have the new inner fenders sitting on the shop floor for Saturday mornings job list.

 

[Ricko1966]

I need a better pic of the front of the carb showing all of the vacuum ports and a white out dot or some kind of marker as to each end-of- the vacuum advance hose. Do you have pcv hooked up I see 1 end of a hose I'm assuming you have going there. Also need a decent shot from the passenger side rear of the carb.

 

[Ricko1966]

The one you marked tvs and the one you marked you don't know are both. Tvs just to control different items if one is open cold meaning you can blow through it cold but not hot,it controls the EFE ,that's the valve on the exhaust manifold,sounds like you are going g to headers so I won't bother explaining hookup or function. If the other one you can blow through hot but not cold it probably controls egr. The port I see with the screw in it is ported vacuum so all good leave it plugged.