Carbureted 350 vacuum line rebuild

[70C10Ginger]

Hey there,

I bought my 1970 GMC K2500 after seeing it sit for over ten years. I’m currently driving it as a work truck, and trying to get the money together to completely rebuild the engine and do a cab swap on it.

My problem right now, is I have no idea about vacuum lines. Ive read a bunch of the forums on the website, and have done some googling, so I think i know where to start asking questions.

Very first thing. Should i get a charcoal canister? Mine is completely missing


From there, the Thermal Vacuum Switch is totally unhooked.


The crankcase vent filter doesn’t stay attached at all.


I think the PCV valve works.....i think

[70C10Ginger]

And that’s all the attachments for that post.

From there, my automatic choke functions, and all the hookups are decent looking. But if you look at the vacuum line just below it, there’s a plugged t and a plugged vacuum line. I don’t think that its manifold vacuum? Also, my EGR is unhooked.


The vacuum advance worked on the new HEI system i installed, and i just want to make sure that I’m not wasting any vacuum, or fuel vapor, or horsepower by being ignorant.


Thanks for the help guys!!

Side note, check out this awesome thing that happened at my local Oreilleys!




Sorry its upside down, cant figure it out. I’m new haha

[57taskforce]

I don’t believe that’s the original engine. I’m quite certain these trucks never had egr or the vacuum switch in the thermostat housing. I would personally throw that stuff in the trash. I can’t help much for that vacuum routing unfortunately, as I don’t dabble in the later emissions stuff much. You don’t need any of that stuff other than the advance line to the distributor and the feed to the hvac, I’m sure you could easily plug the line coming off the choke or even swap it to an earlier style choke. Your truck, being a 3/4 ton I’m confident didn’t have the charcoal evap canister from the factory. If it did there would be a line coming off the gas tank Going up into the top of the passenger cab corner. The emissions on these trucks from the factory were pretty sparse. As far as I’m aware, the most they could have had was A.I.R and the evap canister setup, but that would have been on a 1/2 ton because the heavier gvw trucks had more lenient emissions standards. As for the fresh air side of the pcv, if the hose is bad you can probably get that from the dorman help line at your local parts store.

[hatzie]

If it's a manual transmission with California emissions it's possible it originally had Transmission Controlled Spark Advance in 1970. That would explain the thermo vacuum valve in the thermostat neck.

You do want the PCV valve running to manifold vacuum and the PCV intake running to a filter in the air cleaner or a filter cap in the valve cover opposite the PCV valve. This will keep the crankcase seals from weeping motor oil.

The 1967-72 trucks had the fuel tank mounted with you behind the driver seat inside the cab. You have to put some kind of hose arrangement on the tank vent to carry the raw fuel vapor out of the cab.
Vented fuel caps aren't an option. The filler is right next to the drivers side door.
Three hose EVAP cans like the 70's F-bodies and Vettes used are simple and easy to install. They have zero negative effect on performance and they'll keep the garage or barn from smelling like raw fuel. They have ported vacuum to the purge control valve, fuel vent from the tank, and purge teed into the PCV valve hose for manifold vacuum to suck out the trapped fuel vapor. In short the charcoal EVAP can is a good thing.
If it's parked outside you could just run an SAE J30R7 fuel vapor hose from the vapor connection on the fuel sender to an axle vent. You'll occasionally smell raw fuel using this setup.
The other simple venting option would be to do what Nissan did with the Datsun 210... put an axle vent inside the air cleaner filter ring and run the tank vent hose up to that.

[57taskforce]

Originally Posted by hatzie

If it's a manual transmission with California emissions it's possible it originally had Transmission Controlled Spark Advance in 1970. That would explain the thermo vacuum valve in the thermostat neck.

You do want the PCV valve running to manifold vacuum and the PCV intake running to a filter in the air cleaner or a filter cap in the valve cover opposite the PCV valve. This will keep the crankcase seals from weeping motor oil.

The 1967-72 trucks had the fuel tank mounted with you behind the driver seat inside the cab. You have to put some kind of hose arrangement on the tank vent to carry the raw fuel vapor out of the cab.
Vented fuel caps aren't an option. The filler is right next to the drivers side door.
Three hose EVAP cans like the 70's F-bodies and Vettes used are simple and easy to install. They have zero negative effect on performance and they'll keep the garage or barn from smelling like raw fuel. They have ported vacuum to the purge control valve, fuel vent from the tank, and purge teed into the PCV valve hose for manifold vacuum to suck out the trapped fuel vapor. In short the charcoal EVAP can is a good thing.
If it's parked outside you could just run an SAE J30R7 fuel vapor hose from the vapor connection on the fuel sender to an axle vent. You'll occasionally smell raw fuel using this setup.
The other simple venting option would be to do what Nissan did with the Datsun 210... put an axle vent inside the air cleaner filter ring and run the tank vent hose up to that.

Not all 67-72 trucks have the tank vent. Tons of them came from the factory with a single feed line from the tank to the carb and that’s it. In which case they came oe with vented caps. Non vented caps came with the charcoal canister setup. I believe 71-72 at least maybe even 70 as well 1/2 tons had the eec canister setup but it wasn’t added to most 3/4-1tons. If the rubber line coming off the sending unit to the hard line is in good condition and the tank filler to cab rubber seal is in good shape, fuel vapor/smell shouldn’t be an issue.
You are correct about the trans spark switch I had forgotten about that. Mine didn’t/doesn’t have it.
Here’s a picture of the sending unit setup his truck likely has.
I do agree there isn’t a huge issue against adding a canister (other than gathering the necessary parts) but it’s likely his truck didn’t have one originally.

[hatzie]

You don't want to smoke in those truck cabs.

Humor aside. I hadn't realized they didn't put a vent hose on them in 1970.

[70C10Ginger]

So, with you guys’ help, i figured out where I’m confused with my vacuum lines setup.

57taskforce is right about not having a stock evap canister, and the only real reason i want to add one is to eliminate the gas smell as much as possible.

Hatzie, you have officially answered my question about how i should set up my vacuum lines, thank you so much you guys.

Also, 57taskforce was correct again. The engine is from ‘75 and has the turbo hydro setup on it. So that’s why I’m looking at a bunch of “extra” emission parts, they all go to the imaginary automatic transmission haha

And no, definitely no smoking in the cab right now