Quadraject Vacum Line ?

[Shugalou]

Hey Guys,

I'm still getting familiar with this truck (78 stepside) and while I am getting ready to rebuild the carb (first time for me) a couple of things caught my attention.

One of the lines is plugged, in a hack job sort of way and the other looks like it's just open - should the open port be plugged as well?

I also noticed that one of the return lines to the emisions canister is cut I'm guessing this was done to improve performance? Does it effect the reliability in any way?

Thanks

[Shugalou]

Here is the canister

[Cherub]

right or wrong i remove the emission stuff when i get a new square.

[Shugalou]

Does it choke performance that bad?

[83GMCK2500]

Originally Posted by Shugalou

Does it choke performance that bad?

No, at least, I don't believe it does. I have left all of my vacuum and fuel vapor lines intact as far as the charcoal canister and whatnot is concerned. The only emissions hardware that I remove is the air pump and it's associated parts, plug the holes in the manifold.

I keep the heat riser valve on the passenger manifold, just make sure it moves freely and that the bellows is sound. I can get in my truck at 0 degrees outside, tap the throttle to set choke, fire it up, let it run, drive it, and after it reaches temp the choke opens and idles fine (after some slight adjustment recently). My truck runs smoothly and warms up quickly and part of me likes to attribute part of that to keeping it almost as Ma General built it.

Do you still have your vacuum hose diagram under the hood? That large port (left one circled) is likely the one that runs to charcoal canister (the cut line you pictured).

[Shugalou]

Thanks 83 GMC

No there is no diagram under the hood and most of that stuff you are referring to in new to me (heat riser valve, air pump) I'm gonna have to dig deeper with my research.

I thought maybe that port was supposed to go the canister, should I block it off

[bnoon]

Yes, that's the vent line for the carb and stock it goes to the canister. Does not need to get blocked. Either run it back to the canister or tap into the vent line that goes from the canister back to the tank vent and bypass the canister. Just keeps it from smelling like raw fuel under the hood and possibly into the cab that way.

The small plugged line I don't remember. My BBC truck doesn't have it and I can't find my pictures of the SBC truck I had.

[SkinnyG]

The circled plugged hose on the right goes to the vacuum advance canister on my '77.

The circled port on the left I do not have on my carb. I do have a port below it on the base, though, which goes to my charcoal canister.

[mrolds88]

Port with the screw in it is for the vacuum advance on the distributor. It needs hooked up.

[cadillac_al]

Port with the screw is ported vacuum. Factory used it for vacuum advance but many people plug it and use full time vacuum for the advance. I'll bet that's the way yours is hooked up.

[Shugalou]

Thanks for the info guys
Looks like ill put the one hose back to the canister and ill locate the vacum advance and see if it runs different when hooked up
It runs ruff sometime and feels like its gonna die at a light at times.

Thanks guys this is an awesome forum!

[geezer#99]

It won't do any good hooking up that top hose to your charcoal canister without setting up the canister correctly.
The Small hose with the screw in it is a ported vacuum source.
Rebuilding your carb might not change how your truck runs now. It runs rough and dies cause of your unheated air to your carb.

[Shugalou]

Thanks Geezer,

I'm gonna have to locate this diagram to find out how it is supposed to be routed. I figure there is no harm rebuilding the carb (provided I don't screw it up). Since I have no idea when it was done prior.

[Shugalou]

OK I think I found the diagram. Does this look like it makes sense?
This is for a 78 350 with 4 Barrel