? Vacuum Circuit 78 350 ?

[solidaxel]

I have a question for anyone with a 78 Heavy 1/2T about the routing of the vacuum lines on a Q-jet.
Everything looks like it is working properly but I have removed some lines over the years and I am trying to get everything back to stock location. There is a Heat switch on the water outlet with three connections, 1 goes to the carb and has vacuum sorce and I am guessing when the engine is hot enough it lets vacuum go to WHERE?? There is a choke pull off that I just installed and it will open the choke as soon as the engine is started (which I dont want until it gets hot) and then it is running very lean in cold weather and stalls. If I could see a photo of this in stock configuration or see a circuit it would be very helpful.
Thanks in advance.

[Mickey_D]

The choke pull off is adjustable. When your engine is off and you set the choke, it closes all the way to enhance starting. As soon as the engine is running, it needs far less choke, so the pull off is meant to automatically open the choke half way. There is a Phillips screw on the mechanism that you can use to set how far the pull off yanks back on the choke plates. Play with that until you get it where you want it. Then when the engine is warm, the electric choke pulls it open the rest of the way once the engine is fully warmed up. This also pulls the fast idle cam out of the way.

As for the vacuum lines, I can only point out what the ones on my 81 with the smog pump do. Almost all of the lines coming out of the carb and going to those two thermal vacuum switches (mine has an orange one on the thermostat housing and a green one in the manifold) are for the EGR and the air injection reaction system. There's also a line that goes to the charcoal canister behind the driver's headlight and a small one that goes to my transmission torque converter lockup.

Any other questions I might be able to help out with will involve me getting dressed (it's 3 deg F outside right now and my hood's frozen shut with ice).

[solidaxel]

Thanks for the help. I will look into the mechanical side of the choke pull off, and work on that first.
And NO stay in where it is warm it is not that important!!!

[Mickey_D]

Your carb might not have the adjustment on it. Yours might be like the one on the left here (notice the red arrow pointing at nothing? Mine is like the one on the right.

[solidaxel]

Not like either one!
It has a thermostat pull off for the choke not electric, and it does not have that phillips screw on the top of the vacuum pull off

[78Chevy]

Here is a few vacuum diagrams hopefully one of these will help. If you can't find it on these I think there may be a couple more, but these are probly the most common. I have the same problem with my 78 excepte the ports on the carb don't match the ports in the diagram, gotta love those vacuum systems. Anyways hope this helps and good luck.

[solidaxel]

........BINGO.... the last one is the one I am looking for,
I will try that circuit,and see if anything changes
Thanks

[solidaxel]

Are there different temp. switches?
At what temp does the air cleaner butterfly start to open and what temp does the distributor go into advance at idle?

[andresco50]

Awesmoe thread

[78Chevy]

Originally Posted by solidaxel

Are there different temp. switches?
At what temp does the air cleaner butterfly start to open and what temp does the distributor go into advance at idle?

I can't remember right of the top of my head, but I'm pretty sure that all air cleaner 'thermostates' open at the same temp. The thermo switchs, I believe do open at different temps with each application being different. You could probly find that info but, if you ever take the switch out it should be stamped on the bottom of it.

By the way is the port coming from the carb to the switch ported or full vacuum?

Thanks

[solidaxel]

-Mine has vacuum at idle on the carb connection to the temp switch and it opens when it gets up to temp, but I don't know at what temp it starts to open!

[78Chevy]

Well, just so long as it opens I don't understand why you would need to know the exact temp, but like I said it should be stamped on the brass part of the switch that is down in the coolent. I'll try to find that info for you tonight when I get off work, but I'm not promising anything yet.

[78Chevy]

Ok, I was both wrong and right . The only info I can find about the thermo switchs is "... when the engine reaches a predetermined value, the thermal vacuum switch (TVS) ports open and allow manifold vacuum to the distributor..."
But I did find the air clearner temperature range. It should open if the temp. is over 90 degrees and no more than 130 degrees with 115 degrees being normal. This is measured inside of the air cleaner where the sensor is located. Sorry about the thermo switch I guess you'll just have to take it out and see sometime.

Good luck.

Al