Seeking Edgeucation on Rochester Carburetor Adjustments & Vacuum Port Config.

[71CHEVYSHORTBED402]

Edit: Images updated with new info. Images for vacuum exclude T.C.S./C.E.C./Idle Stop configurations

Working on a 71 402 with a factory set-up or trying. This is a 71 Rochester Quad. Hoping to get some basic knowledge, it's that or trust someone who's never seen a carb. before adjust it in our future

Think I know where some of this goes, but to save some composure this time around, if some in the know can fill in the blanks (they're mostly blanks) I'd appreciate it, basic instruction how to set the mixture right appreciated as well. When done I'll throw it on a picture and we'll all be "experts"

By the way, what's the lever on the carb. called, where the Remote Choke Bi-Metal Thermostat Spring Coil Assembly (cough!) link attaches, or do I run with "lever"?

Thank you.

You may find this useful to copy/paste:

A. Screw for fast idle? RPM?

B. Vacuum connects to?

C. Vacuum connects to?

D. Vacuum connects to?

E. Some air/fuel stuff, general description and perhaps adjust recommendations.

F. Vacuum connects to the PCV, and I think a Tee runs off to the Charcoal can. Can't wait

G. Vacuum connects to?

H. Same as E. Interesting there are two, but I've never seen the core.

I. I believe for base idle speed. Think I read 1000 RPM is about right.

J. Screw is for?

K, Vacuum connects to?

Other: Mystery lever where the choke coil link attaches onto the carb. is called a lever or something

[HotWheelsFan]

Quick reply here.

A. Fast idle screw.

B. Vacuum port B connects to D. This vacuum actuator gets full vacuum. This is the choke pull off. Choke pull off is the vacuum operated actuator that opens the choke blade a certain amount so the engine does not flood out when started cold.

C.

D. This should be full manifold vacuum, connect this to the choke pull off as mentioned above (B).

E. Air/fuel mixture screw. Two of them... E and H as in your pics. These screws are the final tuning of the engine. These screws only adjust the air/fuel mixture when idling.

F. Large vacuum port, full manifold vacuum to PCV valve.

G.

H. Air fuel mixture screw, as mentioned above (E).

I. Idle speed screw.

J.

K.

Vacuum ports C, G, and K would be for distributor, trans controlled spark, and any of the emission devices on your truck. If you have any or some of the emissions systems that you need to hook up, then you need a vacuum diagram, or double check the vacuum ports to what each device needs.

Distributor needs vacuum, can use manifold vacuum or ported (off idle vacuum). Many discussions on this matter. Full manifold was used before emission devices were added.


Bob

[71CHEVYSHORTBED402]

Originally Posted by HotWheelsFan

Quick reply here.

A. Fast idle screw.

B. Vacuum port B connects to D. This vacuum actuator gets full vacuum. This is the choke pull off. Choke pull off is the vacuum operated actuator that opens the choke blade a certain amount so the engine does not flood out when started cold.

C.

D. This should be full manifold vacuum, connect this to the choke pull off as mentioned above (B).

E. Air/fuel mixture screw. Two of them... E and H as in your pics. These screws are the final tuning of the engine. These screws only adjust the air/fuel mixture when idling.

F. Large vacuum port, full manifold vacuum to PCV valve.

G.

H. Air fuel mixture screw, as mentioned above (E).

I. Idle speed screw.

J.

K.

Vacuum ports C, G, and K would be for distributor, trans controlled spark, and any of the emission devices on your truck. If you have any or some of the emissions systems that you need to hook up, then you need a vacuum diagram, or double check the vacuum ports to what each device needs.

Distributor needs vacuum, can use manifold vacuum or ported (off idle vacuum). Many discussions on this matter. Full manifold was used before emission devices were added.


Bob

Thanks a bunch Bob, that was real helpful, especially the pull-off business, that one had me stumped.

I threw some info. below together, though after thinking about it, I suspect "G" is intended for TCS as opposed the distributor, or rather for the Idle Stop Solenoid, I don't know how that business hooks up, and don't really care.

Removed paragraph, too confusing & wrong.

[mr.mud1]

Originally Posted by 71CHEVYSHORTBED402

Thanks a bunch Bob, that was real helpful, especially the pull-off business, that one had me stumped.

I threw the info. below together, though after thinking about it, I suspect "G" is intended for TCS as opposed the distributor, or rather for the Idle Stop Solenoid, I don't know how that business hooks up, and don't really care.

I think "C" is supposed to go to distributor, they're the same size ports. "K" to "CARB" on the charcoal can are the same size ports as well.


Always open to corrections or more details, thank you.

hi,your mystery screw "J" is holding the fast idle adjustment apparatus to the throttle plate.there's nothing to do with that screw,only the fast idle screw that is at right angles to "J" facing forward.i know there's discussion about fulltime or ported vacuum for the distributor.right or wrong,i've always used ported vacuum and on my 67 327 with a 7027218 quadrajet,which i realize isn't your 71 402,the distributor vacuum line is connected to your outlet marked"G" which is ported vacuum.Greg

[Tomchev]

The 71 trucks had a solenoid that was used for anti dieseling. Some of the vacuum ports were used for that. One vacuum port was used for the hot air door on breather. The screw on the throttle shaft with bracket holds off the back barrels from opening when choke is on.

[71CHEVYSHORTBED402]

Anyone know what the vacuum source is for a vacuum reservoir? Here's a diagram, though the vacuum control on my temp. control has three vacuum ports. The vacuum wasn't set up when I dismantled the reservoir beyond this is how I found the vacuum control:

• Port #1 runs to the water valve. The valve closes under vacuum.

• Port #2 runs to the inside and outside air door actuators. The inside actuator opens and the outside actuator closes under vacuum.

• Port #3 runs to the Vacuum Reservoir.

Diag. of a two port vacuum control:



Found a diag. for a three port vacuum control:

[HotWheelsFan]

Vacuum source for AC/Heating system can be run off the intake manifold behind the carburetor. You should find a vacuum port or a plug. Vacuum port could be hooked up to trans modulator (turbo 350 trans).

Gets full vacuum at all times.

[71CHEVYSHORTBED402]

Originally Posted by HotWheelsFan

Vacuum source for AC/Heating system can be run off the intake manifold behind the carburetor. You should find a vacuum port or a plug. Vacuum port could be hooked up to trans modulator (turbo 350 trans).

Gets full vacuum at all times.

I have a question what distinguishes full vacuum and not?

My 71 manifold has a three port vacuum fitting. I think it's correct. The ports run to:
>400TH modulator
>Brake Booster
>Air cleaner, though I'm not 100% sure. If the air cleaner doesn't run to that then not sure. Edit, see, that's why it's better I keep it shut, the air cleaner goes to K

So you guys straightened me out, especially our friends in Nova Scotia Distributor does go to G. Years ago I pulled this off my 72 thinking my 71 might need it more, and forgot about it. It's been hanging on my tool board, I look there I don't know, 30 times a day


So now I know my 71 needs it

I think it's missing a part at the moment, supposed to go in front of the sleeve, just guessing. Seems a 1/4" to 3/16" reducer of sorts. The pic. in post 9 shows a long vacuum hose, but the vacuum tube I have seems to disagree.

Makes me wonder what other vacuumed components took hard lines. I think the vacuum mod may have on top and bottom. The tubesl have flare fittings for what worth, same with what I think goes to the air cleaner off the manifold fitting. Nonsense

[71CHEVYSHORTBED402]

This sure clarifies some things, another 402 off this very cool thread, though I'm not sure "K" is quite factory due the long hose at the carb.: http://67-72chevytrucks.com/vboard/s...d.php?t=367725

Hot wheels you nailed it. Vacuum reservoir to the intake fitting and the air cleaner goes to K. Thanks for all your help, the top images should reflect it.

[mr.mud1]

hi,you're making progress,my 72 402 truck is stuck in covid prison at the Canada/US border in Maine.i hope to have it home in the next week or so??i'll get you some pictures then if need be.i have included a couple pictures of the hoses with the hard line from my 67 327 impala wagon.i realize not a truck but it might help.

[71CHEVYSHORTBED402]

That carb looks more like the one off my 72 402, for example, no "C" vacuum port.

I realized the tube at the end of the boot in the picture (post #8) came off the 72 carb That boot makes for a nice fitting, on the condition the port is 1/4", which my 72 Rochester takes. Not sure it's an original 72 Rochester, but it probably is (edit: it is).

All the vacuum ports on my 71 Rochester are 3/16", so it would seem there were different boots or fittings, or perhaps just hose in cases.

Anyone have an original 71 402 vacuum tube? No really.

[71CHEVYSHORTBED402]

After I lost my fear of running the boot on the vacuum line I realized it's just a rubber hose I don't know, maybe not but close enough. If you slide the 3/16" hose on from the distributor side then yeah, by the time you get past the flanges it will probably fit on a 1/4" port

The mystery to me is why two flanges on the the distributor line nearest the carb? Seems to imply a fitting of some sort, but seems to be just the same to insert a 3/16" hose over one or both.

[pjmoreland]

My truck has a clip like that on the purple and yellow wires that run to the starter motor.

[71CHEVYSHORTBED402]

Originally Posted by pjmoreland

My truck has a clip like that on the purple and yellow wires that run to the starter motor.

I wonder if there were three of those clips.

[71CHEVYSHORTBED402]

On that question I had post 13 for installing vacuum hose right, I think, and that's where I get in trouble so often those flanges for lack of better term on hard lines serve one of two purposes. If the line converts from 1/4" to 3/16" such as for the transmission vacuum modulator connection, then the hose slips over the flanges. If not then the flanges are more/less just a stop. If that's true then the picture in post 13 is correct, because it's a 3/16" hardline running to a 3/16" carb port.

[mr.mud1]

good day sir,i have a few pics of my 327 in my old wagon,again i realize it's not your 402 truck but it might help.my 402 72 truck is still at the border,trying to get home and as soon as it does,i'll have you some pictures of that and how it's set-up.that hard line on my engine,fits into the port on the carb and the rubber hose holds it all together.take care.Greg

[DT1]

Originally Posted by Tomchev

The 71 trucks had a solenoid that was used for anti dieseling. Some of the vacuum ports were used for that. One vacuum port was used for the hot air door on breather. The screw on the throttle shaft with bracket holds off the back barrels from opening when choke is on.

My 71 still has the anti dieseling solenoid. It was disconnected when I took possession of the truck and the throttle screw was adjusted to compensate.
The truck never had a dieseling issue even on the hottest days in the Mojave desert. I plugged the vacuum ports on it and wired it to my A/C clutch switch. I then adjusted the solenoid to up the engine RPM slightly (70-100 RPM) when the A/C is on. This overcomes the compressor load when the truck is idling.

[71CHEVYSHORTBED402]

Any chance you guys are referring to the Combination Emissions Control (C.E.C) valve? I think some call it an idle stop solenoid. Might be a form of or same as transmission control spark. If not then how was an anti-dieseling assembly vacuumed? Just curious is all, I ditched all of it.

I think the CEC was a 71 deal, though 72 may have been stocked with the same or similar.

[71CHEVYSHORTBED402]

Originally Posted by pjmoreland

My truck has a clip like that on the purple and yellow wires that run to the starter motor.

Going off new harnesses, those clips were crimped on the braided wire loom and/or cloth high temp. elect. tape.

Downside of pics is limited days to change or remove them. I deleted that post because the clip I was showing wasn't the factory vac. advance line clip. I used one for it though, no complaints.

I little bird from Canada led me to the right vac. advance clip. Something or exactly like this off the intake bolt behind the choke coil https://www.topflightautomotive.com/...uum-line-clip/

Pic of what I installed, not factory. Someday I may install the right clip, but it's clean as is (eg, no stress on valve cover bolt) pending unforeseen, and I'm not messing with an intake bolt.