Vacuum line routing

[nabeshin]

When I replaced the vacuum lines, I only took off one line at a time and copied what was there with new lines. Wondering now if it is hooked up right? No driveability issues at all. Just wanting to make sure it is all alright.

This is how it is routed right now: (i have a stock quadrajet)

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If that is correct, then is this following description right? If this engineered drawing is right, then my port and manifold lines are opposite and should be switched. Since you'd want port vacuum on the distributor for quick warm up then it will change to manifold once warmed up.

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I think EGR Vac is "Manifold Vac", and "to Carburetor" or "Spark Port" means "Ported Vac"

Does the diverter on the snorkel need full manifold vacuum or could that function on ported vac?

Any help is appreciated.

 

[C10MixMaster]

When I replaced the vacuum lines, I only took off one line at a time and copied what was there with new lines. Wondering now if it is hooked up right? No driveability issues at all. Just wanting to make sure it is all alright.

This is how it is routed right now: (i have a stock quadrajet)

You must be registered for see images attach

If that is correct, then is this following description right? If this engineered drawing is right, then my port and manifold lines are opposite and should be switched. Since you'd want port vacuum on the distributor for quick warm up then it will change to manifold once warmed up.

You must be registered for see images attach

I think EGR Vac is "Manifold Vac", and "to Carburetor" or "Spark Port" means "Ported Vac"

Does the diverter on the snorkel need full manifold vacuum or could that function on ported vac?

Any help is appreciated.

First follow this link to https://www.gmsquarebody.com/threads/1973-1991-service-manuals.14499/ this has the factory manual for your truck down load it and keep it. Look at page 6t-3 that has the vacuum diagrams for your 76 k10.

looks like your manifold and ported lines are switched if im looking at the correct vacuum diagram.

leave the air cleaner connected to the manifold vacuum source.

Depending on the temperature set point of the vacuum switch, it most likely draws ported vacuum all the time unless the engine is running hotter that it is supposed to, once its running hotter than it is supposed to it switches to manifold vacuum advancing the timing at idle to help it run cooler.

Personally i like to run manifold vacuum all the time without a PVS.

 

[75gmck25]

If you want to simplify things you might try using the HD emissions vacuum setup from my '75.

- Vacuum tree screwed into the intake near the back of the carburetor. This tree provides manifold vacuum to the HVAC, the TH350 and the air cleaner. Carburetor is not part of this set of lines.
- Carburetor has three connections:
-- Large threaded metal fitting and large hose on the back running over to the Power Brake booster
-- Large (3/8"?) fitting on the front running over to the PCV valve
-- Manifold vacuum line running over to the distributor vacuum advance (stock I believe it used ported)

I prefer manifold vacuum since it makes the engine run smoother with A/C at idle, so I don't need any idle compensator. I have a slightly more aggressive cam (213/217 @ .050) and aluminum 64cc heads, and find it pulls much better off-idle with about 15 degrees base timing, although factory recommendation is 8 degrees BTDC. My centrifugal advance adds another 20 degrees by about 2200 rpm, and my current vacuum advance unit is 15 degrees (stock vacuum can added 22 degrees). With the 64cc heads and thin head gaskets I estimate I have about 9.2-9.4 compression.

Bruce

 

[nabeshin]

I'll check out the manual and maybe just bypass the vacuum tree thingy and port vacuum.

 

[nabeshin]

Seems like my truck never had EGR, AIR, or the fuel vapor canister.

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But yeah, seems like C and B are flipped on mine. I'll experiment with this set up and then try out putting manifold vacuum to everything and bypassing the Spark Control Switch and capping the port vac port.

 

[nabeshin]

Ended up with the port vacuum capped off and the spark advance and air cleaner things hooked to manifold vacuum; the Spark Control Switch is now completely bipassed.

I adjusted timing with a vacuum gauge (don't own a timing light) to get the highest reading from 15" of Hg to 20", then down about 1" to avoid ping under load. Tried to tweak idle mixture to get max vacuum as well. They started at 3-4 turns out (never touched it before), and ended up in the same place. Is this bad? I might need to rebuild the carb. Also I can't get to the 600-700 rpm idle speed. The throttle linkage won't go all the way closed to meet the adjuster screw, idling at about 1000-1100 rpm. Not sure if the linkage ever touched, but the thing is idling faster with this new configuration.

At any rate it seems to run way better on manifold vac for spark. More power off the line and no pinging under load. More lively all around. I need to order a rebuild kit from Cliff.

 

[C10MixMaster]

Ended up with the port vacuum capped off and the spark advance and air cleaner things hooked to manifold vacuum; the Spark Control Switch is now completely bipassed.

I adjusted timing with a vacuum gauge (don't own a timing light) to get the highest reading from 15" of Hg to 20", then down about 1" to avoid ping under load. Tried to tweak idle mixture to get max vacuum as well. They started at 3-4 turns out (never touched it before), and ended up in the same place. Is this bad? I might need to rebuild the carb. Also I can't get to the 600-700 rpm idle speed. The throttle linkage won't go all the way closed to meet the adjuster screw, idling at about 1000-1100 rpm. Not sure if the linkage ever touched, but the thing is idling faster with this new configuration.

At any rate it seems to run way better on manifold vac for spark. More power off the line and no pinging under load. More lively all around. I need to order a rebuild kit from Cliff.

look down the carb make sure you can close the throttle plates, both primaries and secondaries. If there are not closing things to check would include the throttle cable, transmission cable, choke fast idle cam or a severely worn primary throttle shaft. 3-4 turns on an early carb (one with a divorced choke) seems like alot.

 

[nabeshin]

At the time I tried backing off the fast idle cam and there is slack in the linkages for throttle and trans. Still not closing all the way. I'll have to climb higher on the engine to look down the throat. The throttle shaft does seem to wobble some. I think it is leaking air from around those, and from everywhere else on the carb. Also of note, full motion of the throttle shaft fails to open the secondaries hardly at all. This is peeking under the top choke flaps. I can rotate the secondaries lever separately to get full motion out of it.