HEI Vacuum Advance

[samkost]

A friend came over to see the latest progress on my 72’ when he pointed out that my HEI and carburetor vacuum ports has been capped/plugged.
The carburetor manifold 3/16” port is also plugged.
Is a vacuum line needed for proper performance, timing and function?

Do I need to install a vacuum line HEI to Eldebrock?
The truck runs well but am I missing proper performance?
Truck runs well but is more power accessible with a vacuum line?

Rebuilt OE 350 V8 with Eldebrock Performer Intake, Eldebrock 1406 Carburetor and a mild Cam.
I never got model/parts numbers from the restoration shop from “Hell “
That’s another story.
Thanks all

[Steeveedee]

You should get a little better mileage at light throttle application with the vacuum advance connected. When you're deeper into the throttle, the vacuum advance drops out.

[truckster]

Connect the vacuum advance to full manifold vacuum. You'll get better throttle response. Your engine might even run a little cooler.

[samkost]

Originally Posted by truckster

Connect the vacuum advance to full manifold vacuum. You'll get better throttle response. Your engine might even run a little cooler.

Pardon my ignorance on this subject.
When you say full manifold vacuum, a 3/16” vacuum line from the HEI to the righthand manifold carburetor port or the lefthand timed port?
Do both need to be plumbed?
Thanks

[Mike C]

Manifold vacuum is where you want the line. The timed, or ported, vacuum increases with fuel flow through the carburetor rather than going to zero at WOT like the manifold signal does.

[Steeveedee]

I've never heard it called a "timed" vacuum port, but that's where the vacuum advance generally connects. That's mostly for emission controls, as advanced timing at idle does produce more pollutants. Though some manufacturers used "ported" spark, which is what I know it as, long before emission controls were invented. That port doesn't "see" vacuum until the throttle plate is opened further off idle. If you connected both to your vacuum advance can, you'd see vacuum to it at all times. Your truck came from the factory with the "ported spark" setup.

[samkost]

Alright, that sounds simple enough.
A 3/16” vacuum line from the HEI to the carburetor manifold vacuum port.
I assume once the vacuum line is hooked up an idle adjustment first and then a possible HEI advance adjustment will need to be performed?
I was told that the idle will automatically increase after the vacuum line is installed?
Would this be the correct steps to take?
Steeveedee, truckster and MikeC, I appreciate everyone’s input and direction.
Thanks

[Steeveedee]

If you connect the vacuum line to the 3/16" tube on the passenger's side of the carburetor, the idle speed should not change. That port doesn't see vacuum until the throttle is opened.

And you are welcome!

[samkost]

Great! Thanks again Steve

[68 P.O.S.]

Connecting your vacuum advance isn’t as easy as just plugging the line onto an available vac port. You have to correctly setup your distributor, carb, and timing for the engine to run properly on either vac type. If it’s for mandatory emissions, you’re running ported, or timed, vac. If there’s no emissions restrictions on your engine, you can still run ported, however performance-wise, manifold vac is the best bet for your engine. Manifold vac assits in the engine running more efficiently, it improves gas mileage (nothing to get excited about), it improves throttle response, and it allows the engine to run cooler. Obviously these gains aren’t gonna be monumental, but they’re there and every little bit helps.

For the distributor, you have to ensure it gives you 22 degrees of timing. You need to replace the advance springs with ones that allow full advance at around 2500-3000 rpm. I set my truck’s to its rpm at 60 mph on the freeway. I also install my springs after I set the timing because the advance weights fully deploy at a lower, quieter rpm. Crane makes a great vac advance kit for the HEI (99600-01). You also have to restrict the vac advance to provide 10-16 degrees advance using a stop plate. 12 degree is probably more realistic, but you have to test and listen for pinging and adjust for your application.

On to the carburetor. The first thing you should do is make sure the carb is clean inside and ensure all settings are at factory recommended specs. It is recommended you get a calibration kit for the carb and a fuel pressure regulator set to 5.5 psi (or the Edelbrock fuel pump). Next, you MUST adjust the transition slots, located in the primary bores, using the idle adjustment screw to form a square. This ensures the carb idles only on the idle circuit and ensures the other circuits are synchronized as well. Once you start the engine (the last step after this tune-up is completed), the idle should already be at 750 rpm in Park because of the vacuum advance and require little, if any, adjustment. You don’t want to adjust it much at all; you want to keep that transition slot square. 750 rpm in P is equal to the factory setting of 600 rpm in D w/ foot on brake. Your high idle speed (choke) should be set at 1500 rpm.

Now for timing. GM engineers (and millions of hot rodders around the globe) have found that SBCs run optimally at ~36 degrees of total timing. Also, Vortec heads are much more efficient, so if you have a set of these, they like a total timing of ~32 degrees. (These numbers are a general guideline. Yes, all engines are different. Mine may like 36 degrees, while yours only wants 34 degrees. Weather/environmental dependent as well. You have to test test test.). If you set the timing with the advance springs inside the distributor, you increase the rpm until the timing stops advancing, then you can set it to 36, or 32, or whatever. If you set the timing without the springs inside, it’s the same operation, only it requires less rpm (and noise) to do the same thing. Once total timing is set, install the advance springs if not already, and you’re done. Take it out for a test drive, put it under different throttle conditions, and look, listen and feel. Make any adjustments needed and test drive until you feel satisfied. Then, once complete, you can then check all your timing numbers, carb numbers, etc., and write them down to reference which settings your engine ran best at.

[samkost]

Thanks 68 P.O.S.
Wow, That is quite the detailed follow up procedure.
It looks familiar from a past post I read a while back.
A little above my pay grade but I will get some help to accomplish this task.
My emissions EEC canister was removed and never reinstalled.
Safe to say no emissions parts on the truck now.
Just another gremlin to deal with and like several other OE parts that went missing by the restoration shop from “Hell “.
I appreciate the concise explanation of the HEI and carburetor vacuum advance procedure.
Thanks again

[samkost]

Update report.
Up and running great!
Installed the missing vacuum line from HEI to my Eldebrock 1406 vacuum mainfold port.
After first start the RPM’s were raised so we bumped the idle down.
Timed the truck, adjusted the fuel air mixtures and idle once more.
The 350 has never ran this good and smooth since the rebuild.
I’ve got excellent throttle response, idle and the promised 325 missing horsepower is now available.
What a difference.
Thanks again to everyone for your input and details.
The forum site comes through again.

[slikside]

Originally Posted by 68 P.O.S.

Connecting your vacuum advance isn’t as easy as just plugging the line onto an available vac port. You have to correctly setup your distributor, carb, and timing for the engine to run properly on either vac type. If it’s for mandatory emissions, you’re running ported, or timed, vac. If there’s no emissions restrictions on your engine, you can still run ported, however performance-wise, manifold vac is the best bet for your engine. Manifold vac assits in the engine running more efficiently, it improves gas mileage (nothing to get excited about), it improves throttle response, and it allows the engine to run cooler. Obviously these gains aren’t gonna be monumental, but they’re there and every little bit helps.

For the distributor, you have to ensure it gives you 22 degrees of timing. You need to replace the advance springs with ones that allow full advance at around 2500-3000 rpm. I set my truck’s to its rpm at 60 mph on the freeway. I also install my springs after I set the timing because the advance weights fully deploy at a lower, quieter rpm. Crane makes a great vac advance kit for the HEI (99600-01). You also have to restrict the vac advance to provide 10-16 degrees advance using a stop plate. 12 degree is probably more realistic, but you have to test and listen for pinging and adjust for your application.

On to the carburetor. The first thing you should do is make sure the carb is clean inside and ensure all settings are at factory recommended specs. It is recommended you get a calibration kit for the carb and a fuel pressure regulator set to 5.5 psi (or the Edelbrock fuel pump). Next, you MUST adjust the transition slots, located in the primary bores, using the idle adjustment screw to form a square. This ensures the carb idles only on the idle circuit and ensures the other circuits are synchronized as well. Once you start the engine (the last step after this tune-up is completed), the idle should already be at 750 rpm in Park because of the vacuum advance and require little, if any, adjustment. You don’t want to adjust it much at all; you want to keep that transition slot square. 750 rpm in P is equal to the factory setting of 600 rpm in D w/ foot on brake. Your high idle speed (choke) should be set at 1500 rpm.

Now for timing. GM engineers (and millions of hot rodders around the globe) have found that SBCs run optimally at ~36 degrees of total timing. Also, Vortec heads are much more efficient, so if you have a set of these, they like a total timing of ~32 degrees. (These numbers are a general guideline. Yes, all engines are different. Mine may like 36 degrees, while yours only wants 34 degrees. Weather/environmental dependent as well. You have to test test test.). If you set the timing with the advance springs inside the distributor, you increase the rpm until the timing stops advancing, then you can set it to 36, or 32, or whatever. If you set the timing without the springs inside, it’s the same operation, only it requires less rpm (and noise) to do the same thing. Once total timing is set, install the advance springs if not already, and you’re done. Take it out for a test drive, put it under different throttle conditions, and look, listen and feel. Make any adjustments needed and test drive until you feel satisfied. Then, once complete, you can then check all your timing numbers, carb numbers, etc., and write them down to reference which settings your engine ran best at.

Great info!

[68 P.O.S.]

Glad to see it's running well

[Rem701]

Originally Posted by samkost

Update report.
Up and running great!
Installed the missing vacuum line from HEI to my Eldebrock 1406 vacuum mainfold port.
After first start the RPM’s were raised so we bumped the idle down.
Timed the truck, adjusted the fuel air mixtures and idle once more.
The 350 has never ran this good and smooth since the rebuild.
I’ve got excellent throttle response, idle and the promised 325 missing horsepower is now available.
What a difference.
Thanks again to everyone for your input and details.
The forum site comes through again.

Does it matter if there is a hose from the front of the carb to the drivers side valve cover? I just have a breather in the valve cover.

[dmjlambert]

Hi Rem701. The hose from the valve cover is for PCV (positive crankcase ventilation). The hose goes to a PCV valve, and the PCV valve plugs into the valve cover. It sucks blow-by gasses from the engine into the intake and burns them. I suggest start a separate thread with questions about this separate topic, so you get more eyes on it, and perhaps include pictures of your engine.

[54blackhornet]

Originally Posted by samkost

Update report.
Up and running great!
Installed the missing vacuum line from HEI to my Eldebrock 1406 vacuum mainfold port.
After first start the RPM’s were raised so we bumped the idle down.
Timed the truck, adjusted the fuel air mixtures and idle once more.
The 350 has never ran this good and smooth since the rebuild.
I’ve got excellent throttle response, idle and the promised 325 missing horsepower is now available.
What a difference.
Thanks again to everyone for your input and details.
The forum site comes through again.

Sam you have one fine looking engine compartment ! Congratulations !..Jack

[54blackhornet]

Originally Posted by Rem701

Does it matter if there is a hose from the front of the carb to the drivers side valve cover? I just have a breather in the valve cover.

You presently have no positive crankcase ventilation with out the connection to the carb and the PCV valve. 10 minutes and a few dollars in parts and you will be all set…

[72SB]

Originally Posted by Rem701

Does it matter if there is a hose from the front of the carb to the drivers side valve cover? I just have a breather in the valve cover.

You can run a hose from carb base, front or rear, to a PCV valve in VC and have a breather on the other VC

[mr48chev]

Originally Posted by Rem701

Does it matter if there is a hose from the front of the carb to the drivers side valve cover? I just have a breather in the valve cover.

You need the PCV valve and line to the carb on one side and a breather on the other valve cover to have it work right.

I'd hope that if Samkost posts a photo of the right side of his engine there is a push in breather where the tube from the valve cover to the stock air cleaner would have gone.

This is a diagram of an "original" PCV system with the tube running from valve cover to the air cleaner to pick up clean fresh air and allow it to flow though the crankcase collecting fumes and moisture to be drawn out by the vacuum from the intake ( hose connected to the port on the carb is still intake manifold vacuum)



If you don't have a PCV system hooked up and just run one or two breathers in the valve covers It doesn't work to draw the fumes and condensation out but allows built up crankcase pressure to blow oil out of the breaters even on an engine in excellent shape. Back in the 70's when I built my T bucket I ran 2 breathers and had oil all over my engine. It was after I pulled the 283 out of the T and put it in my 48 with a 4 barrel rather than the 2 4 barrels that I fiigured out that I needed to hook up a PVC system.

[samkost]

Originally Posted by 54blackhornet

Sam you have one fine looking engine compartment ! Congratulations !..Jack

Thanks Jack

To answer mr48chev,
Here is an earlier photo of my passenger side valve cover breather.
I am seriously contemplating powder coating and installing my original air cleaner or a dual snorkel with the correct decals.

[RichardJ]

The drawing that mr48chevy posted is a "Closed PCV System". With a valve cover breather, you have an "Open System".
The arrows shown in the drawing posted above, show the direction of air flow when you have high to medium vacuum levels.
When accelerating and especially at WOT, vacuum levels are very low and at the same time, blow-by past the rings is at its highest. During this period, the arrow in the hose, valvecover-to-air cleaner would be pointed toward the air cleaner. Blow-by is still going into the engine to be burned, just not as much through the PCV Valve.
Image when under heavy acceleration or WOT is below.

I don't want to restart an argument about manifold vs ported vacuum, but I attribute your improved performance and drivability to the fact that you previously had no vacuum advance at all. You had mechanical advance alone.
Your Edelbrock carb is a newer version of the Carter AFB, they started making in 1957 for Olds and Cad engines.
The Carter AFB did not have a small manifold vacuum port on the front of the carb. They also made the Carter AVS for '67-71 Mopar engines and a '66 Chevy 327. One year only AVS on the 327 ?

I'm using a Carter AVS from a '71 383 mopar on my 292. No small manifold vacuum port. Runs great on ported vacuum. My 292 originally had a Rochester B. No manifold vacuum port and it ran fine with the ported vacuum port. Chevy used the same Rochester B all the way back to about 1950 and with ported vacuum. No manifold vacuum port.
Chrysler also used two Carter AFB's on the 426 HEMI. They were not fitted with manifold vacuum ports in the casting either. One carb had a nipple for ported vacuum and the other carb had the ported vacuum port capped.
Some of those Carter AFB and AVS had a small manifold vacuum port on the pass side that was used for a choke pull off, depending on the style choke. No manifold port provided for vacuum advance.