Manifold and ported vacuum

[Ricko1966]

Yes, of course. Im talking about which of the ports people actually use. I’ve never seen one in real life use manifold vacuum, but alot of guys online sure does.

Damn near every car made in real life, prior to emissions concerns ran on manifold vacuum. Pics from Edelbrock installation guide.

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[13matsc]

I’ve ordered myself a timing light with adjustable advance and will try to do it myself this time. Can anyone help with what the markers on the timing tab should be?

 

[83Stepper]

I’ve ordered myself a timing light with adjustable advance and will try to do it myself this time. Can anyone help with what the markers on the timing tab should be?

First large indent is 0 deg (TDC), first tab going to the right in the pic is 4 deg before top dead center (btdc), next is 8 and bet you guessed the third? Going to the left, tab is -4 btdc, or 4 after top dead center. Small lines between the indent and the tab are 2 degrees each iirc.

 

[13matsc]

Okey. So the timing is now set to 8* BTDC. When running manifold vacuum the truck feels sluggish and slow when taking of and on low speeds. When running ported vacuum the truck is more «peppy» and wanting to pull along the whole register. Feels more where it should be.
I’m I missing something? Just borrowed a simple light to verify the base timing, have yet to receive mine with dial back settings.

 

[Ricko1966]

Okey. So the timing is now set to 8* BTDC. When running manifold vacuum the truck feels sluggish and slow when taking of and on low speeds. When running ported vacuum the truck is more «peppy» and wanting to pull along the whole register. Feels more where it should be.
I’m I missing something? Just borrowed a simple light to verify the base timing, have yet to receive mine with dial back settings.

Is the vacuum advance hose disconnected and plugged when you set the timing? You are not readjusting timing when you move your vacuum source are you?

 

[13matsc]

Is the vacuum advance hose disconnected and plugged when you set the timing? You are not readjusting timing when you move your vacuum source are you?

Both vacuum sources on the carb is plugged when adjusting the distributor. Im setting the timing and then connecting the vacuum advance before test driving.

Idle was adjusted when switching from manifold to ported. Also the truck doesn’t «slam» into reverse and drive when using ported vacuum.

 

[Ricko1966]

Both vacuum sources on the carb is plugged when adjusting the distributor. Im setting the timing and then connecting the vacuum advance before test driving.

Idle was adjusted when switching from manifold to ported. Also the truck doesn’t «slam» into reverse and drive when using ported vacuum.

I do not understand what you are saying. Did adjust the timing more than once? Adjust it just one time then hook up your advance whether manifold or ported,if you switch from manifold to ported you do not readjust the timing. You will need to readjusted idle to stop the slamming when you put it in gear. The things I told you to check in my first post to you were not just for fun. Those things will tell me is the vacuum advance working,is the centrifugal advance working,how much timing is in the distributor and how much is in the can. Then after total is set I can determine if you have the right can. As for vacuum accelerating,there is no vacuum unless the carbureator is too small or the air cleaner is too small or dirty. As for base timing,that is a generic number,when the engine was new and as it left the factory. You really should be setting total,let base fall where it will. This is the timing your engine wants,at your altitude,jetting,amount of timing chain stretch etc. Just because 8 works good for someone else,what is his altitude,camshaft profile how much chain stretch,what distributor do they have,there are like 100 variants of GM HEIs, etc.

 

[13matsc]

I do not understand what you are saying. Did adjust the timing more than once? Adjust it just one time then hook up your advance whether manifold or ported,if you switch from manifold to ported you do not readjust the timing. You will need to readjusted idle to stop the slamming when you put it in gear. The things I told you to check in my first post to you were not just for fun. Those things will tell me is the vacuum advance working,is the centrifugal advance working,how much timing is in the distributor and how much is in the can. Then after total is set I can determine if you have the right can. As for vacuum accelerating,there is no vacuum unless the carbureator is too small or the air cleaner is too small or dirty. As for base timing,that is a generic number,when the engine was new and as it left the factory. You really should be setting total,let base fall where it will. This is the timing your engine wants,at your altitude,jetting,amount of timing chain stretch etc. Just because 8 works good for someone else,what is his altitude,camshaft profile how much chain stretch,what distributor do they have,there are like 100 variants of GM HEIs, etc.

Timing was adjusted once/verified to be at 8* BTDC without vacuum advance connected. When switching between ported and manifold vacuum AFTER the adjustment I need to adjust the idle. If I use the same idle settings it gets way to low in drive when using ported vacuum. When using manifold vacuum it slams into D or R when idle is 650 in P. The vacuum advance is working as I mentioned earlier; I pulled the cap and applied vacuum to the vacuum advance clock and the inside of the distributor moved.

I’m just confused as this video and other are saying just the opposite. And I’m experiencing just what’s happening in this video:

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[Ricko1966]

That video is wrong,what the man is saying is incorrect. I wrote this all up real good,but for some reason it wouldn't post,security error. refresh screen,poof everything was gone and I don't feel like rewriting it. Look up and read timing 101,learn why we have vacuum advance,when we need it,why we need it.

https://www.camaros.net/threads/timing-vacuum-advance-101.150344/

 

[13matsc]

Well, I don’t know… For me it works the best with ported as of now.
This video also explains the same thing:

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[Ricko1966]

Well, I don’t know… For me it works the best with ported as of know.
This video also explains the same thing:

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When you set the ignition up right it will run better on manifold vacuum. He says right in his video manifold and ported are the same once you are moving,just like i told you. I believe he also said like I said vacuum advance does nothing on acceleration,at WOT you don't have any vacuum,none ported,none manifold,vacuum goes away unless your carbureator is to small,your air cleaner is too small,or your air filter is dirty.

 

[13matsc]

Yes, I know. I’m talking about throttle response, pulling out from an intersection and other scenarios when you need good throttle response and acceleration.

Earlier today I bumped the base timing to 12* BTDC. Truck is now way more «willing» when taking off and small drives around town. I’m pulling about 21 inches hg in idle. When giving throttle in idle and connected to the ported vacuum, the timing advances as verified by the timing light.
I will switch the ports tomorrow (if the weather is good) and try the manifold vacuum to both see and feel the difference. I should then get about 27* base timing when switching to manifold vacuum and in idle. When the throttle is opened the timing should go back to 12* before increasing with the mechanical advance and increasing even more when cruising at highway speeds when the vacuum advance kicks in again.

 

[Ricko1966]

Yes, I know. I’m talking about throttle response, pulling out from an intersection and other scenarios when you need good throttle response and acceleration.

Earlier today I bumped the base timing to 12* BTDC. Truck is now way more «willing» when taking off and small drives around town. I’m pulling about 21 inches hg in idle. When giving throttle in idle and connected to the ported vacuum, the timing advances as verified by the timing light.
I will switch the ports tomorrow (if the weather is good) and try the manifold vacuum to both see and feel the difference. I should then get about 27* base timing when switching to manifold vacuum and in idle. When the throttle is opened the timing should go back to 12* before increasing with the mechanical advance and increasing even more when cruising at highway speeds when the vacuum advance kicks in again.

Remember when I told you to set the timing at 36 degrees and how-to do it. That's what you need to do. You aren't worried about idle timing. How are you getting your 27 and 12 numbers on your vacuum advance? Do you know what can you have?They are not all the same which is why I asked for all the different numbers from you in the beginning,so I could figure out what can you have.

 

[mlparkey]

I have an 86 C10 with a stock Goodwrench 350, vacuum operated Quadrajet, and a vacuum and mechanical advanced HEI distributor. My initial base timing is 10⁰ BTDC at 500 rpm with vacuum advanced disconnected. I run with full manifold vacuum advanced at idle for a total of 27⁰ BTDC at 750 rpm. My full mechanical advanced is all in at 3000 rpm with a total advanced of 36⁰ BTDC. During idle, low load cruise as well as deceleration, the fuel mixture is very lean and it takes longer for the mixture to burn. Having advanced timing and allowing the spark to occur earlier during these conditions is preferable to promote complete combustion. Using a manifold vacuum source for vacuum advanced, which has a high vacuum signal at closed and part throttle, helps accomplish that requirement. My off line throttle response and acceleration are instantaneous and my fuel economy is very good as well.

 

[13matsc]

I hooked the truck up to manifold vacuum today. The fast idle goes way up, to about 1800-1900 rpm, from about 1200. The normal idle is about 100 rpm above what it is when using ported.
The timing is now advanced when in idle, this I can see with the timing light. When comparing to the tab, its about 6-8 degrees advanced. So about 18-20 total in idle. When revving the engine, the timing should fall back to 12 (which is the base timing) if what you are saying is correct. But, it does not. When revving the engine, the timing increases with the RPM and I’m reviving up to 2000 rpm. The higher the revving, the more the timing increases

This does not seem in line with what you guys are saying.