I had been running a holley on my 68 327 with a direct vacuum source to the vacuum advance canister. Now I'm running my original Q jet which my mechanic set up with a ported vacuum source. I used to have to plug the vacuum tube going to the vacuum advance canister when setting my timing with the holley but with the Q jets ported setup it doesn't pull a vacuum at idle. So when timing my car now how do I proceed with this ported set up?

 

Is your engine pretty much stock? If it is, I would change it back to manifold vacuum. Your engine will run cooler at idle with more ignition timing at idle and have better throttle response.

Ported vacuum was more used for emission control, but it is useful for engines with large cams, big ports and single plane intakes. Using my own engine for example, if I have the vacuum advance hooked to manifold vacuum, when I put it in gear the engine slows, even with the high stall converter. As the engine speed is pulled down, the vacuum drops. As the vacuum drops, the engine speed drops. It's a vicious cycle that quickly has the engine trying to die.

When I hook the vacuum advance to ported vacuum, I run more initial timing and when I put it in gear it slows a little and just stays there. If I give it little throttle the butterfly crosses the vacuum port and it kicks up my timing for cruise or coming off idle. It makes it more responsive, it runs cooler at cruise, and gets better mileage. If I start using more throttle, the vacuum to the port goes away and timing backs off as the mechanical timing is kicking in as it accelerates.

As I said though, if your is stock or close, use manifold vacuum. If your car has a manual transmission, use manifold vacuum.

Mike

 

I don't think there was a ported source used in 68 on a 68 quadrajet. There should be one on the lower front left for the vacuum advance and the one out the passenger side by the choke used for the smog pump source. Both are manifold vacuum and the only two vacuum sources that were available on an original 1968 carb.

 

The original port for the VAC on the front of the carb is ported. You can use the smog port with a tee if the pump is still there. If not just plug in there for the VAC. Even though the original source is ported, you still want to plug it for tuning since only a small amount of increased idle setting will uncover the slot and the vacuum advance will react changing your timing and make it difficult to adjust properly. You can slightly modify your Quadrajet from the inside to be able to use the original port with full time vacuum and not be detectible if this matters.
Jeff

 

On the original 7029215 carb on my 69 L36 both the front drivers side port and the port by the choke are manifold vacuum so I assumed the 68 was the same. Both produce the same amount of vacuum at idle and the passage for the drivers front port starts below the throttle plate at the bottom of the carb base. I was under the impression that this was changed to ported vacuum some time in 70 or the early 70's with the entrance slot moved to just above the throttle plates. I have no ported vacuum on mine.

 

So if I am understanding this correctly my 68 Q jet only allows for ported vacuum. Since I dont have the smog pump connected where is the smog port located on the carb to plug into for converting to direct vacuum?


to

 

Hook up a vacuum gauge to the ports on your carb. At idle, the vacuum is zero if it is a ported vacuum source. If the gauge reads anything higher like 18" vacuum, then it's a port for manifold vacuum.

 

Unless you have a special engine/cam build that requires it, you will be better off using manifold vacuum to run the vacuum advance can. Ported vacuum is the same as manifold vacuum, EXCEPT at idle or near-closed throttle conditions. With ported vacuum you have NO vacuum for advance at idle. The engine will run hotter and [generally] worse hooked to ported vacuum.

That 'system' was utilized to meet EPA emission requirements. At idle, ported vacuum caused the engine to run retarded several degrees. That would make the combustion chambers hotter and burn more hydrocarbons when idling. But, poor idle quality, detrimental to engine, worse mileage, etc. It was "no win" for the car; some help for the environment.

Dump the ported vacuum idea for a normal build engine.

 

I have understood the cons of ported vacuum vs direct vacuum. Basically my inital question was to find out how to correctly time my car with the ported vacuum set up. The mechanic that hooked up the Q jet for me found that the ported vacuum allowed the car to actually run better. I would consider trying a direct vacuum source just to see how it effects the engine but on the 68 Q jet I am not certain where to locate a direct vacuum source on the carb to try it out.

 

We have no idea which quadrajet you have or if it is original to the year which makes a big difference. Look a Resdoggie's post above. The side port by the choke is manifold vacuum and depending on the carb or year, the front left port can be either. Early were manifold and later were ported. Does the front port have vacuum at idle? If so, that's manifold vacuum. The right side port by the choke that is normally used for the original stock smog pump diverter valve is manifold vacuum on any quadrajet produced with that tube.

 

Sbc like lots of initial timing. The problem then is limiting the mechanical and vacuum advance. I have a programmable electronic ignition so I can easily make changes on a computer. My initial is ~22* with the vacuum disconnected. Mech'l is ~12* and vacuum advance is limited to 8*. I use manifold vacuum. Engine idles smooth.

You may try this: Using direct manifold vacuum, advance your initial to 16*. Keep the vacuum advance plugged/disconnected. Adjust your idle mixture. Go for a scoot with the vacuum advance still disconnected and see how it runs. If it runs better then hook up your vacuum advance. If the engine pings at cruise, you need to limit the vacuum advance until the ping is eliminated.

 

If the car runs better with your vacuum advance connected to ported vacuum I would suspect something else is wrong. I have yet to see an engine run better with the VAC connected to ported vacuum.

 

What if all my motors run better on ported?

You have to understand carbs and timing. In my case with big numbers everything I remove the vacuum advance, hook up a vacuum Guage to the full port, set the initial advance to @20 degrees, and turn the warm motor up to @1100 rpm

1100 keeps the vacuum needle more steady around 11 inches

I adjustthe 4 corner idle screws to get the highest vacuum and rpm. I might even have to turn down the throttle to lower back to 1100 rpm then I rotate the dizzy to get the highest readings. Most of my builds seem to like 18 to 22 initial degrees and all in about 3000 to 3200. These are all solid roller 7500 rpm over 11 to 1 c/r

I then turn the the hot idle to about 1000 and hook the vacuum advance back to the ported port

Since I have already got the highest readings and best idle quality it would be stupid to hook it up to full vacuum and ruin what I just setup

I've tried full and ported even using adjustable vacuum advance units over the years my dizzy is the msd e tech so I can change all the various setting with dial switches. I can try 100 different combinations in a day at the track or out on the street

 

All of you need to read the below related post by Lars

I have tried the full port vacuum to the dizzy and if you watch the timing on a digital timing light you have an timing all over the place

The correct way is to recourve the distributor mechanical advance to match your highest vacuum after setting up the carb and initial timing advance.
So with old stock iron heads let'sjust say 16 initial with vacuum advance blocked off. You will want 20 more mechanical to give you 36 total above the all in rpm

You only want additional advanced at light throttle opening cruising

 

You can tap into the manifold directly, no need to use a carb fitting at all. There should be a T fitting behind the carb.

 

Generally, the only engines that idle better on 'ported' vacuum are those with big cams which NEED to idle at higher rpm's and with more advanced timing. And, if the timing is advanced already and the idle speed is increased to where mechanical advance is starting to work, you wouldn't need additional vacuum advance from a can! If you hook up such an engine to 'ported' fitting, it would be exactly the same as if it were connected to 'manifold' vacuum. Why? Because once the throttle plates are cracked open a bit [remember the higher idle speed required by the engine?], ported vacuum is nearly the same as manifold vacuum!

So, lots of folks who tout using ported vacuum don't really use it. To have a ported vacuum fitting with little or no vacuum, the throttle plates have to be almost CLOSED.

Soooo, again.... If you have a stock or mildly modded engine which idles at stock rpm and is street driven, you need to hook your distributor advance can to MANIFOLD vacuum fitting for best engine operation.

 

No No unless you modify the springs and weighs to have some crazy fast mechanical advance your typical ignition doesn't add timing until the 1500 to 1800 rpm range

You see this with quality digital timing lights

The other thing is radical cams have a lower pull on the vacuum advance, but you are going to have a bouncey additional at idle. Above idle is what you are concerned about

Idle is idle. You set up for the best no matter what motor you have

 

I will concede that if you don't have the ability to change the amount of total mechanical advance that you could use additional full vacuum advance as a crutch to prop up the initial

 

The typical motor doesn't need the mechanical advance until the 1500 -1800 range. Besides, the moment the throttle is cracked open the vacuum signal is the same regardless of ported or manifold vacuum. The OP didn't mention a radical cam, I get the impression its a pretty stock 327.

 

Why would you need to "crutch" the initial? It's static, set it anywhere you want.