JB

My '69 qjet only has three vacuum ports, one big one on the passenger side that clearly has to power the PCV valve, one small one on the passenger side that clearly has to be hooked up to the choke pull off, and one small one on the driver's side that by default has to be for the vacuum advance.

At idle, when I pull this hose, there is a faily strong vacuum, and the idle gets very poor even with this hose blocked, which tells me it's getting some definite vacuum to the advance.

Am I wrong in thinking that the advance can should be hooked up to ported vacuum, which means no extra advance at idle? If that's so, why is there vacuum at this port?

JB

shoptek

Correct----Ported should have no vacuum at idle. It's "source" is under the throttle plates. https://www.corvetteforum.com/techti...D=86&TopicID=3
Check in the C3 tech tips. There is a wealth of info there, especially tech papers from Lars. Check his articles about vacuum advance and timing.

JB

Thanks--I did read through Lars' paper on ported vs. unported, but I wanted to clarify things a bit. When you say "under the throttle plates," does that mean that the fitting I'm using for the vacuum advance should be located on the base so as to be sourced underneath the throttle plates? It's currently hooked to a vacuum fitting on the lower driver's side of the body of the carb. I was thinking that the port on the base would be the unported one, since it would be getting straight manifold vacuum?

JB

Jvette73

I could be wrong but I thought the source of ported vacuum was from above the throttle plates. Hence it only recieves vacuum when the throttle plated are opened and incoming air rushes past the source opening. After all the pcv port is constant vacuum and its below the throttle plates.

Also dont know if this would help you or not but every q-jet ive seen has the ported vacuum source comming out the front of the main section, well above the base. Comming straight out the front closer to the driver side. Maybe the tube has been removed on yours and a plug installed?

shoptek

Jvette73,

You are correct!!! My bad :D

JB

That's exactly the source I've got the advance hooked to. So any idea why I would have vacuum at that port at idle?

JB

lars

JB-
The amount of vacuum you have at idle on the ported vacuum source depends on the amount that the throttle blades are cracked open at idle. If you have a bumpy cam and require a little more throttle opening at idle, the ported slot above the throttle blade will be exposed to more engine vacuum. Various factors affect how much throttle opening you need at idle, including timing, vacuum leaks, camshaft and resulting manifold vacuum, and overall engine efficiency. Having some vacuum on the ported source at idle is not necessarily bad. Are you have specific drivability problems, or are you only concerned about the vacuum being present at the ported source?

JB

Well, the idle seems rougher to me than it ought to, though I'm told that with the stock cam, and L-36 should be a little "lumpy." So, I was tooling around, checking for vacuum leaks and such, and happened to pull the advance hose and felt vacuum and thought, "hmmmm."

I guess what concerned me is that with the vacuum advance hose disconnected and plugged, the idle gets awfully rough, I suppose because the timing advance drops way back.

Could well be that I'm looking for problems where none exist.

JB

Flareside

JB, the only way this vacuum hookup could affect your idle were if it was pulling your timing way out of spec. Where's your timing at idle?

I've found that my L36 doesn't have a bad idle, but it will shake the car a little bit. I don't think you'll ever get it totally smooth. I've had 3 different intake/carb combos on my car (the current Q-jet is by Lars), and all have idled about the same way. You could probably smooth it out by running a bunch more initial advance, but my engine won't tolerate much initial timing with pump gas. Too much compression, lame gas (94), and too little cam... I think I'm running around 6 degrees right now.

Bobchad

Aren't you supposed to have vacuum advance to at idle, which disappears as the throttle is opened and replaced by mechanical advance?

JB

I've got initial set at 4 degrees, since that's the factory spec. I may bump it up a couple of degrees and see if that helps without setting off pinging.

JB


[Modified by JB, 9:54 AM 10/15/2003]

Wrencher

I'm going to add a couple of things here. A vaccum gauge is pretty helpful here and would give us a better idea of how much vacuum we're talking about, exactly. Both an idle reading at the manifold and the small amount at the "ported" outlet would be helpful.

Also, over the years I've seen a problem that happened fairly often. The car would present the symtoms that you describe (rough, inconsistent idle, vacuum at idle at the ported source, etc.). Often the timing would be a bit retarded, or the mixture would be slightly lean, or both. To compensate, the idle had been raised slightly, exposing the ported vacuum source to a few inches of vacuum, thus raising the idle a bit more and smoothing things out. Problem is, driveability and performance would usually suffer. So, set the timing where it belongs with vacuum advance disconnected, get your mixture and base idle right BEFORE you hook it back up, then check and see if your vacuum problem went away at the ported source. Automatic transmission equipped cars with A/C can make it tougher to get right, especially without an idle kicker solenoid for the compressor. These require a higher base idle compared to a 4 speed car.

Hope this helps.

Hans

Fevre

IM'd you Lars' paper on timing to clear up how all 3 work.

https://www.corvetteforum.com/techti...=109&TopicID=3

JB

Wrencher--I did as you suggested and plugged the vacuum and set timing at 4degrees and idle at 600rpm. I'm pulling a steady 17" of manifold vacuum at idle, and there's about 15" of vacuum at the "ported" outlet to the advance. When I hook up the vacuum advance the idle jumps considerably and the timing goes off the scale (probably something like 20degrees BTDC).

A couple of other things I should mention. The last time I set the timing it was with a different carb. It's got a new carb on it that Lars rebuilt, with the factory rods & jets installed. It was running very well, pulled strong, just a bumpy idle. Now with the advance disconnected, it still pulls strong & runs really well & the idle's smoother. Also, before it was pulling more vacuum, but was fluctuating (now it's 17" and steady, before it was fluctuating between 18" and 20").

JB


[Modified by JB, 7:50 PM 10/17/2003]