JB

It took me a while to check some things, so my previous topic dropped off the page, so I'm starting a new one. I'm wondering why I'm getting a lot of vacuum at the "ported" outlet on the front driver side of my '69 L-36 carb.

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). If my idle speed isn't excessive, the timing's spec, and its got the stock rods & jets, shouldn't I be getting little or no vacuum from that outlet?

A couple of other things I should mention. 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 before I reset the timing and idle and disconnected the vacuum advance. 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").

What am I missing?

JB

DJ Dep

"What am I missing?"

A Holley carb and aluminum manifold.
Sorry...couldn't resist.
:D

Dep

Smokehouse69

JB, I won't even pretend to be in the same league with Lars, Wrencher or Tom454, but if you are getting nearly full manifold vacuum from your pprted vacuum source, then it seems to me that your primary throttle blades are too far open.
Ported Vacuum is supposed to be engine vacuum that is available above the throttle plates of the carburetor. I don't have a Q-jet handy but I do have a Holley spreadbore setting on my desk, with a piece of vacuum line attached to the "ported" vacuum source, I can barely blow any air through it. If I crack the throttle open even slightly the ported source opens enough to get full flow through it. The vacuum port inside the throttle bore on this Holley is right in line with the edge of the throttle blade, it is like the blade has the tiny hole blocked reducing the amount of available vacuum to the advance at idle.
It could be that your primary blades are slightly too far open.
This is just a guess on my part, hopefully someone who really knows the ins and outs of the whole deal will pipe in!

JB

:lol: Already got an aluminum manifold, tho--came from St. Louis that way!

Steve, right where you left off is where I get beyond my own understanding of the system. So, I get it that the throttle butterfly is open a little, just enough probably to give that port vacuum. Any idea what that's the result of, esp. given that the car runs so good? I guess I don't understand the relationship of idle to throttle opening--how is it if I got the idle set right and the right fuel mixture, the butterfly could be open too much?

I ain't smart enough for this kinda voodoo. Carb stuff is only a hair less confusing that electical stuff. Gimme grease and bolts any day. :D

JB

JB

ttt!

gary6696

Five stars and you're asking questions?
That's not allowed.
:D

JB

Stars? We don' need no steeking stars! Shoot, look how many stars Smokehouse has got & he don't know, neither. :D

Hell, Gary, I was counting on you straightening me out on this one! :D

JB

page62

Dumb question: Have you sent Lars an email? And why is not having your vacuum ported causing you a problem?

Lars hasn't been around much since he started restoring that C2...

JB

Naw, I don't want to bug Lars if he don't want to be bugged. It's really no big deal, to tell the truth. Performance-wise, the car has never been better. I've found it idles better with the vacuum advance disconnected, though, so I'm running it without any vacuum advance at the moment (which is how I used to run it anyway). Mostly at this point it's just irritable curiousity--why's it doing somethin' it ain't supposed to be doin'?

JB

DJ Dep

If it ain't broke...DON"T FIX IT!!! :thumbs: :thumbs: :thumbs:
Or if it's a Quadrajunk, tie it to a rope and drag it ;)

Dep

page62

:withstupid: Curiosity killed the cat :smash:

Fevre

Have tried measuring at other ports? Might be looking at the wrong one, on Holleys the port is above the throttle plates, not sure on a q-jet. With Lars having done the carb I would suspect it is not adj improperly so I can't see why it would be pulling vac. Just as a side note some people prefer to run the vac adv unported so they get extra adv at idle, helps smooth it out. Like others say, if it runs great then just keep :steering: .

Rich's78$pit

I don't know about the earlier versions, but the later model Qjets can have 3 types of vacuum - manifold, ported manifold and ported. Ported manifold has a slot that extends from below the throttle blades when they are closed at idle, to above the blades. That sounds like what you are hooked up to. It works at both idle and off-idle whereas ported vac. only works at off-idle (cruise). My car (non-Vette) had the vacuum advance hooked up to the ported vacuum so you had advance at all times other than during mid-throttle (and harder) acceleration. You set the base timing (4 degrees) with the vacuum advance disconnected (plugged) and then hooked it back up so you got 24 degrees at idle (which increased idle speed so you reduced throttle opening to get the right idle speed). I think you have less vacuum now because the throttle is opened more without the advance hooked up. The idle surging was probably from too lean an idle mixture. Most guys seem to disconnect the vacuum advance so they can dial in more initial advance but if you take out as much vac. advance as the amount of initial advance you increase it by, you can get the best of both worlds (say 12 degrees of initial instead of 4, and reduce vac. advance by 8 degrees).

Wrencher

Sorry I got in late on this JB, I rarely check the board from home. 15" at idle is a bunch for a ported vacuum outlet and would require the primary plates to be quite a bit "off idle". Check your other ports first, but it's possible that the base gasket is wrong at this point. The fact that it runs well, you have great manifold vacuum, and your able to adjust everything to spec. wiould make me want to look at the base of the carb and check how the gasket fits it. There are a bunch of different gaskets for these.

Before you go there, a picture of the carb and various ports your checking would be helpful (to me, anyway) and might clear this up.

As far as using "direct" or manifold vacuum to the distributor, GM used it for a couple of reasons from the late '70's until on-board proccessors took over.
First, they used it on cold start-up to provide more advance on a cold engine, which then switched to ported vacuum (using a 3 port vacuum swith generally mounted on the thermostat housing) as it warmed up. Then later, it was used exclusively to provide better economy and at cruise speeds (more advance) generally in overdrive equipped, high geared, low compression engines (sound familiar?) on engines equipped with EGR valves.

The down side was generally lower off-idle performance, especially when the early EGR valves got weak and opened early. Talk about your basic bog off line! Many cars were switched over to ported, but it was a hassle at smog time switching it back, and often they ran into pinging (detonation) problems.

Hans

JB

Working on the pics (borrowed a camera, waiting for my buddy to email 'em). Basically, though, there are only two outlets on the carb (the 3rd one is right by the choke pulloff and is obviously meant for that). One is a small diameter outlet located low on the main carb body, driver's side. The other is a larger diameter on the base plate, passenger side. The smaller one pulls about 15", the larger one about 17". I've got the PCV going to the large one, the vacuum advance to the smaller one.

The gasket is a possibility. I was using a three piece set I got from Lars, but I replaced the top one of the three when I changed the carb out with a piece I got from Autozone, and I'm not totally certain it was identical to the one I took off. I'll pop the carb off tomorrow and take a look.

JB


[Modified by JB, 7:09 PM 10/20/2003]

Smokehouse69

JB, the Q-jet that was on my car originally had the vacuum advance port on the front left (driverside.) I thought I had a picture at one time of which port to use. If I find it, I'll post it.

Smokehouse69

JB, This page on the Manuals website I found has a pretty poor picture of front of a Q-jet, showing the distributor vacuum advance port.
http://www.tocmp.com/manuals/Carbs/R...73_4M_0018.htm

Wrencher

Sure sounds like your on the right port. Very curious as to why you have so much vacuum there. When you get the carb off, look at the plates (all of them) to verify that they're closed. I'll assume that you don't have an electric idle solenoid (would only be for A/C anyway on yours) so the primary plates should be almost completely closed when they are on the set screw (make sure the fast idle cam is off). If you can see a gap of any size, that would explain the vacuum at that port. I think you'll be able to see the slots or holes that feeds it if you open the primary plates.

Basically an engine needs fuel and air to run. if you have to have the primary plates open far enough to get a decent idle, your already getting out of the idle circuit, thus the vacuum present at the distributor port. The idle circuit works with the plates essentially closed and uses the idle air screws and a fixed amount of fuel to operate.

If I remember right, I saw a nice Q-jet book at Borders a while back that shows this in detail. Should've purchased it.

Hans

Tom454

..... and if you do find that the throttle plates are open too far... and this is the ONLY way the engine will idle, then check the throttle shaft to carb body clearance. If it is excessive, the shaft & discs need to be removed, a bushing installed, and the shaft & discs re-installed. With excessive clearance, air is drawn in past he shaft and a lean condition occurs. To keep the engine in the correct air/fuel ratio, you have to open up the throttle to get to the intermediate fuel circuit. But this also puts the disc near the vacuum port and activates "ported vacuum". I fix a lot of these suffering from this ailment.

Another symptom is erratic idle... the loose shaft wil not always settle in the same spot... so the severity of the leak changes all the time. Thus, an erratic, unadjustable idle.


Tom