Where is manifold vacuum port on Quadrajet 4mv? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 01-05-2004, 10:23 AM
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Where is manifold vacuum port on Quadrajet 4mv?

Hey all,

I have been reading (and learning) a lot about manifold vs. timed (ported) vacuum, and wanted to try running my HEI vacuum advance from a manifold port to see if it might clear up an annoying stall/hesitation that occurs when you try to ease the throttle open after a full stop.

I have a newly rebuilt (stock/SMOG legal) SB 350, with a mild cam, in my 78 blazer. The motor breathes through a (also rebuilt) Quadrajet 4MV.

The issue is that my Quadrajet does not appear to offer a manifold vacuum port. I am wondering if pulling vacuum from the intake manifold itself, or from the PVC/power brake ports on the carb is workable in such a case? I did some quick testing; nothing blew up, or improved, so I reattached the VA to the timed port. However, I am still interested in determining if I have a usable manifold vacuum source with my current setup because many on this forum recommend running the VA from manifold vacuum.

I am leery about using the vacuum from the power brake or PVC ports on the carb; both seem rather strong, and it seemed that the vacuum was not dropping off (significantly) with increased RPMs, as I understand a manifold vacuum port should do. I am concerned about having the VA active all the time if I pull vacuum from either of these carb ports. I also have a vacuum port on the manifold itself, which currently runs to the transmission modulator (and inoperative cruise control unit). I have the same concerns about running my distributor VA from this vacuum source as well. The only other ports on the carb are for the choke pull-off and the thermostatic switch in the air cleaner. Neither of these seem suitable to the task.

Thanks in advance!

PS> I have found pictures of 4MV-ish carbs that show a manifold port just above the fuel filter housing. I have inspected my carb and there is no such port (and no hole for one). I am including the specific carb number below in case this sheds light. From what I can tell the code breaks down to a Federal version of the 4MV made in 1978. I note that the part number indicates a manual transmission, while my vehicle has a TH 350. Not sure what the difference between the manual/AT versions is; this is the carb that has always been attached to my truck.

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Old 01-05-2004, 11:12 AM
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if you have a vacuum gauge, hook it to any of the ports on the carb and take a reading. If one shows a higher reading than the other use it. you can draw vacuum from any port below the throttle plates. If need be you can splice into the one port on the intake manifold.
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Old 01-05-2004, 04:08 PM
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you don`t need vacuum advance at idle, what good would it do at idle? if it did clear up the problem, then you could advance the spark timing alone and cure the problem, vacuum advance is needed at part throttle cruise. even so your Q-jet has been rebuilt, take a look at the accellerator pump shot, make sure it gives it a shot the very instant the throttle is moved, if it checks out okay, then I check the whole carb out, I`ve seen many times where a carb will stumble and it was nothing that a good removal and cleaning wouldn`t cure. I`d suggest you take a closer look at the carb and the ignition since many ignition problems dignose the same as a carb problem.
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Old 01-06-2004, 10:50 AM
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Thanks to all who respondedů

Just to clarify, I was not looking for vacuum advance at idle. I am concerned about pulling manifold vacuum from an intake manifold port, or from a PVC/power-brake port on my carb because my quadrajet does not seem to offer a manifold vacuum port. The engine runs great at idle, and if you gun the motor it revs nice; the issue is when you do a gradual opening of the throttle, and then only at the point when the throttle just begins to open. As I understood, this is when VA tends to kick in (Above idle and before WOT), hence my desire to test with a manifold port (which did not help, BTW).

The accelerator pump appears to activate as soon as throttle is applied, and gas is definitely squirting out. However, because the stumble/flat spot is chronic I am not sure that the pump is the culprit. When I hold the throttle open just a wee bit, the engine no longer runs smoothly and starts to lurch/stumble; at this point the pump will have done any squirting associated with this degree of throttle opening as I understand.

Anyway, I did not wish to convolute this thread when general carb tuning; my primary question appears to be answered in that I can use an intake manifold vacuum port to run my VA, if I wanted to.

Again, thanks all!
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Old 01-06-2004, 01:36 PM
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Any vacume source that is not ported will give you advance at idle, no port on the carb will give a different reading than another, or on the manifold, if all are true manifold vacume ports. Sounds like you stated the problem only exists at part throttle, try testing with the vac line disconnected, I have found sloppy dist that fall apart when vac advance moves the plate to a new location, as it must change air gap or relationship between pick up and rotor shaft, could be a problem in transfer ports in carb[ small slits in throttle housing just above idle ports] there function is to flow fuel untill enough air velocity is reached for the venturi to take over. good luck
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Old 01-06-2004, 01:49 PM
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Thanks for the insite.

I agree that manifold vacuum provides advance at idle (despite my last posting) but note that serveral on this forum suggest better performance was to be had by running VA from manifold vs. ported vacuum.

FWIW, after the rebuild I droped in a brand-new HEI distributer with 50K coil. The swap did not impact the stall/hesitation one way or the other, nor does running w/o VA.

When I bought the vehical the owner had the VA disconnected, citing that the ignition module would blow out if the VA was connected. I swapped in a used unit, which I have recently replaced as noted above. This sounds sonmewhat as you described.

Again, thanks all.
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Old 01-07-2004, 07:18 PM
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It sounds to me you have the dist and timing thing checked out completly. my guess is back to the carb, sounds like you have a lean stumble, check things like low float level or those transfer ports I mentioned, they have really small oriface that deal with air bleeds and fuel delivery, just blow through them both ways with a good spray carb cleaner, then air . could just be a low float level too
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Old 12-13-2005, 11:11 AM
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Stumble

You said:
...FWIW, after the rebuild I droped in a brand-new HEI distributer with 50K coil. The swap did not impact the stall/hesitation one way or the other, nor does running w/o VA...
If this was a universal "perfomance" distributor, the advance curves is probably very conservative. It is almost surely not tuned for your combo. If you crack the throttle and hold it, and the stumble turns into a rough running condition, then you have to rule out the primary side of the carb. If you have a cam, the idle/off idle might be too lean. when you blast the throttle the pump shot covers this up. Moving the throttle slowly will allow to observe operation without the pump. What is your manifold vacuum?
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Old 12-13-2005, 12:36 PM
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Ahhh, the fun of carbs! LOL

I just finally fixed my off idle stumble on my qjet, after much trying this and that.

I'm using a later model electric choke qjet. I've already done some idle transfer tube changes, drilled them out another .004 to get my idle screws to respond better. The later model carbs are lean to begin with so idle mix can be a challange on a non-stock engine. Doug Roes Qjet book is a must for making the carbs stand up and scream. He has lots of performance tips and shows the weak points and how to fix them too.

Anyways, when gas hit $3 a gallon, I decided to do some re-jetting in hopes to boost my pathetic gas milage some. I did manage to up it about 10% overall but now that its cold out, once my choke is open, but the engine is still cold, my drivability was suffering.

I had noticed that I had excessive primary throttle shaft slop, even though it was a newer carb. I know that lots of people say that the slop is normal and won't cause problems, but my experience was much different. I removed the carb and took the base plate to a local carb shop with a good rep and had them drill out and re-bush the plate. I noticed the difference the next morning. It was much much better. I still had a small amount of stumble, but nothing like it was.

I then began re-thinking my jet changes and have decided to up my jets one size richer. Its just too lean now that its cold. Anyways, its running great now. You may try just upping your primary jets one or two sizes and see how it goes. You can order them from Checker as the jets in the Edlebrock Qjet are the same as the GM Qjet. Or you can order them from www.carbs.net. They have a big selection of qjet parts.

Take a look at the throttle shaft and if its loose, in my opinion, its worth the $45 to have it fixed before doing any other mods.

Mark
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Old 12-13-2005, 02:40 PM
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Cold? In
Phoenix?
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Old 12-13-2005, 04:41 PM
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Quote:
Originally Posted by USA1
Cold? In
Phoenix?
Yep, gets into the high 20's in the winter.

Hmm, my number of posts is the same as the year of my truck! 1977. Guess i'd better stop posting.
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