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adryan16 10-13-2004 08:41 PM

Q-Jet Confusion
Okay, before anyone tries to negate my use of a Q-Jet, please be tolerant and accept my infatuation with them. That being said...

Well, I've become very familiar with Q-Jets over the past few years, rebuilding, adjusting, collecting jets and secondary rods etc. So far, I've only run into a few stinkers. Last year, before I finished the 350 for my firebird, I purchased a 795 cfm, electric choke Q-jet from an '85 454 Chevy truck. I received it and promptly rebuilt it. I matched up all the gaskets and such, and put it back together. I got it on the car, and at idle and partial throttle it ran great. A little rich, but that's okay. I figured I'd tune it out of it later. Well, the time came to dip into the secondaries, and.... nothing, just a lot of popping and hesitating. Assuming an incredibly lean bog, I readjusted the air door, and put some richer secondary rods in. Went for the secondaries again and nothing. So, I pulled 'er back apart, and couldn't find anything wrong. Curiosity got to me and I changed the throttle plate to main body gasket, to see what that would do. Well, I sure had secondaries now, and was able to get it tuned okay. However, I could never tune the incredible richness out of the idle. The sun of a gun pulled great on the top end, but there was a part idle hesitation, and was always running rich at idle. I tried everything! Even bottoming out the idle adjustment screws didn't do squat. Then one time I lifted up the secondary hangers to get a better look at what letter they were, and the car about died. That's when I found out it was pulling out of the secondary wells at idle. A little more messing around, and I figured out why my idle always changed after changing secondary rods. I've never run into this, and no one in my area can give me a reason. The only two gaskets that were in the kit gave me the option of no secondaries, or full-time secondaries. If someone else has run into this problem or has and idea what might remedy it, I'd be greatly appreciative.

p.s. sorry for the novel, I just thought I'd be thorough

Restore60s 10-13-2004 09:19 PM

Just being curious was the Q-jet off the truck a power enrichment type?
Also I have had fuel being drawn into the secondary ports by being jetted to lean on the main jets. Looked like a trickle of fuel. Like the engine was starving and vacuum pulling it thru.
You must have that Pontiac 350 built pretty strong running that big of a carb. I'm running a 780cfm Q-jet on a 455 Pontiac and that feeds it thirst for fuel great.
To be honest on the gaskets I really never paid much attention. I just always "matched up" what I took off.
Sounds like you will become the Q-jet wizard. Good Luck!

adryan16 10-14-2004 04:59 PM

No, the Q-jet that I got was not one with the power enrichment circuit. They're just plain trouble and annoying. As far as the power goes, my 350 runs well with a modified "068" cam and a set of ported cast 16 heads. I've run a 750 cfm, and it just didn't pull at the top end like this one does.
I did just like you said, matched up the gaskets, and ran into problems with the secondaries not activating. I think what the deal was, was that the 1st gasket I put on didn't have a vacuum signal port for the secondaries. So I could open them up, but the air doors didn't open, and subsequently didn't meter any fuel. I don't know what the deal was, I was hoping someone had ran into it before and had figured it out. Thanks for the input Restore60s.

DoubleVision 10-15-2004 07:54 AM

Out of curiousity, what`s the fuel pressure set at?

adryan16 10-15-2004 08:39 AM

I don't have an external regulator on it, it's just from a new stock mechanical pump.

DoubleVision 10-15-2004 11:03 AM

You need to know the fuel pressure, even a stock pump can put out more than many give it credit for, when there`s a fuel pressure issue it overcomes the idle since fuel is getting in from another source, anytime the idle mixture screws have no effect the first thing you should look into besides the float level is the fuel pressure. Q-jets don`t like anything over 5 psi, anything over 5 and they have all kinds of mysterious problems, I went through a fuel pressure issue for 6 months before I figured it out, I had stock pumps putting out 14 psi, I had one Q-jet blowing fuel from under the gasket on the front of the carb but it never leaked a drop out of the boosters or the top of the carb. Here`s a test you can do, with the car at idle, shine a light in the carbs throat and see if you see any fuel coming out anywhere, if you do there`s a fuel pressure issue. I traced my problem down to the gas tanks vapor line that I had blocked off, it never had given me any problems and it had been blocked off for years, when it decided to it gave me problems in spades.

Wrencher 10-15-2004 01:03 PM

Just a few thoughts, that gasket should have nothing to do with operating the secondaries, vac doesn't activate them, possibly a blocked vent tube may hinder the operation, is it possible the choke lockout was keeping the throttle plates from opening originally? Are you sure the secondary plates are closed all the way? And, do you have ample vac. to hold down the power piston? Maybe someone changed the rear hanger height holding the rods too high?

docvette 10-15-2004 02:36 PM

sec. Jet height.
1 Attachment(s)
Doc here :pimp:

Had something like that before on my Vette...It kept failing smog on the low speed dyno pull, but was good at high speed dyno..

Turns out the secondary Needle Hanger (part #12 on the photo)was bent (both rods wern't at the same height) so when it was seated on one the other was passing fuel.

I assume the stock air cleaner had something to do with the deformation, since after I got the needle hanger level again, and installed an eldebrock air cleaner, it never happened again...and It passed smog after...and got much better mileage!

Just a thought..


adryan16 10-15-2004 05:19 PM

Well, in lieu of a problem due to fuel pressure, I may get a regulator. But my question is, why did it work so good when I had that 750 on it? I've done your test before, and didn't see anything. Eventually, as a quick fix, I just tuned the idle with the secondary rod height.

Also, I realize that the secondary throttle blades don't open because of vacuum, but the secondary air valve doors do operate on vacuum needs, and because of a cam that also lifts the secondary hangar, and therefore the rods, technically the vacuum does have an effect on the secondaries. Hence the fact that they area vacuum secondary carburetor. As far as hangar height I've got about a dozen different hangars, and have experience tuning with them, and I've gone to both extremes, and no difference - I still have no idle adjustment. I've even trued all my hangars, knowing that both sides area at the same height.

The only other thing that I can think of, but I can't see externally, is that the air horn is cracked somewhere causing problems.

DoubleVision 10-15-2004 08:42 PM

It`s quite possible the main body is warped, which could have caused the first throttle body not to activate due to the butterflies being stuck closed, and I`ve seen this problem once before. Another test you can do is strip the carb all the way down, place the main body over a bucket and fill the bowl with fuel and see how long it takes for the main wells to start dripping, the last Q-jet I got leaked so bad the idle wouldn`t adjust, they had already been epoxied and still leaked, I repeened them and recoated them with epoxy and solved the leak, this method also lets you see if there`s a crack or leak from another source.

adryan16 10-16-2004 03:24 PM

Well, I'll take the carb apart, and check the trueness of the body. I guess I never figured on that. I did gently reseat the plugs, but maybe not hard enough to seat them well. I suppose reepoxying them couldn't hurt. I'll give it a shot, and if it doesn't work, forget it, I'll tune on the 750 some more.

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