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Late Qjet idle mix screw range?

General engine tech -- Drag Racing to Circle Track

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turbo camino
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Late Qjet idle mix screw range?

Post by turbo camino » Mon Oct 07, 2019 6:08 pm

350 Chebby, weird mix of stock parts. 9.5ish:1, L98 aluminum heads (no exhaust crossover, may be an issue?), Bowtie ZZ3 dual plane, super mild OE truck roller cam (190/196 @ .050). Timing is 20* initial, 20"hg at idle. No vacuum leaks, I can slow it down to 400rpm with the idle screw and I've sprayed everything with no result, same result whether PCV/trans modulator/vac advance (ported) is connected or blocked. No other vacuum accessories on the car.

Carb is a 1980 Vette auto 17080204, all stock jets/rods. Float level .400". Stock mechanical pump @ 7psi. 14x3 open air filter. Carb was low mileage and clean when I started, it's now clean-clean, like ultrasonic spotless clean.

The issue (finally!): It's not even slightly happy unless the mix screws are 5.5-6 turns out from seated, it would probably be really happy at 7-7.5 turns. Is this normal for these later carbs? Anything else to check?
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Re: Late Qjet idle mix screw range?

Post by ProPower engines » Mon Oct 07, 2019 6:23 pm

I just had a similar issue with a 71 olds 455 carb. Had to twist the mixture screws way out. Could be the jets were always wrong or an internal vacuum leak some where.
I could turn the throttle screw out till the butterflies were hard shut. had an internal vac. leak.
fixed that and viola =D> .

There is 100's of different carb gaskets for 100's of Qjet models and all it takes is a single gasket to be not quite right
and you got issues. Sounds like the carb is trying to tell you it wants more idle fuel through the idle restrictions.

If you have a good selection of number drills you can go up .002 at a time till its much better or .001 after you are close
But remember go slow and don't get crazy cause its easier to remove then put it back [-X
Real Race Cars Don't Have Doors

turbo camino
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Re: Late Qjet idle mix screw range?

Post by turbo camino » Mon Oct 07, 2019 6:27 pm

I'm pretty sure I could kill it with the idle speed screw, I've only dropped it down low enough to be sure it wasn't getting air from somewhere it shouldn't.

Kit came from Cliff's place ordered per the carb #. New gaskets matched the originals, I checked. This carb showed no evidence of being apart before I got it.
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Re: Late Qjet idle mix screw range?

Post by turbo camino » Mon Oct 07, 2019 6:30 pm

I have the ability to make my own thread-in idle tubes, just didn't think I would need to 'go there' with a combo like this.
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Re: Late Qjet idle mix screw range?

Post by ProPower engines » Mon Oct 07, 2019 6:48 pm

Again just because it was from a car does not mean it was even correct from the factory or rebuilder at some point.
While there is tons of similar applications for them carbs they had subtle differences that can piss off the most well versed carb guys at times.
Also just saying just cause its new in a box don't make it correct. It may be worth a tear down to confirm as much of a PITA as that can be.
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Re: Late Qjet idle mix screw range?

Post by Tuner » Mon Oct 07, 2019 6:56 pm

What size is the curb idle discharge hole in the bore wall where the tip of the idle adjustment screw is located?

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Re: Late Qjet idle mix screw range?

Post by turbo camino » Mon Oct 07, 2019 7:07 pm

Honestly no idea on that Tuner. I only did a stock-type rebuild, I didn't map anything out. Didn't think I would need to. Didn't pull the idle tubes either, as I said it was clean to start with (it'd obviously been put in storage before the crap corn gas came along and screwed everything sideways). I did do your secondary tube & linkage mods but I'm not even to the point of driving the dang thing yet, tires are dry rotted from sitting abandoned for the past 12 years. I'll get around to seeing how it works on the road later this week or early next.

Since I reminded myself, how much will this corn-gas-water mixture lean the idle? There's no local source for non-adulterated real gas.
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Re: Late Qjet idle mix screw range?

Post by turbo camino » Mon Oct 07, 2019 7:33 pm

Curb idle discharge = .080"

The tips of the needles still protrude into the bores at 6.5 turns out, though only by about .010".
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Re: Late Qjet idle mix screw range?

Post by turbo camino » Mon Oct 07, 2019 7:42 pm

Oh, also... both screws seem to have the same sensitivity. 1/2 turn in on either one will give the same amount of change. So it's not acting as if something's clogged in one side. I can kill it by turning in either screw one at a time.
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Re: Late Qjet idle mix screw range?

Post by 77cruiser » Mon Oct 07, 2019 7:47 pm

In my combo ethanol makes a difference, if it's on the lean edge & you fill with corn gas it won't run good. ARF will go up .5 to .8.
If it won't cause hard hot restarts maybe raise the fuel level.
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Re: Late Qjet idle mix screw range?

Post by turbo camino » Mon Oct 07, 2019 9:35 pm

Air cleaner on or off makes no obvious change though just for best practices I have been adjusting with it on. Maybe that's part of it, the less restrictive filter? I do have a stock '80s-era snorkel air cleaner I can throw on there and see if it richens it up.

Just to be clear, it runs great (as long as the mix screws are out far enough). It starts every time as soon as the first cylinder goes over TDC, idle is stable, doesn't stink, throttle response like a friggin' laserbeam. When I get it on the road, if it runs good everywhere else I'll stop worrying about the number of turns.
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Re: Late Qjet idle mix screw range?

Post by Schurkey » Mon Oct 07, 2019 10:38 pm

turbo camino wrote:
Mon Oct 07, 2019 9:35 pm
f it runs good everywhere else I'll stop worrying about the number of turns.
Uncommon wisdom.

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Re: Late Qjet idle mix screw range?

Post by MadBill » Mon Oct 07, 2019 11:45 pm

The fix/kludge I've applied many times is to drill out the idle discharge orifices to 0.090"-0.100" (#43-38)
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Re: Late Qjet idle mix screw range?

Post by cardo0 » Tue Oct 08, 2019 4:22 am

Use a vacuum gauge and adjust idle mixture screws for highest vacuum. I've tried adjusting for best rpm on a qjet before until I got frustrated, tired and quit.

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Re: Late Qjet idle mix screw range?

Post by Tuner » Tue Oct 08, 2019 8:20 am

MadBill wrote:
Mon Oct 07, 2019 11:45 pm
The fix/kludge I've applied many times is to drill out the idle discharge orifices to 0.090"-0.100" (#43-38)
Might be OK in this case ,but it may not be wise to arbitrarily jump to .090" in all situations like this because not all the QJet idle circuits are the same to start with. Some have smaller hole for the needle and finer pitch on the threads (58 per inch thread pitch?) instead of the original 32 per inch. The old '60s and early 70s carbs the .090 isn't far off, but the newer units might use a smaller hole.

If you did the secondary mods, and you have the screw-in IJ setup, why don't you also do the primary idle circuit mods to add the adjustable down channel jet and move the IAB to the lid?

Probably a reason it has to have so many turns open is the fine thread pitch and the taper on the needle is long and pointy, not as blunt as the old '60s carbs.

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