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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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Re: Late Qjet idle mix screw range?

Post by turbo camino » Tue Oct 08, 2019 2:31 pm

No, I didn't do the threaded idle tubes on this carb, I just mentioned I have the ability to make them if it comes to that. I did the secondary tubes because I found one fallen out when I took it apart, and I had a spare set of tubes that'd already been modified so I stuck those in while I was there. Front half of the carb is still stock.

If it turns out the mix screw adjustment is an indicator of a real problem I'll for sure dig into doing the primary tricks. It may run OK, and that's all this one has to do, be 'just OK'.

It's been 10+ years since I messed with a carb of any kind, though I did tons of Qjets, stock rebuild/repair stuff, back in the early '90s when they were still common. Oddly, probably better than half of them were E4MEs, I know those better than the late non-electronic carbs. I just didn't remember ever seeing one that wanted 5-6 turns out on the screws.
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Re: Late Qjet idle mix screw range?

Post by rfoll » Fri Oct 11, 2019 1:01 pm

I don't know where you live, but it could be related to cold weather and no carb heat. I recently ran in to a dramatic indication of how important it can be. My car is a 79 impala with a Vortec 350. To provide carb heat, I use an Edelbrock Performer
RPM manifold that has crossover bungs and a path under the manifold. The heater circuit is plumbed through the manifold and if the heat is shut off, there is no water circulation for carb heat. With the engine up to temp, and on a warm day, the circuit is closed. Our current temperatures are about 30 degrees overnight and 60+ during the day. I parked the car with heat turned off and drove the next morning with out turning it back on. When started the car ran and idled really rough. While sitting at a stop light 3 miles down the road I realized the heater was off, and when I turned it back on, the engine cleaned up and started running smooth before the light changed.
So much to do, so little time...

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Re: Late Qjet idle mix screw range?

Post by turbo camino » Fri Oct 18, 2019 11:16 am

Well, don't ever buy tires off ebay. I got sent a "set" made up of 3 tires made in one country, and one tire made in another. Same brand and model and pattern and everything, but the one outlier measures 12 points harder than the others. Seller doesn't understand why that's a problem. FML.
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Re: Late Qjet idle mix screw range?

Post by Rick! » Fri Oct 18, 2019 11:55 am

turbo camino wrote:
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?
The 1980 vintage carb is early emissions which means the idle tube orifices are squeezed down. I customized a pin punch to drive the tubes down a little in the collar and then used the idle screw tamper proof cap slide hammer and yanked them out. Then it's just a matter of sizing it with the right numbered drill bit and go a bit bigger. Tap the tube back into the collar and reinstall, easy peasy.
.090" is really big and I've never seen them that big. It would make off idle on the IR CO meter get pretty large on the rollers. It doesn't take much change to fix off-idle issues or idle screw sensitivity (or lack of it), in my experience.

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Re: Late Qjet idle mix screw range?

Post by turbo camino » Fri Oct 18, 2019 12:11 pm

I wouldn't say there's a lack of sensitivity, 1/2 a turn in either direction makes a detectable difference. They're just farther out to get a good idle than I remembered seeing. They are the super fine thread pitch and smallish curb idle holes in the baseplate (that's the .080" mentioned earlier in the thread), I think that's the only cause. Maybe I'll even get to drive it someday...
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Re: Late Qjet idle mix screw range?

Post by 77cruiser » Fri Oct 18, 2019 12:43 pm

As long as it runs & drives good enough for you, it's all you need. Sounds like you know your way around a Qjet.
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Re: Late Qjet idle mix screw range?

Post by novadude » Fri Oct 18, 2019 5:59 pm

77cruiser wrote:
Fri Oct 18, 2019 12:43 pm
As long as it runs & drives good enough for you, it's all you need. Sounds like you know your way around a Qjet.
What he said.

The fine pitch on those late model screws and the taper of the needle often has the screws ending up out quite a bit more than you'd find with an earlier carb (and way more than a typical Holley).

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