General Category > Quadrajet Carb Talk and Tips

Q-jet with 3 mixture screws? 17085213

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Glutenfreecarbs:
This is my current daily driver, P/N 17085213. It has a hidden and capped adjustment screw, which somehow connects to both the throttle body and the secondary fuel wells. I have no idea what any of it does, but it's been a great summer carburetor, mainly because it has an A/C idle solenoid to keep the RPMs up when my giant axial R-12 compressor kicks on during a traffic jam.

This carburetor also came with big cone-shaped "61S" primary metering rods and .078" jets. It's hard to imagine a heavy truck/van/RV would be happy cruising with such a lean mixture, but I guess that's what The General prescribed...

tayto:
It's for a hot idle compensator. I rebuilt a Qjet for a '80 Firebird and it had one. I believe Cliff recommended to block it.

Cliff Ruggles:
Those are HD truck/motorhome carburetors and used special castings with an adjustable secondary airbleed/enrichment system.   Most likely due to the fact that those engines would spend a lot of time on the secondaries towing and climbing hills and steep grades.

Most showed up on HD 454 truck engines but I've seen a few used on 350 HD engines as well.

The special full tapered 61 primary rods are correct but I've never seen 78 jets in one to date.  I suspect those were added by a well meaning tuner that didn't under stand the APT system, which on that model is a little different than others being adjustable for height and travel/limit of the PP.

They have a pretty rich idle set-up for an emission years carb.  Most are part numbers 17085212 and 17085213, but they were made with a couple other part numbers as well.  17085000, 17085003 comes to mind or something close to that.

Be warned that they use different parts and gaskets and will NOT work well if the wrong ones are installed during the rebuild.......Cliff

Glutenfreecarbs:
Ohhhh, so that's what this mechanism is! I thought it was just an anti-tampering measure for the APT.

Thanks for the info; maybe I'll reinstall the big 61S rods and the upper PP travel limiter (?) just for fun...

Glutenfreecarbs:
So I reinstalled the original 61S rods, .081 jets, and the upper power piston travel limiter. It's been a lot of fun to drive around and tune this unique APT system; you can go waaaaay off the spectrum either lean or rich, and shrink or expand the range of PP travel, all without removing the air horn. Thanks Cliff for explaining this feature!

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