Author Topic: Q-jet with 3 mixture screws? 17085213  (Read 2807 times)

Offline Glutenfreecarbs

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Q-jet with 3 mixture screws? 17085213
« on: May 08, 2021, 11:29:27 AM »
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...




Offline tayto

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Re: Q-jet with 3 mixture screws? 17085213
« Reply #1 on: May 08, 2021, 07:28:21 PM »
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.

Offline Cliff Ruggles

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Re: Q-jet with 3 mixture screws? 17085213
« Reply #2 on: May 09, 2021, 05:32:06 AM »
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

Offline Glutenfreecarbs

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Re: Q-jet with 3 mixture screws? 17085213
« Reply #3 on: May 09, 2021, 10:19:55 AM »
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...


Offline Glutenfreecarbs

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Re: Q-jet with 3 mixture screws? 17085213
« Reply #4 on: May 20, 2021, 02:01:59 PM »
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!

Offline Cliff Ruggles

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Re: Q-jet with 3 mixture screws? 17085213
« Reply #5 on: May 20, 2021, 04:04:01 PM »
.081" jets would be huge in that carb.

The biggest jets I've ran in one to date were .076" on a pretty "hefty" 454 engines and they were plenty.

I like the 61S rods as they are full taper instead of "stepped".

My 50C rods are the same basic design tapering from .050" down to .036" tips.  I use them in all carbs built here that originally used "M"  series primary rods......

Offline Glutenfreecarbs

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Re: Q-jet with 3 mixture screws? 17085213
« Reply #6 on: May 29, 2021, 10:32:42 AM »
Hey Cliff, does anybody else ever struggle with quick transitions to WOT on these units? This carb has always had a lean spot if you randomly stomp on the gas pedal, which I've been unable to correct with any combination of air valve settings and hanger heights.

Maybe this is due to the uniquely small secondary accelerator wells?  I tend to avoid drilling or modifying castings, but should I delete these partitions to get more of a squirt when the air valve begins to open? Note the fuel  trapped in the "void" spaces after draining the float bowl.
« Last Edit: May 29, 2021, 10:39:16 AM by Glutenfreecarbs »

Offline Cliff Ruggles

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Re: Q-jet with 3 mixture screws? 17085213
« Reply #7 on: May 30, 2021, 06:09:47 AM »
The choke pull-off is a BIG player there.  Make sure that it is holding the secondaries tightly closed at idle and that it releases smooth and not too quickly. 

I have the OEM style straight inlet pull-offs in stock, the bent tube variety everyone else is selling these days are HORRIBLE and there is no restriction in them........

Offline blazer74

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Re: Q-jet with 3 mixture screws? 17085213
« Reply #8 on: July 04, 2021, 11:31:28 PM »
I have one of these 17085213 carbs in my collection. Good Info.

Offline Cliff Ruggles

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Re: Q-jet with 3 mixture screws? 17085213
« Reply #9 on: July 07, 2021, 04:06:38 AM »
I personally wouldn't use one outside of it's original application.  They were specifically built for big block engines and Heavy Duty trucks, motorhomes and towing applications. 

Much like Marine carburetors they are calibrated to work on engines that see a lot of heavy load.

Kind of a cool story but a good friend of mine converted his late 1980's Chevy crew cab dually from TBI to a late 70's Cadillac Q-jet some years back.  He did an HEI at the same time.  He tuned and tuned on the big Caddy carb but was never overly happy with it.  Even with APT and playing around with jets and tapered metering rods it just never did as well as he thought it should.

I recommended switching to a 17085213 and had one in the shop.  He made the switch and it worked better for everyplace with improved fuel economy empty and towing......FWIW......Cliff

Offline Glutenfreecarbs

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Re: Q-jet with 3 mixture screws? 17085213
« Reply #10 on: July 07, 2021, 04:48:13 PM »
I've put my 17085213 back on the shelf for now; I was never able to cure the nasty lean spot when quickly going to WOT, even with my stash of old OEM choke pulloffs. Also had a pretty decent lean backfire while trying to impress Mrs. Gluten on a date night...

I still want to try removing those little partitions in the secondary accelerator wells to get more "squirt" as the air valve is opening, but I haven't thought of a civilized way to do this. This unit has worked great for me otherwise; I ran it most of last summer...with 38J rods and .070 jets because Reasons.


Offline Kenth

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Re: Q-jet with 3 mixture screws? 17085213
« Reply #11 on: July 08, 2021, 12:49:39 AM »
I have built several of this model and always removed the extra wall in the secondary initial POE well. I have also plugged the holes for the adjustable extra air channels with set screws (were the toothpicks sits in your picture) in the secondary circuit as they were never used for high performance use.
Also, since the carburetor has APT, I have not found any reason to change the main jetting.


Offline Cliff Ruggles

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Re: Q-jet with 3 mixture screws? 17085213
« Reply #12 on: July 08, 2021, 03:31:56 AM »
Considering they are somewhat rare and application specific I save them for big block Chevy truck and heavy duty applications.  That is where they were designed to be used at and fair best in that scenario. 

Just like Rochester Marine Quadrajets they work best where they were designed to be used.  Since there were literally tens of thousands of 1979-up SMAB carbs produced that don't have that specific secondary bleed system and no goofy APT limiter, etc, it's just best to source out one of those instead if you are looking for the latest design as a starting point for your application.

I'd also mention here that the 17080212/213 carburetors are excellent units but don't use that system, if one is looking for a really nice starting point for a big block Chevy, heavy duty, or truck/motorhome or other towing application.....etc......

Offline 73ss

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Re: Q-jet with 3 mixture screws? 17085213
« Reply #13 on: July 17, 2021, 02:32:07 PM »
The DCR's look huge, Did you measure them?

Offline Cliff Ruggles

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Re: Q-jet with 3 mixture screws? 17085213
« Reply #14 on: July 18, 2021, 05:30:07 AM »
They are huge, over .060" nearly as I can remember.

Those carburetors are NOT lacking for idle fuel and work reasonably well on big block engines even with slightly larger than stock cams in them.

They have other features that make them less than desirable for some milder applications and why I don't recommend using them much beyond what the factory designed them for.......