Technical Quadrajet - idle mixture screws do nothing

Seem to remember working on one Mopar with the factory Q Jet. Wasn't very common.

 

I'm thinking that some of the later model GM truck Qjets also had a variable or dual-displacement accelerator pump. Don't recall just how that worked though. May have been only on C3 feedback carbs ? ? ?

 

When I looked at one of the quadrajet model number guides, it only listed GM cars. Maybe the carbs that got sent to Mopar didn't have a number, or a different I.D. system.

 

I work for San Jose PD our Dodge diplomats had Q Jets on them

 

I had a brand new Mopar 360 Q-jet for a while, it came out of the local Mopar training center. It was a '85 or '86 model year.

 

I know that many Mopars came with qjets. I was only suggesting that the model numbering/ ID info may not have been the same as the carbs that went to GM autos/trucks.
Who has info on identifying the source year/model for Mopar Qjet?

 

Got the new lifter in, Inspected the old one. it has some extreme wear on the bottom(and a hole at the bottom too, which explains why it collapsed!), it sits like the leaning tower of Pisa on the bench. Common sense would say that the cam is most certainly flat from the amount of lifter wear. But hey, the new lifter *really* spins like a son of a ***** when the engine is cranking/running. I broke in the single new lifter at 1800-2400 RPM for about 20 minutes today. I'm praying to the Chrysler gods that everything works out alright, even though I know there's a 99.999% chance that it won't. The lobe itself surprisingly didn't look worn through the borescope, but that doesn't mean much. You certainly can't judge if it's flat or not through the low res camera. I wish I had taken pictures of the lifter before I closed up the garage. It's pretty gnarly.

As for the quadrajet on there, it responds very nicely to all of the adjustments. I have the APT ballparked in to where it doesn't bog and cough when you blip the throttle anymore, and since the idle mixture screws now work, I can get a vacuum reading of ~17-18" at idle now, and the throttle screw is barely touching the linkage at all.

As to what solved the rich idle, I think it was just the APT yoke bent out of adjustment? Even with the APT adjustment screw all the way down, I had to bend the yoke down a fair bit to get those rods to look/sit right. If it wasn't that, then it was the float, as I set that at ~.400". It was sitting quite high before just by eyeballing it.

I definitely would have just thrown a squarebore adapter and edelbrock on there if it weren't for this forum. Now I know what to look for in the future if something like this ever shows again. Thanks everyone for the help.

 

I have a Ac Delco master parts book at work. Pretty sure it lists Dodge. Will check tomorrow.
If you had to bend the primary rod hanger on the APT anything other then level, its possible the metering rods are wrong. Two different lengths between early and late quads. Something like .040-.060 different.
Did you ever find any stamped numbers on this carb?

 

No numbers. There's an obvious grind/file mark where they're supposed to be on the main body. I'm not sure why someone would do that.

What with the price of gas lately I've been thinking of getting a wideband O2 kit, so I wouldn't be adverse to experimenting with different rods and jets. Would like to see how many MPGs I could squeeze out of a 1972 coronet with a 318.

 

The supply of used primary rods is very limited. Did you notice what size/number was stamped in your rods?

 

The carb looks like a CCC carb for a 82-84 Buick 4.1 liter engine. You have a Franken Carb there. I not only had one but I worked at a Buick back then also. Rebuilt more than a few of those. That carb is just scary looking!! Whoever rebuilt it looks like they used a chisel to take it apart. They did use the good vacuum pull offs though. If that carb was off a Chevy or a Caddy it would have different vacuum ports. If I was you I would look for one from a 305 or 350 Chevy and rebuild it. The calibration should be pretty close, just don’t grab a computer feedback one. Just my opinion. For better throttle response use the aluminum check ball for the accelerator pump.

 

I didn't even know there was a number stamped on them. That must be a damn tiny die they used to get the numbers on there.

I thought so. I have a CCC carb sitting in the other room. That blanking plate definitely wasn't factory. Looks like someone hand filed it to shape. Also, can I not just calibrate this one to work with the motor? What's the difference between a big block quadrajet and a small block quadrajet? Surely it's just the jets and stuff, right?

 

There were actually 2 different sizes. A 650 and an 800 or 850cfm. All the engines v6 or v8 if they were lower performance or gas sipper they got the 650 with calibrations to match the engine specs. Now the 800 or 850cfm (can’t remember what size exactly was on the 454 on the higher performance and big trucks. There were dozens of metering rods, fuel pump shot springs and rods, secondary cams and rods and probably other stuff I forgot about. As far as I am concerned the only downfall of the Q-jet is the small float bowl. You could spend years trying to calibrate one. Try to find a good manual and you will see. The bigger cfm carbs are very hard to find in the wild.
Like I said it is easier to find a Q-jet off a 305 or a 350 Chevy. This way you would not only be in the ball park but be in the game too.

Also I think you are missing the retaining springs for the primary needles. Like I said that carb you have has had a rough life. I never liked store bought rebuilt carbs. Seen some real **** ones. Rebuild a junkyard one yourself or buy a new one. New ones from GM in 1988 were over $800.00.

 

That little spring clip on the ATP yoke? It might not have shown up in the pics but its definitely there. Its a very fine wire spring.

And it just so happens i did pull a quadrajet off of a 305 in the junkyard recently, I guess my compulsive hoarding of car parts pays off every now and then. Its in pretty rough shape though. Got rained on for an unknown amount of time, rusty ****erflies, oxidation galore, etc. I'll get around to a deep cleaning one of these days, nowing what i know now. Thanks for the info.

 

Yes..Beat me to it here.. If you take it apart again , look for the rod number, and post the overall length of it...I have some of both lengths, but I'd have to measure a few.
My old 454 RV wouldn't pull a hill worth a damn without 75-76 jets and a rich rod. Maybe somebody else found that out with yours too

 

Just thought of something else.
If you screwed the APT all the way down, then the spring will be operating at a stiffer rate against engine vacuum..Meaning that it will want to go rich sooner /faster. Of course they make several different rate enrichment springs.. Which one is now in yours is anybody's guess.
You would be better off having an APT adjustment available, IMO

 

Hell, Ford even put Q-Jets on 429 Cobra Jets.

 

I thought everyone used them at one time. I think even AMC used them for a year or two.

 

Calibrations included upper and lower air bleeds.
Some with just one set. Idle tube restriction. Down channel restrictions. Idle air bleeds. A****st a host of other differences.
Just changing rods and jets to far can screw up the original calibration a bunch.
Without knowing what the original carb number was and being a franken carb, you dont know what the original rods and jets that were in it.
750 and 800 CFM capable castings. The limit for such was set with how far the secondary air valve opened.
If it runs good you got lucky. I would keep my eyes out for a good core. They are out there.
Snag a copy of Cliff Ruggles Quad book and you will have lots of good info. I believe there is a new one from Everyday Performance that just came available.

 

Later models with 1.21 primary venturi are referred to as 800's.. Also the 72 Pontiac 455 HO with the single ring boosters are called 800's too.

 

Don't get hung up on the "rating" numbers. Flow testing may have been under the watchful eye of engineers, but published ratings are established by publicity folks to whatever value they want.

As Mark stated, the later models with the 1.21 primary are referred to as 800's.

Most earlier carbs are referred to as 750's.

Mark's finger slipped in that the Pontiac single ring versions are 1971, not 1972. Somewhere, I have a flow test on one of these and it flowed 829 CFM. Some folks refer to them as 800's, others as 850's.

Other "ratings" may exist as "stops" were often used to limit air-flow to pacify various organizations.

Don't even think about one of these Pontiac single-rings, unless you are building a numbers-matching Pontiac, are independently wealthy, and don't plan to stay that way. Besides, they are not H.A.M.B.-friendly.

Rochester did not make a lot of these. 3/4 of these were butchered by the drag racers, and the other 1/2 are either in the hands of, or being actively sought after by restorers. The math is correct, as due to scarcity, some restorers are reworking junk just because of the number. Decades ago, we reworked some of these butchered carbs; what an absolute nightmare; but the customers wanted the result, and cost was not considered.

If changing applications, best to start with one originally used on a similar application. As several others have mentioned, one from a 305 Chevy would be easier to tune. Yes, I know, we are all hotrodders, and capable of making a Holley Dominator work well on a Sears riding lawnmower; but it really is easier to start with something closer.

As far as the differences between small block carbs, and large block carbs; remember "carburetion 101, lesson 1": a carburetor is a device for MEASURING and mixing fuel and AIR. Most folks only consider fuel, when air is just as important, often more so.

Jon

 

I love that line Jon...but in my case, I'd try running the Sears lawnmower carb on a big block, and it would still run eyeball burning fat.

 

I have tried "preaching" starting at a reasonable point with levity for decades, with maybe a few results. The second line in my signature block suggests the same thing.

Another gentleman on another forum is somewhat more blunt. His comment: "the Laws of Physics are NOT the Suggestions of Physics" which is so true.

Jon

 

You mentioned you were going to get a wideband O2 sensor setup. I did and put one on my 383 700 R4 set up in my daily driver 64 El Camino. I bought a Jet remanufactured Quadrajet from Summit. Well it was better than the last one that was worn out but it still wasn't right. I got the wide open, cruise ,and idle dialed in perfect went from 12 1/2 mpg cruising to 18 cruising. Well worth it

 

I'm looking forward to installing that thing once I actually buy the proper exhaust pipe and make a full system for it. Right now I just have an obnoxious pair of purple hornies on there. It'll be my first time tuning with an O2 sensor. Since the 318 still has it's factory compression ratio(and factory open chamber heads) I wonder how lean I can get the mixture before I run into pinging. I'd like to see if I could squeeze 20mpg out of it for starters. It's always been a hog on gas, but the only time I actually ran the numbers was after doing 40 mile runs on the interstate with it, came out to 7 MPG. No, I didn't miss a keystroke. But that was also "going with the flow" of traffic, 90mph or so.

 

Get it running and driving comfortably.
Unless you are actually going to drive it 5,000 annually, you'll never save enough on fuel to pay for the O2 sensor.
If you want to check highway fuel ecenomy, you need to fill up, drive the freeway at a comfortable cruising speed until tank is almost empty. Fill up and repeat a couple more times. THEN check your overall mpg.

 

I don't know about that, but a good sharp tune will often almost double the highway fuel economy. Not so much of a % increase around town, but it's overall definitely a welcome increase in range. Wasting fuel is stupid, because it isn't going to run any better or faster. It may well run worse, And O2 sensors are pretty inexpensive now.

But a proper fuel "curve" also keeps plugs from fouling, and crankcase oil contamination from excessive fuel. It's noticeable that the motor oil stays honey colored a lot longer, in much the same way more modern engines do.

 

With 7 MPG driving till empty comes alot sooner than you think.

Also, I disagree about not tuning for maximum MPG as a money saver, especially with gas prices now. At 10 MPG, it costs $200 to go 500 miles, ***uming $4/gallon.
At 15mpg, it costs $133 to go the same 500 miles.($66 difference from going from 10 to 15mpg)
At 20mpg, it costs $100 for 500 miles.
At 30mpg, its $66 for 500 miles.
As your MPG increases from there, the fuel bill difference per 500 miles diminishes rapidly, at 40mpg it costs $50.

The starkest effect on your wallet is improving low teens MPG gas guzzlers to get even 20MPG. The next car I'm working on is a 63 valiant with a 170 slant 6, and I dont think 30MPG would be an unattainable goal.

 

So how many miles will you be driving this year?
I'm all for saving money, and my 350 SBC gets 17 - 20 on the highway with a quadrajet. But I'm not about to drop a couple hundred to measure my O2, although it might help to get a little finer tune.

If you are getting an accurate 7 mpg on the highway, you can easily get to the next level without the heavy equipment. All you will learn with an O2 measurement at this point is that your afr is pretty low. I think we already know that.

 

Its like driving 10 miles too save 10 cents a gallon