Q-jet stuff

[Cecil14]

Ok. I got my Q-jet today. It came off a '71/72 caddy. The guy thought it was a 400 or a 425. It had an egr valve. It looks like it's in pretty decent shape, everything moves ok, nothing seems bound up. Got a rebuild kit coming tomorrow.

Couple q's now:

Anybody know about what size this one is? I'm guessing around 750cfm? Kinda thinking that might be bigger than I really want but we'll see.
Next, is there a book on these carbs I can get? Something that details a rebuild really well and maybe which metering rods and jets and stuff I'll want to use? Or are you guys that book? smile.gif

Oh, this is going on my 4.7L stroker, not the 360. Need the 6 done soon, V8's gonna have to wait a bit longer.

Hmm...can't think of anything else at the moment.

Thanks guys,
Anthony

[J Series Trucks]

This discussion is probably best suited for the tech area, but here is the scoop on QJets:

There were a couple different flavors of QJets. The pre 75 units numbers started with "70"
if you have a 75 and up the numbers start with "170". They all work the same and many parts swap even up into the 1980s where they became somewhat electronically controlled Let's talk about the "70" series.

The differences in the 70 series were two: GM division and CFM size.

The differences between the GM divisions were fuel inlet type and choke type.
Caddy used the 90 degree inlet as did Chevy, Buick used a straight in type of fuel inlet, and I'm not sure what Pontiac and Olds were doing. The choke styles were different- some used a divorced "spring in a well", some a hot air set up, some electric. No matter, they are all bolt on, and ultimately work the same moving the choke air door.

The CFM size was this:
small blocks all got the 750 size, and so did the big blocks pre 71. In 71 on some big blocks got the 850 CFM QJet. No matter, the beauty of the QJets is that they work on demand, and an 850 QJet will work on a small motor. You can;t use a 750 on a worked big block because of internal differences that restict fuel flow unless you modify them. It only has THE POTENTIAL to flow 850. To tell the difference, you look in the top of the air horn past the choke door at the venturis. The 750s are cast with a bigger "ring" restricting air flow more than the smaller "ringed" 850s, as well as the internal passages on the 850 which are larger in certain circuits.

Since you want to run this on a 6, either is fine to start with.
I could tell you which one you have if you send or post a clear picture.

In any case, what you will have to do is to change the primary jets and primary rods to a combo that is closer to what you need for the 6, and then also the secondary rods for the same reason. Other than that, you can take the number off the side of the Qjet,
704123X where x is some even number IF it came off of a 71 Caddy with auto.

Take that number to a NAPA and get a Standard (brand name) rebuild kit.
Also get a new secondary pull off (also called a vac break) Borg Warner has them in thier
fuel catalog (send me a pic if you need help)

Rods and Jets
Here is probably a good starting combo for your 6 with Edelbrock part numbers:

.068" #1968 Primary Jet
.045" #1946 Primary Rod
CK #1952 Secondary Rod

This is at least ballpark, and you will know better once you get it broken in, and can tune.
Tell me what rods and jets you have now, I may want to trade for stuff you can't use, I collect
QJets parts for tuning, and have rods and jets you can't get anymore for when you DO start to tune your Qjet to that motor.

The best book hands down for QJets is "Rochester QuadraJets" by Doug Roe, the engineer who designed the QJet. You can get it on Ebay all the time. Just bear in mind that some of the mods in the book are for drag racing only, big blocks at that and will not be nice to do to an innocent Qjet! smile.gif

Any questions write me direct as I don't check this board all the time

Brad Feick

[porkchop]

I moved this to the tech forum for you.

[J Series Trucks]

Cecil14- are you there?

Brad Feick

[davez26]

I got the Doug Roe book from Barnes and Noble about 9 mths ago. It is an excellent source of Q-jet info, both theory and application.

[Cecil14]

Originally posted by J Series Trucks:
Cecil14- are you there?

Brad FeickYah, I'm here. smile.gif Thanks for the reply. I went and ordered the rods and jets today and picked up my rebuild kit. How do I tell which rods/jets I've got now? They marked?

I also ordered the book today. Should have it in a couple days.

Thanks for the reply,
Anthony

[J Series Trucks]

The jets are stamped in the top of the face which you can read (sometimes only after rubbing the tops with steel wool) I'm guessing in a Caddy Qjet there will be at least #73
or #74 jets, and something like a #42 rod. The secondary rods are anyones guess, GM made a SLEW of them, but I'll guess around AK code secondary rods which is a .0570
rod in a scale of .03 to .1something.

The rods have the code stamped in the on the sides.

Make sure you use a BIG screwdriver to remove the jets that fits firmly in the jet's slot or you will tear them up rendering them useless.

Later, if you like, I can run down the mods that are worth doing in your application after you read the Roe book.

Brad Feick

[Cecil14]

Cool. It's probably going to be a bit yet before we tear into the carb. I'm still working on getting the rest of the motor under control.

I'll let you know what we find.

Thanks,
Anthony

[Don S]

Originally posted by Cecil14:
Cool. It's probably going to be a bit yet before we tear into the carb. I'm still working on getting the rest of the motor under control.

I'll let you know what we find.

Thanks,
Anthony..
Cecil14:...
Here us a link to Q-jets I just found.
http://autorepair.about.com/gi/dynamic/offsite.htm?site=http%3A%2F%2Fkarlmonster.tripod.c om%2FRochcarbs%2Findex.html
Good Luck. ;) .CUL… ds..
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