I love a quadrajet. It's great when people dump them for Holleys because then the rest of us can buy their old quadrajets cheap.

They are a lot of fun rebuild, very satisfying.

 

I vote for the QJet . I ve owned many vettes most all with QJets Really no complaints . None were race cars . .....requiring quick track mods or adjustments to get the last bit of HP dialed in for a specific track .
I have a Cliffs rebuilt QJet ready to go on my Camaro. Kinda excited to see how it will run.

Once you have the QJet dialed in it's pretty much worry free. For pleasure driving .

It's not like you have to rebuild it every other year . It should last the rest of you life or till you can not drive.

Just pay Cliff to build it how you want and be done with it. According to your engine spec , trans , rear end , etc.

You have a chance to get one of the best to build you a QJet . He won't be around doing this forever .

 

'76 and later Q-jets also are set up to run leaner than the earlier units; but that's not a good thing. Select your carb wisely....

 

I have an O2 gauge in my car and have fun tweaking and tuning both a Holley and Qjet on my car.

If you are not familiar with the APT screw system on the 1705xxxx carbs, it is pretty genius. I would recommend one of these for the tunability aspect. I have the 17058204 like the one Shark Racer built here https://www.corvetteforum.com/forums...t-rebuild.html

Basically, you swap primary jets to set your WOT air fuel ratio. At WOT, only the rod tip is in jet so the variable is the jet size. (beauty of a qjet is you can swap the jets with the carb on the car and without disconnecting the fuel or spilling a drop.)

Then you need to gain access to the APT screw to dial in your cruise or part throttle A/F ratio. This screw raises and lowers the lowest setting for the rods. Under high vacuum, the rods are pulled down so that the tappered part of the rod is in the jet. The APT screw allows you infinite adjustability. Its like an idle screw for the mains.

So its easy to adjust the mixture for altitude, winter, summer, or fuel economy.

I would give Cliff a call. The first thing he said was (as 7T1 said) my carb was calibrated lean. The fix for that was to tap in air bleed restrictors. He has the kit you need including the following special items:

* custom lathed rods
* air bleed restrictors
* APT port access plug


John

 

again thanks all,
And it goes to show that the mystique and opinions surrounding the Rochester qjet will be longed lived, even which year ones are the best cant be agreed on in this little thread...

I hate to say it but yeah, when my car is running again many parts like the carb will likely out last me,

But where I gave the impression I am playing this cheap I do not know while I make fun of my budget build status I will pay between 4-500 for a top name rebuilt unit, ruggels, murphy, jet not sure which one but all agree and do many things most of the smaller or hobbyist re builders do not do,

I will also pick up cliffs book and a few cores but where I get them isn't about being cheap or a 50 buck difference but rather the fact that most agree with that all qjet cores are hit and miss and unless you buy it from a guru or are savvy and break it down at the sellers house there is simply no way to prove a 100 eBay core is automatically better than the cheap or free some one gives me,

I am excited about this and it's very motivating.

 

What you want in a good core is one that has not been abused...meaning, not a Frankenstein (made up of parts from many carbs) or one that has been heavily modified. The ideal is one of vintage close to that of your car, and that has never been rebuilt. Then you know it is as it was built from the factory with matching parts.

Sometimes you can glean that info from the seller or the ad; or you can ask the seller whether it has been rebuilt or not. If the original owner is the seller, you have a good shot at getting a good core.

 

in my case my car is a frankenvette , 454 block from the 80's heads 70's? hooker side mounts hei...etc etc,
This is a great example where I am good surfing coat tails and asking each big rebuilder what they recommend,

Also in 69 was the same qjet on sb and bb's?

One thing I am seeing is most of the big names for the most part agree what is what yet step down to the ones I respectfully call "hobbyists" and they seem to each have a different opinion,

 

Somewhere in Lars' papers he has a recommended rod/jet and secondary rod size. I used (after many attempts to "better") them in my 427 and a 454 that had moderate cams, and the engines would break the tires loose from a 20 MPH roll in 1st and pull very strongly to 5500. If I remember 74/41 BG are the numbers, but when I sold my last car I shipped all my notes and manuals to the buyer.

 

Different carb #s for sb and bb. Do a Google for correct carb # for 1969 QJ.

 

The only significant differences between sb & bb Chevy Q-jets was their primary rods/jets and secondary rod selections. There were other linkage differences. But a Q-jet can fit on about any engine large enough to use all of the primary flow. You can re-rod/jet a BB carb to work fine on a SB powered vehicle.

ANY 1969 Chevy Q-jet would be a good candidate for ANY 1969 Chevy vehicle.

 

The later Q-Jets are easier to deal with and they can be tuned to run as rich as you'd like. You can wash the oil off the cylinders at a 3000RPM cruise if you'd like.


Why do I say they're easier to deal with?
1. APT, as mentioned. This makes tuning cruise much easier.
2. More reliable - GM figured out most of the problems by then.
3. On-carb choke, anyone who's set up a divorced choke knows how much fun that is. This opens you up to using pretty much any intake manifold you'd like - as long as it supports a spread bore or you have the space for an adapter (not ideal).
4. They're dirt cheap because most people think the 4Ms are better since they're from the "performance era". They both flow 750 CFM, with some of the M4Ms actually flowing 795 CFM.

 

Since 1981 was the first year of the electromechanical QJet, I am guessing "later model" would be the 80 or 79 (M4MC). According to Corvette by the Numbers the 80 numbers are:
17080204 = 350/190hp AT
17080207 = 350/190hp ST
17080228 = 350/230hp AT

 

Hence why I said "M4M". 80 was the first year for the E4M, but CA only. M4Ms started in 1975, but the 75 carbs have some extra circuits that make them more complex, so more specifically I'd look for 76-78 carbs. 79-80 have different idle mixture screws (and passages which necessitate the screws) which require a different different screwdriver bit (DD, I believe).

The earlier M4Ms us a combination head, you can get at them with a slotted screwdriver or a 1/4" socket. A 1/4" socket on a flex head makes them very easy to tune idle with.

Functionally, they'll behave the same (have the same level of reliability and use the same parts to tune otherwise).

You can tell an M4M from the top because they'll have two major clues:


1. The choke flap has a wide notch in it that looks like a "D" with an "O" towards the center of the "D" to fit the casting rather than a much narrower "O" shape.
2. There is a 3/8" steel dowel plug in front of the choke flap that blocks access to APT. In the carb I photo'd above, I removed the plug and tapped the hole for a 3/8-16 set screw so that APT can be adjusted without removing the airhorn.

Here's a pic of a 4MV for comparison:

 

I don't think I would reject a 79 or 80 carb because of a different idle screw. The last time I needed to adjust one at a friend's garage, I just made one in a few minutes. Also, these carbs can be picked out right away by the casting (for the future solenoid) on the left front of the top of the carb.
I also think those folks in CA only got a 305 back in 80?

 

In Cliff's book, it seems like the only issue with 79-80 is the loss of the primary choke pull-off, as they went to a rear-only choke pull-off. "The rear choke pull-offs have a very slow release time and can be somewhat difficult to modify...they are not as advantageous as the models that use a primary choke pull-off." (Quoted from Cliff's book, and note he is speaking regarding "high performance use" not typical street usage.)

Cliff's book agrees with Shark Racer: 1976-79 large bore with appropriate inlet placement (front or for Chevrolet, side inlet) are the practical sweet spot.

 

Sure. I agree with you.

I'd put my preference list for Cores as something like this (and all Chevy Cores, BTW):
1. 76-78 M4Ms
2. 79+ M4Ms
3. 4Ms - but at this point there's a BIG gap between 2 and 3 in terms of desire.
4. E4Ms.

I agree that sockets can be bought or made, but 1/4" hex is such a more common socket and pretty much everyone has one, not to mention they're commonly used as the shank size for most replaceable bit screwdrivers. And you have 6 different ways to get the orientation right vs 2 for a DD. IMO, they're also more positive "feel" on the turning so you're better to appropriately feel the 1/8 turn finesse you need for adjusting idle mixture screws.

But not a dealbreaker.

Other clues for an E4M are the TPS plug and idle air valve.

 

Another you tube video, I thought this one was pretty good. Lots of resource stuff out there.

http://www.bing.com/videos/search?q=...8C7E&FORM=VIRE

Hope this might help.