Okay guys...I've searched the forum, read the posts, and think I'm ready to go for it...but just want to make sure before I give up the cash. I've got a basic fresh rebuilt 327, 2004r transmission, 3/4 headers, dual 2 1/2 exhaust, Edelbrock Performer intake, and an Edelbrock 1406 carb. I'm thinking of converting to a q-jet, but wanted to know if one is better than another, if there are models to stay away from, or if the newer models had improvements I might as well have?

any thoughts or insight is much appreciated. I'm not particular to having one that originially came on a '68 camaro, so if a newer model with all the bells and whistles would work better for my daily driver, I'd appreciate the info.

thanks,
clay

I would recommend a later model with electric choke. They also have easy access to the APT setting screw. You can connect the choke to the wiper motor power wire.
What kind of choke will manifold work with may help you decide. The early ones with divorced choke will need the choke coil mounting surface on the manifold.

TTFN
Mat

And conversly, I'd recommend a pre-smogger carb; 74 or earlier. Though they won't have the APT, they have a wider range of adjustment.

FWIW.

definitely not the 74 or earlier,absolutely not a 75 or some 76 carbs, there are alot of reasons why which I just don't have the time to write a book about. I suggest you buy Cliff Ruggles' book on quadrajets or speak with him. I put on a 1977 model and rebuilt it myself after buying the kit from Cliff. It did require resizing some passages but it runs better than any ather carb I have had

Are you suggesting the use of a 1977 and newer Q-jet over a 69-74? You'd possible be the only one with that recommendation. The Ruggles book you cite conflicts with your recommendation (see the chapter on performance tuning). Again, I assume the goal is performance; not MPG or clean emissions.

Also, here's an excellent white paper from Randy Frazier and Jim Hand, a couple of respected and winning quarter-milers (SE class) here in Texas who run Ponchos.

http://www.geocities.com/gtopercy/Pictures/quadrajetTech.html

Here's a quote from the above from Randy Frazier:

"This condition (lean-out) is usually worse on '75 and later emissions model Q-Jets and those equipped with EGR valves (exhaust gas recirculation). The '75 and later Q-Jets also used different power pistons and rods which will not interchange with '74 and earlier models, but the jets and other parts will. Also, on '75 and later model Q-Jets the factory used a couple of versions of APT devices (adjustable part throttle). The models can be identified by the extra bulge cast into the front of the air horn. The purpose of the APT was to provide a leaner than normal mixture under high vacuum conditions. The factory literature that I've seen advised the GM mechanics to not change the settings, and really contained no other service information than to replace the device if it was malfunctioning. My experience with '75 and later Q-Jets is that they are not as good for performance applications as the earlier models, and may not run as smoothly because the APT may be out of adjustment. For performance applications, I always suggest replacing them with '74 and earlier models, but if this is not practical, you may can change to a slightly larger jet size and smooth out the lean condition somewhat.

Here's another quote from Jim Hand from the same white paper:

"Whenever engine modifications are added that tend to adversely affect idle quality, or a later Q-Jet with more emissions controls is used, it is sometimes impossible to obtain satisfactory idle."

Interesting info on the Q-Jets. I believe the earlier ones are better, but I did use (just now swapping carbs) a 1977 Pontiac Q-Jet on my 454 that ran 11.95 1/4 mile in a 3940 pound car(with driver). With the stock power-piston spring (that's all I ever used) adjusting the mixture screws didn't seem to have much effect on the idle. I've read that you need to change the stock power-piston spring when used with a large cam, but I never got around to it.

hmmm... there is no chapter on performance tuning, there is however a chapter titled selecting the right carb and another titled high performance modifications. after rereading the book I stand firmly by my recomendation. On the other hand Doug Roe's book is a bit dated. The 77 and newer design is superior in many ways including the APT if you know how to calibrate it correctly, there are several folks out there who have over complicated it. It does not provide a leaner mixture on the 77 and newer design, the 75-76 design IS a nightmare and should be avoided. What the APT does is so simple, it is a stop screw to limit the downward travel of the power piston. Therefore allowing proper positioning of the needles in the jets, under high vacuum. The most effective way of adjusting the APT is outlined in the book but is basically doing a lean tip in at whatever your cruise rpm roughly is. So it doesn't lean you out, it allows you to set the rods right at the verge of going to the rich part of the needle when at cruise, this in turn gives you the economy and and great transition to the power circuit. Run the strongest power piston spring that lets the piston stay down at idle. If the idle mixture screws have no effect it is a problem with either the idle air bleeds being oversized or the idle uptubes being to small or both. I followed the recipe in the book and it was really close, all I needed to do was open it back up and enlarge the idle air bypass some more. Also the float design(float fulcrum, etc) are superior on the newer carbs. Like I tried to say previously, it requires some passage resizing to get the qjet to work better, In my opinion nobody knows a qjet better than Cliff Ruggles.

"Here's another quote from Jim Hand from the same white paper:
"Whenever engine modifications are added that tend to adversely affect idle quality, or a later Q-Jet with more emissions controls is used, it is sometimes impossible to obtain satisfactory idle." "

Jim Hand apperently has no idea how to modify the idle circuit to get a satisfactory idle, actually read the next paragraph, he does. As for the different needles and such on the 77 and newer, this is good news as the tuning kit from edelbrock was designed for this style

my 67 camaro has a qjet on sbc with weiand 142 supercharger, performer rpm cam, 4 speed, 4.11 gears. It previously had both a holly 750 dp and a 750 edelbrock.

It was also pretty easy to modify to boost reference the power valve

Where do I start my disagreements.

The APT's sole function is lean out the mix during high vacuum conditions (cruise) for emission purposes. It does nothing for performance.

Secondly, Doug Roe was the chief engineer at the Rochester Products Divison of GM in the 50's and 60's who R&D'd the q-jet during it's design, so I'll take whatever "dated" information he offers.

"Run the strongest power piston spring that lets the piston stay down at idle"??? Wrong. Performance engines make less vacuum and need a LIGHTER spring which will keep the power piston down (which keeps the primary rods in the jets) during idle and other high vacuum conditions. Otherwise the carb reads the low vacuum signal (caused by a cam with longer duration) as being under a load and enrichens the mix at idle which obviously causes several problems. A stock power piston spring is usually too much for an engine with a performance cam. And each application will have a specific spring designed around the vacuum requirements. The PP spring installed on an L-36 Vette (390 HP 427) in 68 & 69 will be set up differently than a 305 Caprice built in 77. And the recommendation to use the "strongest" or "lightest" spring is suspect advice. You'll pick the color coded spring based on your vacuum reading at idle.

You can get lighter PP springs here in a color coded kit:

http://www.carburetion.com/weber/weber.htm

If the idle mix adjusting screws have little or no impact on idle quality, you probably have a 75 or newer q-jet, which has limited adjustability for emission reasons. Go back and either read my first post above or the white paper. I don't follow your fix for this problem, but,..

The only major float change happened sometime in 68 when brass was replaced with cellular plastic. If there were minor design changes in the late 70's, I've no knowledge of their performance impact.

Jim Hand has probably 2000 passes down the quarter with a q-jet supplying the fuel. I'll yield to his expertise.

But, if MPG and emission concerns are the buying criteria, a later q-jet will do great.

sl 69, omg, do what you want, but the apt is not what you think, what you are describing is on the 75-76 models, again to be avoided

Yes, Doug Roe is a smart guy,, people have learned a lot since then.

yes, the strongest spring that stays down at idle may be lighter than stock.

as far as the idle screws, on most qjets many of the circuits need modified, its not that hard to do after you read the book if you understand how carbs work. How can you not follow the fix? So the adjustment is limited, not after you modify it.

The float changes rsult in less fuel displaces by the float and more leverage on the needle, result is more bowl capacity and the ability to run a larger seat orifice and higher fuel pressure to eliminate fuel starvation.

I find Jim Hands site to be informative but brief.

"But, if MPG and emission concerns are the buying criteria, a later q-jet will do great"
not true at all, aside from calibration, the later models have iimprovements, including APT that makes them more tunable
Are we talking about slapping a qjet on without tuning it? I wouldn't do it.
If I were to go through the trouble of putting one on a High performance car, which I have, and have modified, which I do run on the street and does see track time. I again stand by my recomendation.

All I really wanted to say is that once you read Cliff Ruggles book it will make more sense.

I was hoping Charlie would jump in here. He seems to have a very good handle on Quadrajets and how they work.

You guys know a lot more than I do, but I want to say that the old, "condum" rubber needle valve Q-Jets are good ones!

My stock '67SS/RS had one on it so know. The big deal is setting the back top butterfly's tip-in suck/rpm range!!!

You get that right and if she is a good match to your engine size, you will have a winner by me.

I have an AC Delco rebuilt 750 Q-Jet off a 403 Olds on my Strong Arm 406 SB and it ran fine before I pulled it to make way for my 496.

pdq67