I was shuffling some parts around in my shop yesterday. One of the items I came across was a Rochester Quadrajet carburetor. While looking at it, I was impressed at how small the primaries are. I have had Quadrajets on a couple of cars and always liked them. There was a Sharp 4 BBL manifold for an 8BA hanging on the wall over my bench that started me thinking. I knew I had a Quadrajet to Holley adapter, so I dug it out and started piling parts on parts. To make a long story short, it looks like I can mount a Quadrajet on that manifold using the adapter by drilling 4 holes and buying 4 allen-head cap screws. After my experience with 2G's on Merc manifolds, I think this is something I might want to try. It looks like the primaries are small enough so they won't overwhelm the engine (my 258" with a MAX-1 etc.), but I wonder about those huge secondaries. Will the engine sort of self-regulate, or will the secondaries flood it out? Any thoughts and suggestions will be appreciated.
The secondary air valve is tunable to a certain extent. If the engine bogs when the secondaries open, you can tighten the spring. It is sensitive and can be very finicky to get right. There are also multiple sizes of Quadrajets. One for a 305 Chevy would be a better starting point than one from a 500 Cad or 455 Olds. I think there were even a few made for the Buick V6 but I'm not sure.
Most Qjets are the same size. The secondary air valve has a spring that you can adjust to control the opening. There is usually also a vacuum diaphragm that works to close it, too. It's a demand secondary system, so it adjusts itself to the required airflow. A carb that is too large does not "flood out"....rather, it will "lean out" because there is not enough air velocity to create enough pressure differential to get the fuel to flow. The Qjet when set up right (to factory specs, in most cases) does not have this problem, because the primaries are so small, and the secondary side has that air valve to limit airflow to what is needed. I expect it will look rather odd, though, seeing that carb on that motor.
Ur better off running a small base Carter or Rochester from the early 50's on a flathead. They bolt right on the 4 barrel manifolds with no adapters, have small primaries, and don't look out of place. If you've still use the original ignition that's all vacuum a Holley 4000 off a y-block will work without doing anything.
I've been looking around, and it seems they made Quadrajets for 262 ci Chevrolets; that seems about right for my 258" engine. Also, from what I've found, you can make just about any Quadrajet run on any size engine. As to looking funny, yeah, I know. The reason I got thinking this way is that I have a rebuilt Rochester 4GC from a '53 Olds I was going to use on the Sharp manifold. I just came across another '53 Olds 4GC, which I bought because I have an Offenhauser dual quad manifold for my '51 Olds. Matched early 4 BBL's are really hard to find and extremely expensive when you do. It looks like the 4GC's are going on the Rocket, so I have to find something to run on the Sharp. Have you guys priced WCFB and 4GC cores lately? If you can find them? This Quadrajet thing is looking better all the time. "jimmy six" - your point is well taken, but I love to fool around with things and I'm cheap; probably a bad combination. I haven't seen a reasonably priced usable WCFB or 4GC core for literally years. Now Q-jets on the other hand, are a completely different story.
I wonder if you might want to consider a Rochester DualJet, which is essentially just the front half of a Qjet. Maybe from a '79 or '80 Caprice, Malibu or Monte Carlo with a 267" V8. Pre-'81 federal emissions models will get you away from the electronic fuel mixture controls. I think there was also a DualJet that looked like a full size Qjet but the secondary throttle bores were cast shut.
Some 4.3L Chevs came with small Q-Jets too. Check early Astros for them. Sent from my iPhone using The H.A.M.B. mobile app
I don't think that would be practical, for a number of reasons. That being said, I am a big fan of small base 2G's on flatheads; they bolt right on and everything but the choke linkage falls right in place. I am running a couple of them on bored out late Mercury manifolds. It's one of the sparks behind this Quadrajet idea.
Just run the right air cleaner that can look proper while hiding the carb from prying eyes. This reminded me I need to get the carb number off the on on my boat and see if Carb King has a kit. I have one of his kits in the carb for my 454 and that works great.
Thanks for the help guys. I may be on to something here.Since I've already got all the parts and won't be modifying the actual Sharp manifold, I'm gonna try it!
The Quadrajet gets a lot of bad press because a lot of guys don't understand them. In the later half of the 60's, I bought a new muscle car and promptly set about cleaning up the usual hairballs to uncork some horsepower and shave 2 or 3 seconds off the E/T times. One of the car magazines had a comprehensive how-to on the Quadrajet carb showing many of the tricks. I'd never touched a carb before and was young, dumb and fearless so I figured I'd learn by doing. (I didn't know that you were supposed to blindly buy a Holley and consider no other options. ) The main thing to understand about the Quadrajet is they didn't have the typical jets that you could upsize or downsize like other carbs (Holley). They had these tapered rods linked to a lobe on the air vane, that raise and lower inside of large, fixed "jets" to control the air/fuel mix. Quadrajet carbs come from the factory with different thicknesses for those little "rods" to match the expected demands of the engines as installed. In my case, the answer was to chuck the little guys up in a collet on a lathe and cut some meat off the taper which equates to a bigger jet effect. Now-a-days, with lots of internet info and sellers that specialize in Qjet parts, it's simple enough to "tune" these carbs. Back when I dove in, I had one magazine write-up in my sweaty fist and the seat of my pants. "Jetting" the carb is just one of the steps. There's a half dozen or more other tweaks and details, too. But once you get your mind around it, it's a pretty flexible carb that performs over the full load range of most any healthy engine.
Back in the 80`s and early 90`s I used to build a lot of them. Pull the well plugs, drill out the ports, drill holes in the pick up tubes etc. Worked really well, ran dual quadrajets on a tunnel ram. Built a lot of hot Pontiac street motors and all were quadrajet carbs.
Up to and including 1974, Q-Jets came in 3 sizes: 750, 800, and 850. The Q-Jet has a "demand" secondary; thus the 3 sizes may be looked at as: 750 = 150/600 800 = 200/600 850 = 250/600 The first number is the CFM of the primary side. The 600 is an "up to" figure, depending on the needs of the engine. Unless building one for a numbers-matching show car, the 1965, 1966, and 1967 versions are best left to those that ARE building number-matching show cars. The need for primary throttle shaft bushing is well-known, as is the stripped fuel inlet threads (especially on the earlier models with the 3-thread inlet fitting). The very best money to be spent on a Q-Jet (if you have not done so already) is to purchase Cliff Ruggles' book "How to Rebuild and Modify Rochester Quadrajet Carburetors". Best aftermarket carburetor book I have read since I started doing carbs in 1959. The problem I see (and it may not be a problem, I don't know) is the lack of availability of an intake manifold for the flathead with a Q-Jet footprint. AIR/FUEL MIXTURES DO NOT LIKE ADAPTERS!!!!!!! - Think NASCAR restrictor plates, which are more effective than a spread-bore to square-bore adapter! Often, placing a larger carburetor on the wrong manifold through an adapter LOWERS the effective air/fuel flow. The adapters are basically good for the used car salesmen, and the posers that have cars that are "parking lot fast". And to "parrot" what others have said in this thread, the Q-Jet is an excellent carburetor; but some are better than others. Jon.
Thanks for responding Jon, I was hoping you would chime in. I really don't know what to expect from this, but I already have all the parts and I was at my shop today, and it sure looks like it will work. Since the dual quad deal, I don't have a carb for the Sharp manifold, and since my luck at finding old square bore carbs has gone in the toilet, I'm going to try this. With the airflow capability of the Quadrajet, I'm not worried too much about airflow (not now at least). I'll let everyone know how this turns out.
From all the reading I have done on the Hamb, plus else where, my understanding is that they run better than multiple carbs, and are easier to set up. Those who run multiple carbs do so, often for looks and traditional reasons.
I, for one am a Qjet fan. Having been a GM dealership mechanic and having worked on hundred's of GM police cars and can not tell you how many I have rebuilt over the years. They can be made to run on anything.
http://www.florida4x4.com/tech/quadrajet/Q-Jet Tuning Paper.doc This is a good article about how a qjet works and lots of factory info. Most magazine articles only talk about wide open throttle. This article takes a more practical approach to a street driven vehicle. Hope someone finds it helpful.
I just ran the number on the carb I have. It is a 7040200 and looks like its from 1970 Chevrolet with a 396, 400, or 454 engine. It's probably not the best place to start for me on this project. Or is it? Does anybody know what I need to do to tone down the secondaries or where to find that information? Maybe it's a rare carb off an LS6 Corvette and I can sell it on eBay for lots of money to one of those "gold-chainers". Seeing it's a big block carb, it's probably one of the 850's. I think I'll start haunting eBay looking for something a little more suitable. (I also have the original carb from my '68 Corvette 300 HP 327 that I could rebuild and use temporarily.)
Buick used quadrants on the 4.1 (252ci) Buick v6's in the early 80's Rivieras and Electra's. I'm running one on a 3.8 v6. The biggest issue is all of the flathead 4barrel manifolds are square bore. Sent from my iPhone using The H.A.M.B. mobile app
"Pontiac used a Qjet on the straight six OHC engine in the 60s...." I believe that was a 230, not the 250 even! Shows the versatility right there. Some of the big block apps...don't know which...actually had a sort of air bypass on the primaries so the air volume at idle did not force a huge throttle opening that might upset the idle/off-idle openings. On a small engine I would think you would want to avoid that. I think it is really impressive that these things were used successfully on engines from 230 to 500 cubes...flathead application has interested me for a long time, so I have my experiment penciled in for about 3 lifetimes down the road. So much neat junk, so little time!
the big block Chevy qjets were nothing special or big...because they used Holleys on the high perf engines. Give it a try, after you make sure it's all in good condition. the secondary air valve should work reasonably well with the stock settings. big engines need a lot of air, so the spring can be kind of tight and it will still open up. Sound like this stuff about air flow vs engine size might be kind of counter intuitive to some folks?
Tubman - as Jim suggested, the Qjets for the larger Chevrolet engines were nothing special; yours is a 750, and relatively common. One from a 327 or 350 Chevy would also be common (cheap) and a better starting point. Bruce - Pontiac used the Qjet on the Sprint Option from 1966 (230 CID) through 1969 (250 CID). However, these bring collector prices so not a great starting point for a non-collector. Still the very best expenditure on any Qjet is Cliff's book! Jon.
Thanks for encouragement guys. I'm proceeding full steam ahead with this project. The hole in the bottom of the adapter is wider than the carb pad on the manifold, so I'm going to make an intermediate plate from 1/4" aluminum to take care of that; other than that, it should be pretty easy. The airflow won't be the best in this initial "quick 'n dirty" version, but it should give me some idea of how well it will work. I can see that spending some money with a local outfit that I have worked with before would result in a better adapter, but I'm not at that point now. When I have this ready to test, I'll probably have about $2 in it. Oh yeah. I ran the numbers on that carburetor and it is off a '70 Corvette with a 454. I owned a 1970 LS5 convertible back in the mid 90's, so that's probably where it came from. The carb looks in real good shape, so I think a better ultimate use of it is to grace somebody's NCRS award winning restoration. Anybody got a generic Quadrajet in decent shape they want to sell cheap? Jon - I have Cliff's book coming and I got a nice comprehensive write-up by a guy named Lars Grimsrud off the internet that looks to be real good as well. It is over 40 pages, and is where I got the application for the 7040200 carburetor I found.
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