OK. I almost have the manifold ready to go. It turns out that the hole in the adapter is just about as wide as the pad on the manifold, so I am going to have to make a bottom plate for the adapter out of 1/4" aluminum plate that overlaps the pad so I can be sure there are no vacuum leaks. I did mount the adapter and while it was tight, I think it fits OK. I used flathead socket head cap screws to mount the adapter to the manifold, and if I line up the primaries, the ones on the secondary side protrude a little. When I get the 1/4" adapter made, I'll trim the screws so the secondary butterflies clear. When I do this again, I will use regular socket head cap screws and counter sink them deeply into the adapter so they're out of the way. Here are some pictures of the manifold and adapter and with the carb mounted. Because "Squirrel" was concerned that the carb would look funny on the manifold, I took the air cleaner off my '68 Corvette and put it on the carburetor. I think it looks pretty good.
If it's the Lars I'm thinking of, he use to have a huge following from the Corvette guys. Any chance you could PM me the link you found his write up at?
Because the throttle lever and fuel inlet line up better that way. I found this out when running 2G's on bored out Merc manifolds. They seem to run fine that way. I don't see why this would be any different.
The carb has no idea which direction it's going. The three Holleys on my car face the wrong way and have done so for more than 50,000 miles. Sometimes ya just have to do it wrong to make it right.
In the mid 70's I was running a dirt track modified that was leaning out in the middle of the turns even with a center pivot Holley. I solved the problem by building a twisted adapter from a 2" block of aluminum. Turned the carb 45 degrees so it thought it was going straight ahead when I had the car all crossed up in the corners.
The Rochester 2G series have the float suspended from the airhorn, whereas the Q-Jet float is in the bowl. This changes the geometry of buoyancy that may be effected by rapid acceleration (if this is desired) with the carburetor reversed. The 7040200 does have the revised (better) float placement. Suggestions: (A) Use the 0.125 fuel inlet valve (don't use the 0.135, and don't even think about the 0.149) (B) Make sure it is the non-windowed design If you run rich under acceleration, this might be a good place to look for the cause. Jon.
This is the best way to make a Ford faster, use Chevy parts! Quadjets are the most misunderstood, under rated carb in our world! My first drag car had a junkyard dog Qjet on it that I built from a box of carbs and it ran great! Being young and dumb, I swapped it for a 3310 Holley because that’s what my buddies said to do. The Holley was no quicker and the Qjet ran much nicer on the street. My O/T Suburban has one that has never given trouble. Sent from my iPhone using The H.A.M.B. mobile app
Thanks Jon, if I get to the point where this is a problem, I will consider this endeavor a smashing success.
FWIW, from my shaky memory...I have a QJet from a more recent small displacement application. It's from a fairly later Chevy truck with a V6, one from the last days of non electronic carbs. If I can find it, I noted source on it... it was something uninteresting but in the general flathead displacement range. As noted above, GM did not need to shrink casings on small applications but did so by working on opening springs on the secondary flap. I suspect the small engine ones probably have a spring with more range at the low end and so might be easier to adapt down around 260. I think all of this is a very promising area of experimentation, a modern (by Hamb standards!) carb with a much better range of controllable/progressive air flow than the Holleys.
Thanks for the comments, Bruce. What you say is all true, but I am mainly motivated by the fact that I refuse to pay $175 for a grungy WCFB core with missing parts (or $500 for a new Holley 390).
A bit more information: One of the minor street, but major performance issues with the early Q-Jets was the pin placement for the float. This was changed over time depending on the application from 1969~1971 (although it is possible some later Olds Q-Jets had the earlier placement). This placement, ESPECIALLY if the enthusiast tried to use the windowed fuel valve seat could cause unpredictable results with float opening/closing. As stated above, the carb you are using has the revised placement, but I have never run one backwards, so just suggesting that if an issue arises, this would be a good place to look. WAY TOO MANY enthusiasts try to use the 0.149 fuel valve (or even the 0.135) on the street. The 0.149 should be considered for trailered race cars only. I know Rochester DID use the 0.135 on many enthusiast carburetors, but dyno testing on a RA II Pontiac showed NO DIFFERENCE in WOT power from the 0.125 to the 0.135; and we have seen a number of street flooding issues with the 0.135 at idle or city (low speed) driving. Racing parts for racing cars! Jon.
Coming from someone who destroyed a Pontiac engine running a Q-jet, I strongly suggest pulling the screws out of the bottom that hold the sections together and adding loctite. One of mine fell out when running a modified manifold at 5600 rpm ( crossing the 1/4 mile finish line). The screw went through 7 out 8 cylinders, split 2 cylinder walls, and mangled a dozen valves. Stock manifolds won't let the screws into the engine, I opened mine up. It would be a good idea to secure the adaptor mounting screws at the same time. Dad modified his with a small roll pin that was pressed in a hole he drilled. The pins were just long enough to hold the screws from falling out even if they got loose.
Let me say up front, that this is a fascinating thread. I love to see people tinker with stuff and try different things. Also , I hate to see people run for the Summit or Jegs catalog whenever they have a problem. That being said, and as much as I like Q-jets, I think this is a very bad idea. The backwards carb, the bad, bad potential flow pattern, all add up to this conclusion. As an alternate take, I'd consider using a 69-70 2GC with the 1.375 venturi with a 2bbl adapter. (see Jet carburetion for circle burner racers) It would be plenty big enough for that flatty. Also, you can bore out a 1.250 carb to 1.375 without hurting it. Cut down the boosters, and you'll have over 400 cfm. I guess , if your deal starts, idles, and goes down the road, then you won. Performance might become a totally different issue. Good luck.
I am probably one of the foremost promoters of 2G's on flatheads you can find (I have two running right now). Also, I bore the manifolds all the way down and don't use an adapter. I appreciate your comments, but neither of us know for sure, do we. I have $2.67 (4 short and 4 long socket head flathead cap screws) and about 3 hours in this whole deal, and am this close to getting this running. I am a bit more sanguine about the potential of this experiment than you are, given the comments of the others on this thread. It seems like Quadrajets are custom made for something like this. Several folks have told me that the 2G's won't run right when mounted backwards. They are wrong.
I'm patiently waiting to hear how this works out. It sounds like something I would do myself given the right set of circumstances.
Yeah, me too. I ran a small Q-jet [IIRC it was spec'd for a 262 Monza] on my 68 chevy pickup with a 292 six banger and it ran great! Loved it.
Rocky, the 75 Monza had a 1.09 2GC, and the 76 was 1.18..No four barrels . The 86 C10 and Astro Van 262 V6 had a Qjet .I doubt it was that one..It's a 1.21 venturi (800)
Yes, the earliest dualjet looks like the full size carb and even was bolted on a 4bbl intake; I had a '77 LeSabre with a 301 Pontiac for a few months 20+ years ago that had one. I found out tearing it down after it threw a rod. It may still be in a milk crate in an old storage van. Secondaries are blanked in the casting.
I'd just turned 16 and my buddy had stepped up from a Merc Flathead to a late model [1956] Pontiac...sold me his flathead intake/carb for $15. It' was just an old 55 Buick carb on an Offy intake but these Rochesters are still around. Worked like a charm on my 255 merc
The Dualjet to fit a 4 barrel intake seems to be the answer to a question nobody asked. The straight fuel inlet probably limits its uses, especially backwards. It will work with low displacement engines though. I bought a '74 Cutlass that had an adapter plate to host a Rochester 2 barrel on the Olds 4 barrel intake, thus proving that a car will run with just about any setup...
The "$" is what I am trying to avoid here. Plus the Quadrajet is a more advanced design than what the Speedway manifold is designed for. If this works, it will provide another alternative induction system for these old engines.
So, if anyone is still interested in this, I have abandoned this project. I didn't have much success, and I need the test stand for my Olds Rocket. I don't know if this was because the carburetor or the manifold was the problem, but it got to be too much. I may revisit this if I can find a known good carb for a 3.8 Chevrolet or Buick..
I seem to be a dollar short and a day late but any way. Yes it's supercharged, but the Q jet is mounted backwards. It came from a supercharged Buick V6 (231ci)(this motor 265ci). This unit only boosts when at full throttle. The carb is mounted to a square hole via an inexpensive spread bore to square bore adaptor. This thing performs great driving around town. 14-18 in of vacuum and 16-18 mpg. I've also run with a 2GC on a Merc manifold. The carb I had flowed around 380 cfm which is plenty for a mild+ flatty. It performed great. Neither required any jetting. The only mod to the Q jet was to drill a small hole in each throttle plate to allow more air for idle. The motor is highly modified.
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