Hello, on my newly acquired 1949 Studebaker 2R5 pickup with a Chevy 307 engine and quadrajet carb, I think the carb may be too big for this engine. I am getting less than 10 mpg. I did rebuild the carb. I believe the engine is stock. Can someone tell me what the CFM is for a quadrajet? I heard there is a site you can go to that will help you determine what carb and CFM is best for your engine, does anyone know of this site?
Nothing wrong with the Q-Jet. They are great carbs. GM spent a bazillion dollars to design them and used them for many years. Yours might need some work, (get a book) or the engine is out of tune. I would expect 15-18 mpg with that combo, depending on gears, tune and driving habits.
You have other issues than carb size with 10MPG. What final drive ratio, OR RPM at 60... Carb could have problems like heavy float, leaking welch plugs, etc...basic tune-up problems, ign or timing problems, whatever
The Qjet will work well on just about any size engine, Pontiac installed them on the 250 ci six, Buick installed them on the 252 ci V6. The primaries are tiny, the secondaries use an air valve so they won't open until there is sufficient airflow, and they only open as far as they need to (assuming it is adjusted properly) If you tell us more about the truck maybe we could help you figure out why the mileage is so lousy. I mean, aside from it being an old truck with a V8, which in general will get lousy mileage. Things like gearing usually have more effect on mileage than the carb type...and of all the carbs you could have, the Qjet is probably the best for mileage. I get 18 on the road in my 57 Suburban with a 350 and Qjet, but it has the right gearing.
Glad to know that the quadrajet came on similar and smaller engines. The rearend is from a 1970 Camaro, not sure what gear ratio but it is not posi. I really do not drive the truck like a maniac so that is not the cause of the poor MPG. It has a 200R4 trans with a manually operated switch that I do not activate until 48mph and above. I did JB weld the plugs in the carb that are known to leak. The truck does run well once warmed up, which takes longer than I think it should. I did install a manual choke.
If you only drive in city, stop and go traffic, then 10 mpg is not really unreasonable. If you get out on the highway, it should be getting at least 15, if it's all set up right. If (for example) the cam is way off for the rearend gearing, or the timing is retarded, then mileage won't be very good.
I live in a rural area where most of the immediate roads are 55MPH. I would say 50/50 split city /highway. The guy I bought it from is an older meticulous gearhead so I guessed the engine is within tuneup specs but I will double check those myself.
If you don't know the gear ratio in the diff, check it now. A 70's Camaro (maybe 2.56 gears) with a 200R4 will "lug" the engine and mileage will suffer. Check advance and total timing and make sure the TV cable is properly adjusted.
Otto this is a general statement and not aimed at you personally but if you have a car that is not running right or you don't think is running right wouldn't a tune up be the first thing that you would do? As for mileage your mileage could be caused as much by gear ratio as tune up. If you are running a relatively short tire like in less than 28" in diameter you still want a 3.73:1 gear or deeper to keep your engine in a usable operating range. Even a 307 wants to turn more revs that 1800 if you catch my drift. Another thing to consider is that you are tripping your lockup manually, that is not optimal and more often than not causes mileage problems, unless you have been doing it long enough for it to be a habit you are probably not being consistent with it. The proper wiring and setup for it to be automatic is the way to go. Anyway things to think about and not just you any0ne else who happens onto the thread.
You mentioned that the previous owner was a gearhead, maybe he changed the needles & jets for "just a little more go". Suggest checking those items, as the stock parts on a 307 Q-jet should give ample fuel flow.. Through the years I've been told repletely by the Holley Boys that Q-jets are junk and don't perform, as they replace another power valve or change out " Too Big/Too Small" Holleys. Ask the Super Stock racers in the 9's if Q-jets work ? Hang in there it'll work fine... Dyno Dave
I personally don't like Q jets because I don't like working on them. Guess I am just lazy. But I also quite changing power valves nearly 20 years ago. The guys you are talking to couldn't tune a banjo. The fella isn't complaining about his car not going fast enough he is complaining about fuel mileage, something that most Super Stock racers are not too concerned with I'll bet. you Q jet guys are about as smart as the No SBC guys. The only Holley mentioned was by a guy who has both a holley and a Q jet and he said he prefers the Q jet. While I prefer Holley carbs I never bothered to suggest one at all. Maybe throw something of value out instead of brand consciousness. That has little to no value at all.
From I remember, most of the puke 80s quadraslobs ran around 650 cfm; but they did open them up to the mid 700s for the larger motors (big blocks and higher HP mouses). Why not lock out the 4 barrels and just run the primaries? Should save you something?
I think that some of the medium horse motors in the '60s had bigger Q jets. Less HP than the L 79 but more than 300 horse motors. I'll bet @squirrel knows more about that as to truth or fiction. he keeps track of that stuff. It is not likely that the 307 is even pulling the secondaries open very far, they don't take a very deep breath. there are a lot of variables there obviously.
750ish cfm on the Qjet. Some were supposed to be about 800, on some of the big engines in the early 70s. Like I said before the secondary air valve is pretty good at opening to the extent that it is needed. there's a spring you can adjust to tweak it, and the vacuum pulloffs are usually connected to it also to make sure it doesn't open too fast. I think most folks never bothered to figure out how they work....which is typical...
I'll bet you are absolutely correct in your assessment. There is really nothing wrong with one and they are a lot like an AFB at least more like an AFB than other carbs. makes them very tunable if you want to take the time.
Are you 100% sure you got the metering rods back in the jets when you put it back together? They can be a pain. Or maybe the secondary air valve not seated properly. If the back screws (long ones) have ever been over tightened, it can warp the rear of the carb just enough to hold the top flaps open. Secondaries cracked open (worn base plate) will cause it to flood and run rich too.
I really don't know how to determine the gear ratio or the correct way to adjust the TV cable. The secondaries hardly open at all with the throttle fully depressed. Of course it is possible I may have not put something back correctly when rebuilding the carb, but I did not rush the job and did my best. Would it make any difference that I used the original (thin) base gasket over the thicker one supplied with the kit? The bolts weren't long enough to use the thicker gasket. As stated earlier, the truck does not run bad once warmed up. I, nor the previous owner, knows what this carb came from, he had it laying around so he thought he would try it. I do believe the original set up was a dual jet carb. Porknbeaner, you are correct, a tune up should only help the situation. I concentrated on the carb, as the previous owner stated that , that is what he felt needed attention. I also agree with you that the lock up, automatically locking up, would be nice, but this is the way I bought it and have only owned it for a short while.
THE BEST at building a QJet Sean Murphy https://www.smicarburetor.com/products/sfID1/85/sfID2/86/sfID3/9
Put the rear on jack stands and mark the wheel and pinion with chalk or something. Count the wheel rotations for 1 pinion rotation, divide. That will give rear gear ratio. Here's a link on TV cable adjustment, google it, there's a lot of instructions, pick 1 you like. http://www.cpttransmission.com/techinfo.htm 200r4 lock up kits so it will work as intended. http://m.summitracing.com/search/part-type/torque-converter-lock-up-kits/transmission-type/200-4r
all a dual jet is , is the same casting of a Q jet body minus the back 2 barrels ( some they removed this part of the casting in the later years ) so it's the same CFM as the q jet primarys, the carb number is on the linkage side on the back verticle bolt stand it starts with a 70 or 17o5 or 1708 number sequence . as for bad milage ,q jets most of the ones I have fixed were primary metering sticking , someone changed out the primary metering rods from a late unit to a early unit ( lates are shorter ) , float problems ( misadjusted , sunk , stuck ) , or using one from a large Cid/ Hd truck and jetted rich . and bad bushings on the primary causing a vacuum leak ( the q jet gurgle ) .
Chevy 307s all came with 2GV carbs, didn't they? the good old Rochester 2bbl. But that is kind of irrelevant. The base gasket you need, depends on which intake manifold you have. So I have no idea which one you need. One of them, the early one, used a composition gasket about 1/16" thick, covered by a thin stainless steel shim. The shim was reusable. When you check the rear gear ratio, you need each wheel to turn one revolution (or one wheel to turn two revolutions), and count the pinion revolutions, that's the gear ratio. For example...Say it takes a tad over 3 pinion turns to get one wheel to turn twice, while the other wheel stays still. You have a 3.08 gear ratio. pictures always help us, you know
The Q-jet is an excellent carburetor. It, like all carburetors are calibrated for specific engine and vehicle combinations. When swapped, you have tossed-the-dice on whether or not the calibration is even close to what your demands and needs are on another engine. This is why when people run to universal carburetors from Holley, Edelbrock, Quick Fuel, Demon and others, they just end up having to accept the end result, be it good or bad. Now, the majority of carbureted Rods running the roads in our area are getting 8 - 12 MPG in normal driving. Allot of this is due to the extremely poor fuel we have today, which by the way must be compensated for by richening the carb 10% or more just to get close to what it was back in the 60's & 70's. A close friend of mine that works at the same Hot Rod Shoppe as I drives a 46 Ford pickup everyday. For a few years it was powered by a good 283 Chevy with tri-power, only the middle 2 bbl was used. It averaged 10-11 MPG every week, and he drives real easy. He finally gave up and we swapped in a 5.3 LS engine and 4L60E transmission, the truck now averages 20 MPG everyday. Can you get your trucks gas mileage to improve substantially? Yes, start by finding the correct Q-jet for your engine, verify the components have not been changed, and begin the long process of tuning the carb and ignition until the results have been achieved you desire. OR Just fill the tank and enjoy your truck !
If you are pulling to stiff a rear gear you don;t have enough vacuum to make the vacuum advance work. The vac. advance is a gas milage idem. Griffin Auto Electric
thing is most people do not know you can tune a q jet as easy as a holley , but finding the parts ( jets and primary rods ) was a pain for a long time till lately since people are using them again , and also the pain of taking off the cover to get to them .. the only problems with a 2 GC/V carbs was you had to run a good filter as the power valve was in the bottom of the bowl and trash would stick them open ,
Check for the proper thermostat. The engine runs most efficiently at the normal operating temperature. Cooler operating temps will require a richer fuel mixture. Manual chokes usually seem to be kind of a Band-aid fix to me. A working, properly adjusted automatic choke is hard to beat for easy starting and better cold driveability. A manual choke is better than nothing. Does the mechanical advance in the distributor work? How about the vacuum advance? Is it connected to ported or full manifold vacuum? How much manifold vacuum does it have at idle in neutral and while cruising at a steady 40 to 50 mph?
If you intend on keeping the Q-jet, get the book "Rochester Carburetors" by Doug Roe. There are a whole variety of secondary metering rods and hangers to choose from. Secondary air valve adjustment is crucial. Also there were two versions a 750 and 800 cfm if memory serves. If you look down the primaries and see a fairly substantial hump in the venturi area its the smaller, the larger one the venturi hump is fairly small. Q-jets are really great carbs when you get to know them.
the easy way to do the jetting, is to get a carb that was made for an engine similar to what you have. Works for me....
The previous owner just told me, he was told that the rear end gear ratio is 3:42. I need to double check that.
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