Inliners International Forums General Discussion Engines Jet Sizing Quadrajet

I have a stock Chevy 250 with an offenhauser 4 barrel strapped to it. The jets that came with the carb were too big. .076 I changed to a .068 primary jetting-too small any ideas? or recomendations? Apart from just guess cause thats as good as I am gunna do.

Size carb?
From the jet size,sounds like a 700-750 Holley Carb?

What size metering rod?

Its a quadrajet. Or am I not understanding your Question Hank?


Actually I don't know about the metering rods. How Would I know? There was now build tag on the carb when I got it. The Jets say the numbers-I really dont know about the rods though.

Sorry, missed the topic title.
You should find out what the carb came from.
P.N. ?
Then find out what size metering rods & jets originally came with.

I never had too much luck when using a spreadbore quadrajet for performance,but then again I did not send it or take it to someone who could set it up properly for my L6.

Is it a squarebore or spreadbore carb?

Are you absolutley sure you want to use that carb?
Just asking.

MBHD

Yeah I am not looking for so much performance,Trying to keep the Fuel ecnonomy while Sticking Genuine GM.

If you can look for the specs on the carb used for the Sprint Pontiac 250 engines & 230's w/the 4 bbl Quadrajet. 1967-1969 IIRC?

That should give you an approx baseline of the size metering rods & jet sizing needed.
Pretty sure you can find the info on the net.

Is it a spreadbore carb,or a squarebore carb?
Or are the squarebore carbs not called Quadrajets,been a long time.

Or just find an original Pontic 4 bbl carb & use that.

Someone posted it here on a previous carb topic.

MBHD

I agree with Hank, as they did use Q-jets on the 230 Pontiacs with no smog prior to 1968, also 250 motors into the smog era. I would find a pontiac ohc lovers site and look at the different jets/rod combo the factory used. The rod are also stamped near the top (bent end) like 41 for .041" dia. for the econo (cruise) position, the standard power position is like .026 dia and never stamped- the stamped number is always talking about the cruise position. An example- a'67 275hp/327 would use a .071 jet with a .044 rod. The pontiac 6 would be some different, either or both the rod or jet. The Doug Roe Rochester Carb is excellent for Q-jets- Read that and you will see there is no need for holleys etc. Folks malign Q-Jets 'cause they dont know 'em!

All the above information is good. Here is some additional, Carbking recommended this book sometime ago. I have it and it is excellent.

"how to build and modify Rochester quadrajet carburetors" by Cliff Ruggles

Here is his web site;

http://www.cliffshighperformance.com/

I called him a few months ago to rebuild a Qjet. His turn around
for a rebuild was 6 months. I couldn't wait.

Here is another article on rebuilding Qjets. It will also tell you how to identify the carb by the numbers.

http://www.lbfun.com/warehouse/tech_info/fuel & intake/Q-Jet Tuning Paper.pdf

Hope that helps.

Larry

.068" sounds big for an L6, but it may have a lean metering rod (like .048") in it.
Unlike Holley, the cruising range is determined by both the primary main jet size and the thick (upper) metering rod step, changing either one changes mixture.
Changing only the jet changes both WOT and cruise. Changing only the rod doesn't change WOT because (as edski said) all WOT (thin, lower) steps are .026".
Which is faster at WOT: the .068 or .076"?

Doug Roe was a Rochester products engineer, his book is published by HP and was first published in the early 70's and probably still somewhere on Amazon. His book has had several printings and covers Rochester models B,2&4 GC,HV, Monojets, Dual Jets, Q-Jets,both std and electronic,even has the F-85 old turbo carb. He has another book on economy minded carb building, with tips, even tactics and tricks used during the mobilegas economy runs. In various hot rod mags over the years i have seen Doug's efforts reflected in two corvair turbocharged roadracers(one fiberglas bodied),a turbocharged & Q-Jet carbed Vega and Chevette road racers. At one time I think he sold Rochester parts too. There aint too much out there on Rochester stuff.

The 76's were faster.

I don't remember the rod or jet sizes but I have been running a Qjet on my 250 for the last four years and it is set up to use on the 4.3 V-6 like the factory used on a 1985 Astro van and a few caddys. That was the last year before they went FI. It is in a 41 Plymouth 2 dr. that I drive all over the country and use it to tow my T roadster. Driven sensibly it gets 19/20 mpg, but if to open it up that falls off quickly. Prior to that I had a Edelbrock that peaked at 14mpg or so.

That means that both your WOT and cruise were too lean with the .068, probably your metering rod is too thin. If you have them, I'd try a 70 or 72 with the same rod, and get the rod numbers.

I did not get that good mileage either until I messed around with the springs for metering rod pistons.
Meaning use soft springs,cut springs so the metering rod would not get pull out of the main jet so soon.

Small CI 6's ,create less vacuum when accerating as compared to V-8's.

Those Edelbrock kits are made for V-8's in mind not the smaller 6's.
Wonder if you messed w/the springs & what size Edelbrock carb did you use?

I could never get great mileage when using any 625 AFB or AVS.
But got really good w/a 400 CFM Carter 4 BBL.

Quadrajet ,when properly set-up can get really good mileage.

MBHD

Larry Griffin that is possibly the best article I have come across about about q-jets very helpful thank you so much.

Hank we tried about every rod & jet comb on the chart and some made more power but about 14 was top mileage.

Does anybody know of anybody who has a quadrajet on their 6 that would know what rods and jets to use?

Someone with your exact engine, manifold, cam, exhaust system, air cleaner, spark advance setting, gearing and chassis weight?
Who would that be?

Okay I get it I am just looking for a rough idea as I was getting 13mpg with the carb in its stock setting and knew I should be getting higher. I have apart from an offenhauser 4 barrel intake a stock engine In my 65 Chevelle which is essentially hte same as the Pontiac Lemans' which came with the OHC engines.

On a Different note. To achieve better fuel economy do I generally want a larger or smaller total fuel area in the primaries?

But you have a rough idea: a smaller main jet than 76, and see what happens, and tell us what metering rods are in there now... as I said.
Your 250 is very different from a Sprint engine, only the size matches.

OK,
I get that,those carbs are not that great for mileage.

Quadrajets are good when properly set-up for mileage & power.

It's just most people think they are not a good carb & just get rid of them.

MBHD

You need to find the number on your Q-jet. It will be on the drivers side toward the rear where the long screws go through holding the top on. its a 7 or 8 didget number.

If you go back to the factory setting, you should be pretty close. Try a softer spring under the primary metering rod holder. Vacuum is what pulls them out of the jet and a stronger spring will get them out quicker. A softer spring will keep them in longer and improve your fuel mileage.

I have a ton of paper work on Q-jets, a couple of books, and years of working on them. Cliff is a good friend of mine and my dad and has been over to the house a few times. He is the best there is for performace tuning a Q-jet.

Joe

All engines are exactly the same, which is why there are several hundred different QuadraJets.
Now, that's settled.

I went through a bunch of Rochester papers a minute ago, the '66 230 Pontiac Q-jet had 71 jets and 46 or 47 rod depending on the transmission. A'67 230 had 71 jet and 41,42,43,or 44 rod depending on the carb number.

A 1967 396 Chevrolet, had 71 jets and 45 rods. The same carb used on the 350,400,428 Pontiac's used 71 jets and 44 rods.

Engine size doesn't matter! The carb is designed to mix "X" amount of fuel with air. Once they got the desired air/fuel ratio, any engine design to run with in that ratio could then use the same carb with minor changes to linkage and secondaries.

A 396 will pull more air down the venturi then a 230 will, but each will still get the same air to fuel ratio.

Thats why I said to put it back to stock, what ever that is for your carb number, not engine size. You will find it to be really close to what you want. The tuning of the metering rod spring is where you will find the most changes is driveability. With out a lot of springs on hand, you are kinda of at a lose so you will have to make do with what you have. A step down in jets or up in rods will help, but don't go more then number or two.

http://www.cliffshighperformance.com/tech_carb_ID_2.html

Get that number and I will help identify the carb for you.

Joe

17080204 is the number stamped on my carb.

I believe this means it is from a Chevrolet build in 1980 for an automatic transmission. This would also be the second generation of these Q-jets and build to be 49 state legal.Meaning I would need the post 1976' metering rods. I do not know the current size of the metering rods I am using-but the Jets had the size engraved on them when I took them out that is how I know that what the were originally. Thank you for all of your help so far.

My Plans of action at this point are
-to move to a set of 75's or 74's jets
-install a softer spring in the power piston.

This a 1980 GM carb, my papers don't tell what make it was on.

Factory primary jet, GM #7031971 (.071") marked 71
Factory primary rod, GM #17056948 (.048") marked 48M
Factory secondary rods, GM #17053659 marked DR

A 76 jet would be really rich, look for a 71 or 72 jet. You can ream out the 68 with a good reamer, its not 100% accurate, but it will get you really close. Just a size or two will make a big difference, so go a little at a time.

A 48 rod is on the lean side of the middle. Keep the rod and change out the spring to a softer one if you have it. You will notice a hesitation when you go from cruise to power circuit if the spring is to soft. When you get the right one you will not notice the change. To stiff will cause a rich condition all the time and your fuel mileage will suffer. This is a tough one to iron out, it takes a lot of drive time to get a feel for how its operating. If your engine is stock like mine, it should make a lot a vacuum, mine has 20" at idle, but since it has little horsepower, I am always in the throttle some for hills or passing. This is where your fuel mileage goes, its like going from a 71 jet to 93 each time the vacuum drops below the spring pressure on the metering rod holder.

Any rod will work as long as its what you want. The later three digit rods like this one (48M) are the same as a older two digit 48 rod. The difference is in the taper from the big end down to the small .026" tip. The older ones use a single taper, the later ones are dual taper so they stay leaner longer.

The secondary circuit probably won't effect much. Just play with the air valve spring to keep from bogging when you hammer down on the throttle. Keep it on the tight side, you should hear it open and feel the engine start to pull, if you feel a quick snap or bog, its to loose.

Just for fun, I figured the square inch of each rod and jet,
76 jet & 48 rod = .00272 sq in opening
71 jet & 48 rod = .00215 sq in
68 jet & 48 rod = .00182 sq in

76 jet & 44 rod = .00303 sq in
71 jet & 44 rod = .00243 sq in
68 jet & 44 rod = .00211 sq in

At full open,
76 jet with .026" rod tip, .00400 sq inches opening
71 jet with .026" rod tip, .00342 sq in
68 jet with .026" rod tip, .00310 sq in

So you see, you can get the same effect with a rod change or jet change, where you get in trouble is the amount of fuel you get through the smaller jets at wide open throttle, remember, the tip of each rod is still .026".
So to stay in the factory air/fuel ratio at full throttle, you need to stay close to the .00342 sq in opening (71 jet). By using a fatter rod, you can lean out the cruise circuit with out hurting performance.

Joe