Primary jet sizes

Discussion in 'The Venerable Q-Jet' started by Gary Bohannon, Apr 28, 2019.

  1. Gary Bohannon

    Gary Bohannon Well-Known Member

    Why are some Buick racers using larger .077 to .80 primary jets?
  2. johnriv67

    johnriv67 Well-Known Member

    That is a good question. I might not be of much help, but here is what I run in a cammed 430
    74 jets 41 primary rods K hanger AX secondary rods
    A long time ago I used 76 jets with a weak weak Corvette 350 engine, thing was so rich.
    Gary if you’re alluding to secondary jetting being the part of the carb you should change for more fuel at WOT/racetrack, you’re right
    I don’t know why such large primary jets would be useful
  3. Cliff R

    Cliff R Well-Known Member

    The size of the jets are NOT the only determining factor in fuel delivery to the engine across the load/speed range.

    Every Q-jet has a specific part number on it and calibrated for a specific application. This means that it will have different air bleeds, idle system calibration, etc.

    Just a "rough" example but I can delivery as much fuel to the engine using a 1969 Chevy carb equipped with small main air bleeds with .067 main jets as you can with a very similar 1972 model that uses the large MAB's using .78" main jets.

    Specifically for the Buick units, the pre-1971 models used smaller MAB's, and smaller main jets unless it was a 244 carb for a 350 engine. A 1970 455 carb will be as happy with .068"-70 main jets as a 1971 455 model would be with .073's-75's......Cliff
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  4. Gary Bohannon

    Gary Bohannon Well-Known Member

    I used a complete Cliff 2nd recipe on mine, and currently using a .074 right, and .075 left with Cliffs tapered metering rods on a B4B intake. Stagger jetting has been recomended for the edelbrock intakes since the late 1970's.
    But some Buick guys have been forever using really big jetting in the 1971 and newer quadrajets and most likely have some various modifications to passages and air bleeds. Just don't understand the big .078, .079, and .080 jets in a Buick carb.
    Thanks Cliff.
  5. Gary Bohannon

    Gary Bohannon Well-Known Member

    Just received some GREAT INFO from David Hemker.
    David uses an AFR meter, and Dragstrip testing, to fine tune his quadrajet powered Buick GS to the maximum.
    ..........I simply asked David if he had switched his old 750 Quadrajet, to an 800 Quadrajet........Now read his answer:

    Hello Gary,
    Actually I'm still running a 750.
    I have experimented with the 800, however, and have hit a bit of a wall. The issue I had was getting it rich enough for my combo.
    I use an air fuel meter and found the sweet spot for the combo which is around 12.75 (air fuel ratio). Each engine build combination is different. I watch the meter more closely after I get into 3rd gear.
    However, I can obtain the ratio I prefer with the 750 (cfm) but still have problems obtaining this ratio in the cold air at the October Nationals.
    I have found and been told that the biggest gains come from the primary side of the carb. So, I install the richest SECONDARY rods that I have, and leave them alone.
    For the 800 Q-jet PRIMARIES, I start out with the factory Stage 1 jetting (75 jets & 45 rods) and go from there. As you most likely know, the primary rods affects part throttle metering only, therefore changing the primary rods makes no difference at wide open throttle due to the .026 tip being the same size on all of the rods.
    ( My note: part throttle and cruise fuel/air ratio can be adjusted with the apt screw ,and works especially well with Cliff Ruggles custom tapered primary metering rods with .026 tips.)
    If your dual plane intake is not modified by having the divider cut out, then 2 - 3 jet sizes higher on the drivers side of the carb will correct metering issues and gain both ET and MPH. But, if your dual plane intake (B4B) has the divider cut down or out then it is essentially a single plane, and stagger jetting will have no effect on the metering.

    Thank you David,
    This is great information.
    Last edited: Jun 9, 2019
  6. Cliff R

    Cliff R Well-Known Member

    We seldom get into "staggered" jetting for street applications, but have tuned some race stuff that way.

    The most drastic tuning I can remember was doing two AFB's for a Pure Stock Class Hemi car. Pretty sure I had different jets in all 8 corners of the carbs being used....but it made fantastic power on the dyno and went 10.70's over 130mph on bias ply street tires.

    The "basic" rule for tuning is to for jet size first. This is done by doing some heavy part throttle pulls w/o the secondaries.

    Once you establish adequate jet size, tune the primary metering rod next. Easist to use APT, all of the early Buick carbs have it, and we make an external APT screw for them.

    Last tune for WOT. My preference there is the drag strip, looking for best ET/MPH. I typically er just a tad rich, as a slight sacrifice in ET/MPH is better than a couple of pistons in the oil pan! In many cases your really aren't chasing that much anyhow, as the run is established on how hard the car launches, then the engine just has to pull hard on rest of the run. You could literally yank off a plug wire half track and I'll bet you wouldn't slow much more than a tenth!

    I've tuned my set-up at WOT from "pig" rich to so lean it started to surge a tad and didn't see much more than a tenth and a half and less than 2mph. Most runs were hundreths apart. Not a big deal for a daily driver or street/strip/bracket car, but really important for Stock/SS set-ups.

    For street tuning I'll also add that the idle system plays a big role, and opening up idle system items effect part throttle. The 71-74 455 carbs non-Stage 1 are really lean at idle, so they respond well to larger jets and smaller metering rods, but if you tune the idle system first to get it up to par for your larger cam (common addition to most of these engines) you'll find it will need LESS jet and primary metering rod tuning to make it happy.

    Engine efficiency is also a BIG player with these things. Increased compression, static and dynamic, tighter squish, etc requires LESS timing and fuel. Don't forget the vacuum advance either, it should be used in conjunction with APT tuning, as leading the timing further allows more effective burning of leaner mixtures.

    With that said, for some reason I see a LOT of big Buick's showing up here w/o vacuum advance, and poor attempts with aftermarket weights/springs to get all the mechanical timing in right off idle. Not sure why this happens but folks need to get out fo the 60's and up to modern times and ALWAYS use a vacuum advance for anything street driven. We should be way past all that, but I still hear 3/4 cam, double roller timing chain, high volume oil pump, camel hump heads, and get rid of the "Quadrajunk" carb just about every single day.

    I'll add however that I NEVER hear any negative Quadrajet comments in staging lanes in final rounds when we've whipped up on every car on the property!...........Cliff

    Attached Files:

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  7. Stevem

    Stevem Well-Known Member

    Here's some common match ups of jets and Rods.

    Attached Files:

  8. Cliff R

    Cliff R Well-Known Member

    Not sure where that came from but jet/rod relationships have little to do with idle fuel.

    The only time they would is when the metering area left between the jets and rods was LESS than the orifice diameter of the idle tube, since all the fuel to the idle tubes must pass thru the jets first.

    The biggest jet I've seen to date in a factory Q-jet was .078", used in the 1970-74 big block Chevy carbs with the huge MAB's.

    1970 was also a pretty hard year on these engines for emissions, and as we know it caused the bottom to drop out of performance by 1971 when they lowered the compression ratios in the 7-8 to 1 range.

    I'd also mention here and don't want to overcomplicate the subject, but one of these carbs can be set up to run flawlessly without any metering rods at all in it. We used to set our Holley's up like that all time and the do fine that was as well. By design a carburetor is a load sensing device, and will increase fuel delivery proportionally when it senses pressure differential above and below the throttle plates combined with air flow past the idle holes, transfer slots, air bleeds, venturi and boosters.

    If it didn't work well then the millions and millions of engines out there getting it done quite well for well over 100 years would be in trouble, right down to the 3.5 HP Briggs and Stratton engine sitting on your push mower!.......FWIW......Cliff
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  9. Stevem

    Stevem Well-Known Member

    Yup! Just passing along some general info that's good to have on hand at times!
  10. Gary Bohannon

    Gary Bohannon Well-Known Member

    This has gotten really interesting. Thanks to all of you.

    Sooo, time for me to throw out a questionable theory that's stuck in my head....
    * Buick limited the opening of the secondary air flap to 80 degrees on their biggest and best engines.
    * Pontiac limited their secondary air flap to 76 degrees on their biggest and best engines.
    * A Pontiac guru who experimented with this flap angle said he got "DIMINISHING RESULTS" with anything over 81 degrees on his big block Pontiac engine.
    My take on this is that the air valve at 90 degrees (strait down) is :
    1. Blocking the secondary fuel nozzles.
    2. Leaning the fuel in the secondaries.
    3. Resulting in, perhaps, the need for larger jets in front to compensate.
    4. And causing high powered engines to never get enough fuel on the top end of the track.

    (Brought this subject up before, but got no input about this 90 degree usage.... Could this, perhaps, explain the need for larger than normal front jets.)
    Last edited: May 10, 2019
  11. Cliff R

    Cliff R Well-Known Member

    The lower portion of the flaps are not at the same angle as the upper portion, so you need to be specific about where you are measuring for 90 degrees, upper or lower.

    I spent a LOT of time on the dyno and at the track testing the ideal open point and when you start to get too far. The factory did it pretty well and on their factory "hot-rod" units they limited the opening so that it you extend the lower portion of the flaps down at a straight line they will point very close to the rear of the opening in the baseplate where the huge secondary throttle plates seat.

    What I think happens when you go too far is that it not only starts to block off the nozzles but it also produces less of a low pressure area to help pull fuel from them. Going further also changes the path of the incoming air in addition to potentially increasing CFM.

    On my own carb I have an adjustment screw and ground off enough of the stop I can set them to open pretty far. If you measure with a dial caliper from the upper part of the flap back to the opening in the airhorn as shown in my book I see no improvement going past 1.315". Even so some factory carbs and the "famous" Edelbrock 1910 "850" cfm units have the flap open closer to 1.280".

    I've settled on a little less than that and have set up quite a few of these cars for racing applications including Super Stock that will be running into the 9's.

    Although we don't do a lot of "racing" stuff (no money in it) I still set one up occasionally for a customer. We just did one recently for a customer so he would have a back-up carb for race days, and it went from the shop straight to the customer and right on the dyno. He told me that right out of the box it was within 1-2 HP of the carb he'd been using for decades and tweaked and tuned to the brink of extinction. I set the angle of the secondaries using my 1.315" measurement........FWIW.......Cliff
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  12. Gary Bohannon

    Gary Bohannon Well-Known Member

    Cliff, you said:
    "What I think happens when you go too far is that it not only starts to block off the nozzles but it also produces less of a low pressure area to help pull fuel from them. Going further also changes the path of the incoming air in addition to potentially increasing CFM."

    I just measured an old 1972 Buick carb at 1.300", and the lower flap area is about 82 degrees.
    1. Cliff, you saw no improvement beyond 1.315" which is close to my measurement.
    2. The Guru saw no improvement past 81 degrees.
    3. My measurement could be off a degree however.
    Looks like we may have something here. Putting the lower area at 90 (straight down), or beyond 81 degrees, may likely cause "diminishing returns" .
    If diminishing returns includes leaning out the secondary, larger primaries could come into play to compensate. This points back to the start of this thread.
    Last edited: May 12, 2019
  13. Cliff R

    Cliff R Well-Known Member

    It's just an area that doesn't really need a lot of attention. Folks get WAY to caught up in modifying things like this to improve airflow and engine performance when their combination doesn't need it right to start with.

    Considering I've set up "stock" smaller castings for Stock and Super Stock applications and some of those cars run well into the 9's in the 1/4 mile, why would one want to mess around with too much opening of the secondary flaps on a street or street/strip application. Aside from a few Q-jets originally released on tiny little Pontiac 301's or Chebby 305's with a LONG stop severely limiting the flap opening most don't need any attention in that area.

    With the upper part of the flaps set at 1.315", for example, the lower portion isn't at 90 degrees or really that close to it. I figure if that setting is adequate for my 455 making over 550hp it's not an area I'm going to mess for smaller CID engines making less power......Cliff
  14. HotRodRivi

    HotRodRivi Tomahawks sighted overseas

    If you just gotta have more CFM, the primary side is ware you can get it. And you must stay ABOVE THE VENTURI. Also you must understand what all the finger drill hole sizes do and how they affect each other. Rarely does increasing one hole alone work. There are other holes that become part of the adjustment. There is nothing to gain from making the secondary air flaps open more. Or people would just remove them.
    Stager jetting makes no sense. On any motor. Unless you purposly run 8.5 compresson on one side of your motor and 10.25 on the other side. Has any one ever run throtle body injection with staggered injectors.? If there is someone out there like that, thats fine. Just dont take any advice from them.
    Also I have 4 Rochester books including Cliffs. Cliffs is absolutly the best. The only part you wont get from Cliffs is how to get more cfm from the primary side, without removing the outer booster ring. How to build horsepower vol 2 carburators and intake manifolds by David Vizard will fill in the blank.
  15. Gary Bohannon

    Gary Bohannon Well-Known Member

    The stagger jet idea came from our Buick heroes back in the kenne-Bell days and were used on the B4B in particular. Don't know about the stock iron and surely not on single plane intakes. Gary Paine said "use 3 sizes larger on the primary driver side of a B4B" and pick up a tenth or more due to #1 lean cylinder. Others confirmed the benefit of B4B stagger jetting at the GS Nationals, Bowling Green, back in the 1980's. I use ONE size larger and a LOWERED DIVIDER, hoping to just avoid detonation on 91 octane.
    I brought up the subject of air flap opening because some qjet guys were thinking straight down was better but some research shows otherwise. I suspected some extra large primary jets were used due to setting over 80 degree angles on the bottom area of the flap. I suspect that crowded the secondary nozzles and leaned the rear, so more fuel in front helped. But also, part of that may have been the fact that many qjet guys did not utilize the adjustable rod hanger screw to help off idle fuel, so bigger jets compensated.
    1976 and up qjets are easier to work with on that piston-rod hanger screw setting and Cliffs tapered rods help too.
    I haven't tried any mods to gain cfm, but I have seen some ideas from David Vizard and others. I don't go to the track often enough to confirm any of that anyway.
    Last edited: Jun 9, 2019
  16. 87GN_70GS

    87GN_70GS Well-Known Member

    Another idea is too "thin" the blade shafts with a grinder by making the shaft cross section smaller when the blades are 90 deg open.
  17. HotRodRivi

    HotRodRivi Tomahawks sighted overseas

    Picking up a tenth of a second could be the wind, reaction time, shifting at different rpm. The best place to test carb mods to me is a long steep up hill freeway run.

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