Quadrajet Tuning For Performance

Discussion in 'Engine Topic' started by 79z Guy, Dec 22, 2018.

  1. 79z Guy

    79z Guy New Member

    Messages:
    1
    Likes Received:
    1
    Joined:
    Dec 22, 2018
    Location:
    Florida
    Recently rebuilt the quadrajet on my stock 79 z28 (4 on the floor trans). Runs good and eliminated any bogging, but I want to get more performance out of it. Idle/fuel mixture screws were set to 5 turns out. I went to 9 and a half turns out by using a tachometer and looking for increased RPM. Went from 700 to 920, then I brought the curb idle down with the idle screw at the throttle. Wanted to get to 700 but with that screw backed all the way out I could only get down to 740.

    Is 9 1/2 turns out on the idlle/fuel mixture screws a normal setting? How far is too far?
    I noticed that there must be a mechanical limitation as well, because the screws are under spring tension and any screw can only be turned out so far before it is just too loose mechanically.

    Anyone have any pointers for further tuning a quadrajet to gain performance?
     
    1980RS likes this.
  2. Zspoiler

    Zspoiler Veteran Member

    Messages:
    3,118
    Likes Received:
    236
    Joined:
    May 6, 2012
    Location:
    Federal Way, Washington
    There are some publications from HP books and others on Q-jets that may help. Since they have used for racing such as Stock and Super Stock NHRA drag racing.Just Google it.
     
  3. 1980RS

    1980RS Veteran Member

    Messages:
    3,013
    Likes Received:
    448
    Joined:
    Jun 17, 2006
    Location:
    MN
    I messed with Q-jets for years back in the 80's and had I ton of rods, jets and hangers which is what you need to tune these great carburetors. Now it all depends if you are working with early or late models. I have always used the early ones like from 67 to 1970 or up to 72, these were used the old rod and jetting system. Most of the time for performance a 71 jet and 42B metering rod worked the best on the primary side. Now if you have a later carb the rods use a different number and lettering on them, just use a micrometer to check them against the old rods. As for the sec. side you will just have to try rods until you get the best performance. Now comes for the fun part, if you are lucky enough to find and old 454 Q-jet from the late 80's those are the ones to get as they are 800 cfm models. You can mod the sec. door opening by clipping the the place where the stop is on the air horn and it will open the door a little extra. There are also pull over holes that can be drilled out to help also. There is an old book called How to Modify your Big Block Chevy, in it you will find a a section where you can to the 30° secondary pickup mod and believe me it works. It makes the car feel like it's got another 100hp LOL, but I have done this on 2 street cars in the past and the gain was pretty impressive. As you might expect there is a down side, you will have to learn to keep you foot off the gas as it's really easy to start getting into the secondaries with this mod. Nice to see some of you guys keep working on there old carbs from the past. The Q-jet may be the best designed carb of it's day.
     
    budro6968 and jeff swisher like this.
  4. jeff swisher

    jeff swisher Veteran Member

    Messages:
    1,203
    Likes Received:
    352
    Joined:
    Apr 26, 2018
    Location:
    Yukon Oklahoma
    Yep there are many things you can do to a Q jet.
    Fine all the books you can on them and research Lingenfelter and Q jet ..you may find things there also.

    Believe it or not my cars responded better when I made the lower secondary linkage mechanical.
    Basically bent the pin that held the secondary linkage closed on the passenger side of the carb.
    Or remove all the junk not needed YMMV.
    Worked for my stuff which was 10.8 or more compression small blocks with less than 180cc heads.
    May not be too happy with a stock low compression 200 HP turd though.

    I ran the Q jet before the Holley for a couple decades if i had my choice.
    Now I run very tweaked Holleys and they run as good as the Q jet :)

    454 was not the only 800cfm one..To tell which is 750 or 800 look in the primary venturi.. the 800 has a smaller raised ring about mid way into the venturi.

    Now the 800 version was also used on the early turbo buick that ran a carb. But the secondary upper air valve door was very limited on how far it opened.. it was about 1/2 way open.

    Like 1980RS stated look and see what limits that door from opening all the way. Easily tweaked.
    If I dug around enough I bet I still have about 10 Q jets and lots of body parts for them maybe 5 Holleys and nearly 40 carter AFB and Edelbrocks combined.
    I like carbs.
     
  5. Rustbucket350

    Rustbucket350 Veteran Member

    Messages:
    839
    Likes Received:
    32
    Joined:
    Sep 4, 2013
    Location:
    Oviedo, Florida
    I bought a book by Cliff Ruggles called How to Rebuild & Modify Rochester Quadrajet Carburetors from Amazon not too long ago. I think it was like $20. It didn't help me modify one from a blow through twin turbo setup (like anyone in their right mind would want to use one for that) but it is filled with all kinds of info for rebuilding the various models and tuning them for naturally aspirated engines. It has way more info than you can dig up on the internet.
     
  6. 77 cruiser

    77 cruiser Veteran Member

    Messages:
    530
    Likes Received:
    23
    Joined:
    Nov 17, 2006
    Location:
    Frostbite Falls MN
  7. Jeep43

    Jeep43 Veteran Member

    Messages:
    1,209
    Likes Received:
    42
    Joined:
    Mar 30, 1999
    Location:
    Connecticut
    As you mentioned the concern with 9.5 turns is the springs don't keep them from moving. You can drill the passages larger like I did to get the same idle fuel with less turns out.

    A couple of other tips I learned from Cliff, Lars (see the sticky in this forum about Lar's Q jet papers and get them, the timing one too) and other sources that really helped me tweak the QJet on my car is.

    Don't get too hung up on well plugs that are spun in aluminum. Check them from what I read the leak thing was from the early style with pressed in brass cup plugs.

    Get a wideband air fuel meter. Its not gospel but is still good info. Without any track time yet I tune WOT to 12.5 and cruise to low 14's. I still set idle as per vacuum gauge tho even though the meter reads mid 13's.

    When tuning the primaries wire up the secondary disconnect to keep the secondary butterflies from opening at all no matter what. You can now dial in the primary circuit rods and jets without the secondary circuit confusing you.

    Unless you buy an assortment of rating power piston springs, you can test the springs you have to be sure you have enough vacuum to overcome the power piston spring to be sure its in the full down position at idle. To do this you need a junk bowl and power piston and hook a vacuum gauge to your running engine. I bought rated springs tho.

    Polishing the Accel pump bore with a brake hone wrapped in fine paper gets better operation than the cast finish.

    A carb with Adjustable Part Throttle makes tuning cruise mixture easy. My Edelbrock 1902 Qjet has a screw in the base plate that adjusts the down position of the power piston to change how much rod taper is in the jets at part throttle. Jet makes a kit to convert later carbs to easily adjust this. Cliff Ruggles sells a special screw to allow easy tuning of the base plate style like I have. Tuning was just a matter of hitting flat highway and adjusting the screw to obtain the AF ratio I wanted.

    I did buy Cliffs book and tried to exclusively use the instructions to tune but eventually broke down and bought components to best "over the phone" calibrate my Q jet to my combo followed by tech support.

    My Qjet from edelbrock featured torx screws throughout including the secondary air door spring. I bought the right tools to easily adjust (torx drivers and tiny allen type torx wrenches).

    That's about all I can think of for now. Its a great carb.
     
    Last edited: Jan 4, 2019
    1980RS likes this.
  8. 1980RS

    1980RS Veteran Member

    Messages:
    3,013
    Likes Received:
    448
    Joined:
    Jun 17, 2006
    Location:
    MN
    Sounds Familiar, we used to play with the APT when a couple of us guys read about it back in the day, helped a ton with fuel mileage and drivability concerns. When I worked at GM in the very early 80's I got more complaints with the duo-jets, 1/2 a Q-jet, I would work my magic and the cars ran great as they were all to lean from the factory and this just made them a little fatter. I did that on my wife's GP with a 301, adjusted it down and she would get almost 30 mpg on the highway, didn't last long though as it burned some exhaust valves, oops too lean. She was not happy with me, don't know why, she got great fuel mileage.
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.