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  1. #1
    Jr. Apprentice charlie268's Avatar
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    Default Rochester Carburetor

    I have a rochester 4bl carb on my 77 pickup. Just had it rebuilt and was wondering about the mixture screw. I remember years ago I used to hear that you screwed the mixture screw all the way in ant then backed it out so many turns. Does anyone remember this?

  2. #2

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    Best way is with a tach but the old tried and true way is to turn it in slowly till it misses <this is a lean miss> then back it out 1 turn.
    Jim

    2004 SILVERADO 2500HD LS
    Pro Comp Leveling Kit
    285-75-16 Cooper ST All Terrains
    Pro Comp Programmer
    Bilstein Shocks
    Linux Bed Liner
    Tinted Windows



    Other rides:

    1967 Landcruiser <350 Chevy>
    1970 Camaro
    1990 Geo Metro Convertible
    2007 FJ Cruiser

  3. #3
    Jr. Apprentice charlie268's Avatar
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    Default

    My carb is a 4MC and I can only find the idle screw on the driverside in the front. Where is the mixture screw located?

  4. #4
    Sr. Apprentice kaiah723's Avatar
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    Bridgeport, CT
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    Default

    i had to do this to my 84 monte ss. i played it by ear and it worked well
    2009 cobalt ss, imperial blu metallic, lsd, dejon powerhouse intake, Dejon bpv spring upgrade, Trifecta tune, clutchmasters aluminum flywheel, clutchmasters stage 1 clutch, 20% tints,

    2008 avalanche ltz, imperial blue metalic, k&n cold air intake, Gibson cat back duel exhaust

  5. #5

    Default

    Quote Originally Posted by charlie268 View Post
    My carb is a 4MC and I can only find the idle screw on the driverside in the front. Where is the mixture screw located?
    They are on the front of the base plate probably under some covers. You have to remove the covers to gain access to the screws. I always start about 2.5 turns out and then do the method described above.
    Gary

    99 K 1500 Suburban
    78 Silverado Big 10 w/factory 454
    00 S-10 Blazer
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  6. #6
    Jr. Apprentice charlie268's Avatar
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    Default

    Thanks, found the screw and did what you said!!!

  7. #7
    Jr. Apprentice charlie268's Avatar
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    Default

    I started this thread and thought I had solved the problem, but have one question. I located the screws on the base of the carb. One is on the drivers side base and the other is on the passenger side of the base. Does anyone know which is the idle and which is the mixture? I looked on line and found one source that told me they were both idle screws!

  8. #8

    Default

    Correct Charlie, they both are mixture screws.
    Jim

    2004 SILVERADO 2500HD LS
    Pro Comp Leveling Kit
    285-75-16 Cooper ST All Terrains
    Pro Comp Programmer
    Bilstein Shocks
    Linux Bed Liner
    Tinted Windows



    Other rides:

    1967 Landcruiser <350 Chevy>
    1970 Camaro
    1990 Geo Metro Convertible
    2007 FJ Cruiser

  9. #9
    Jr. Apprentice charlie268's Avatar
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    Default

    Should they both be set the same?

  10. #10

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    Yes they are exactly the same. There's one for each primary. Adjust them one at a time then recheck them. When you turn them in you are going lean. Once the idle is good you're done!
    Jim

    2004 SILVERADO 2500HD LS
    Pro Comp Leveling Kit
    285-75-16 Cooper ST All Terrains
    Pro Comp Programmer
    Bilstein Shocks
    Linux Bed Liner
    Tinted Windows



    Other rides:

    1967 Landcruiser <350 Chevy>
    1970 Camaro
    1990 Geo Metro Convertible
    2007 FJ Cruiser

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