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Rochester Carburetor

Discussion in 'General Chevy & GM Tech Questions' started by charlie268, Mar 24, 2009.

  1. charlie268

    charlie268 New Member

    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. Jimmiee

    Jimmiee New Member 1000 Posts

    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.
  3. charlie268

    charlie268 New Member

    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. kaiah723

    kaiah723 New Member

    i had to do this to my 84 monte ss. i played it by ear and it worked well
  5. GaryL

    GaryL New Member Platinum Contributor 1000 Posts

    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.
  6. charlie268

    charlie268 New Member

    Thanks, found the screw and did what you said!!!
  7. charlie268

    charlie268 New Member

    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. Jimmiee

    Jimmiee New Member 1000 Posts

    Correct Charlie, they both are mixture screws.
  9. charlie268

    charlie268 New Member

    Should they both be set the same?
  10. Jimmiee

    Jimmiee New Member 1000 Posts

    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!

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