adjustment for buick GS 350 71

Discussion in 'Small Block Tech' started by French-GS71, Jul 24, 2007.

  1. French-GS71

    French-GS71 Member

    Hello,
    i don't have a book manual on my engine 350 ci.
    And i need to adjust my rochester carburator 4bb and ignition too.
    But to do these job i need the original value of carburator and ignition for a good adjustment.
    i need your help.... :pray:

    thank you.
     
  2. bob k. mando

    bob k. mando Guest

  3. sean Buick 76

    sean Buick 76 Buick Nut

    To start with look for a 71 chassis manual on e-bay it has tons of info for you!

    The factory timing spec is a little low, about 4 deg above spec will be the smoothest for you.

    If you don't already have these you will need:

    timing light
    tach
    vacuum gauge

    Read the powertiming thread in the FAQ section, and set timing first.

    Since you want to get the motor running right let me clue you in on what it takes. Timing is very important and must be close to right before the carb can be set up. You'll need to adjust the idle mixture on the carb to get the proper air/fuel ratio at idle. I use a electronic measuring device called an LM-1 to measure this ratio but you can use a vacuum gauge, rpm gauge and some tinkering to set it up. Reading the spark plugs is a really good way to see how the mixture is.

    Set the idle mixture screws on the bottom front of the carb to get highest vacuum readings (and smoothest idle) with the wheels blocked, vacuum advance plugged, at the rpm at the spec. for that year and motor. I can elaborate if you need.....

    Have you set the timing yet? If not, do that first

    Check total timing as well as the intitial using a dial back timing light.

    These Buick motors like 14 degrees intial, 20 degrees of mechanical advance, and 8 degrees of vac advance on top of that. The vac advance disapears under high load and leaves the 34 which is best for performance.

    I had to do some work to the dist to get this setting though:

    I used MR Gasket bushings to limit the mechanical advance from about 30 to 20. This allows more initial timing which makes it run better without any detonation (ping) once the timing is all in as rpm climbs.

    I use lighter springs for the mech. advance from a Crane cams vacuum advance kit for a chevy 350 (it works on all buick, pontiac ect as well) Kit includes springs to let the timing come in earlier than stock which helps performance and economy as well as an adjustable vac advance canister (which I've limited to 8 degrees to eliminate detonation (ping) on light throttle.

    To properly set the idle mix screws:

    1.disconnect and plug the vacuum line to the vac advance canister

    2. block all 4 wheels

    3. connect vacuum gauge

    4. start car and put it in gear

    5. adjust idle mixture screws to achieve max vacuum and smoothest idle (make sure they are equal turns out and start at 2 full turns out)

    6. re set idle speed (it should rise with the right idle mixture setting)

    7. go back and check initial timing

    8. As an extra step I then go back and fine tune the idle mix screws again but this time just adjusting a 1/8 of a turn to get it perfect

    This works well for me, but what do I know....
     
  4. French-GS71

    French-GS71 Member

    hi,

    thank you for all.
    I ll try with all your informations my adjustment.
    i will try to find a shop manual on ebay, it ll be better to take care of my buick GS.
    have a nice day.
     
  5. sean Buick 76

    sean Buick 76 Buick Nut

    Your welcome, but we want pics of your car!
     
  6. French-GS71

    French-GS71 Member

    I repair my error.

    soon, i ll post other pics of my buick.

    and i ve two friends who have too buick model,

    the first is a skylark coupe with a 300ci of 1967.
    the second a GS 400 convertible of 1967.
    i ll post too some pics of these cars.

    see you soon, :)
     

    Attached Files:

    walts72 likes this.
  7. Tripple7

    Tripple7 Well-Known Member

    Sorry to bring this old post up, but where do I connect the vacuum gauge using stock 350 intake and a 2GV rochester carb?
     
  8. Nailhead in a 1967

    Nailhead in a 1967 Kell-Mnown Wember

    How to Adjust a Two-Barrel Rochester Carburetor
    by Carl Pruit
    The two-barrel Rochester carburetor is designed with two adjustment screws, located in the front of the carburetor near the base.
    Adjusting the carburetor is fairly simple and the adjustment screws are easy to access.
    The idle fuel mixture screw on the left side and the idle speed screw on the right side should be adjusted to read the same on the vacuum gauge
    at the highest readings possible to get the best performance from your engine.

    Step 1
    Locate the two idle adjustment screws in the front of the two-barrel Rochester carburetor and turn both screws clockwise with a screwdriver until they are completely closed.
    Be careful not to over-tighten the screws.

    Step 2
    Turn the adjustment screws back, counterclockwise one and a half turns with the screwdriver to use as a starting point for your adjustments.
    Start the engine and let it warm up for about five minutes.

    Step 3
    Remove the rubber vacuum hose from the base of the carburetor and attach a vacuum gauge to the vacuum port at the base.
    Start with the idle fuel mixture screw on the left side of the carburetor, looking at it from the front of the vehicle and turn the screw a quarter turn.

    Step 4
    Turn the idle speed screw on the right side a quarter turn and alternate back and forth, adjusting the screws until you get the highest rpm readings possible on the vacuum gauge with the engine running smoothly.
    Continue the adjustments, alternating between them until both are reading the same rpm while running smoothly.

    Adjust both screws one final time by turning them a quarter turn counterclockwise and verify that both are reading the same rpm.
    Remove the vacuum gauge and reattach the rubber vacuum hose to the vacuum port. Shut the engine off.
     
    Tripple7 likes this.
  9. Tripple7

    Tripple7 Well-Known Member

    Ok, so that would be the tube that leads to the vapor canister?
     
  10. sean Buick 76

    sean Buick 76 Buick Nut

    any vacuum line from the carb that sucks at idle.
     
  11. alec296

    alec296 i need another buick

    Main question is why you feel they need to be adjusted? As basic tune up or are you having a problem? You may need more then adjustments.
     
    Tripple7 likes this.
  12. Tripple7

    Tripple7 Well-Known Member

    Looking at the plugs the mixture seems too lean.
    Want to try that before rebuilding the carb next Winter.
     
  13. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Before you even think about trying to adjust the timing, you must make sure that the engine has the distributor it left the factory with. For a 1971 Buick 350-4 barrel, the distributor part number would be 1112080. The car is close to 50 years old, and you are likely not the original owner. That distributor 1112080 may be long gone years ago, and if it is, the factory specifications are absolutely useless. If you do have the 1112080 distributor, the factory settings will not get you the best performance, not even close. Find out the distributor part number first. Then we will go from there.
     
  14. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Didn't realize this thread is 13 years old.:D
     
  15. Tripple7

    Tripple7 Well-Known Member

    I apologize..
     
  16. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    There are two types of vacuum connections on most carburetors. It is VERY EASY to tell the difference between the two. One is PORTED vacuum, the other is MANIFOLD vacuum. PORTED vacuum feeds from above the throttle blades, so there is no vacuum at idle, only when you open the throttle. MANIFOLD vacuum feeds from below the throttle blades. It is present anytime the engine is running, at idle and part throttle. Both PORTED and MANIFOLD vacuum will go to ZERO at wide open throttle. With the engine running, if you pull a vacuum connection off and it hisses, it is MANIFOLD vacuum. If it doesn't hiss, it is PORTED.

    The vapor canister is just a vent tube. There is no vacuum present.

    You can't really judge mixture by looking at the plugs with unleaded fuel. You are better off using a wide band O2 sensor and gauge.
     
  17. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    You must connect the vacuum gauge to MANIFOLD vacuum, see my previous post.
     
  18. Tripple7

    Tripple7 Well-Known Member

    Thanks Larry. I found a constant vacuum port at the back where the AC control valve is connected.
     

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