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engine troubleshooting please

Discussion in 'The Garage' started by 78Suburban, Sep 4, 2005.

  1. 78Suburban

    78Suburban 1/2 ton status

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    please help me diagnose what could be wrong with my bone stock 350 4barrel from 1978.... Its got an automatic choke....

    I crank it up and it idles really high (the choke is engaged I assume).. so I blip the throttle it idles so low that it cuts out.. its even hard to get into gear without it cutting out (I have to give it a little gas).. the problem seems to continue, as I drive it, if I come to a stop, it idles ridiculously low.. then on occasion it seems to idle high.. I wonder if the slowspeed Idle for cranking it isn't staying on or something..

    I know nothing, please make all explanations in idiot's terms.. I'm hoping its something easy to fix and obvious..

    thanks
    :bow:
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

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    Sounds like the choke is not adjusted correctly.

    You've got an adjustment on the passenger side of the carb, a small flatblade screw in amongst the choke linkage. That determines what it idles at with the choke "on".

    Drivers side of the carb at the base is where you set the "curb" idle speed.

    When the engine is hot, check the aircleaner and see if the choke is fully open. It SHOULD be. If it's not, then you've got to adjust the linkage so that it IS fully open.

    Once you are certain the choke is fully open you can worry about setting curb idle. It's pointless to do so unless the choke is opening fully.
     
  3. Mad-Dog

    Mad-Dog 1/2 ton status

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    Ok, when the engine is cold a bi-metal spring located inside the choke stove contracts and pulls down on a rod which is connected to the choke plate, when you first depress the go pedal in the morning to squirt a little gas in the manifold you also set the choke in the same fell swoop because of the pressure the choke stove has been applying the rod that is connected to the choke plate.
    On that type of carburetor there is a stepped fast idle cam that is connected to other end of the choke plate that drops in place in front of the idle speed screw to hold the engine rpm up while the engine is warming, as heat builds in the engine and manifold the choke stove also begins to heat up and that bi-metal spring we talked about before begins to unravel and slowly pushes that rod in the opposite direction forcing the choke plate open which in turn rotates the stepped fast idle cam up and away from the idle speed screw.
    The reason it is stepped is so that idle speed slowly drops to lower rpm gradually as the engine warms until it is at normal operating temperature at which point the fast idle cam should no longer be holding the idle speed up.

    The idle speed screw should be adjusted so that the engine idles at a comfortable rpm in gear when the engine is warm, there are 2 screws located at the front of the carb all the way at the bottom which control the idle speed air/fuel ratio, these may need to be adjusted and can be set by turning each screw CCW a 1/4 turn at a time until the idle speed fails to rise any more........now rotate each screw CW 1/4 turn at a time until the idle speed begins to drop........
    STOP THERE and rotate each screw back out CCW 1/4 turn and your done, readjust the idle speed screw if needed and roll on.
    if the idle speed varies a great deal thruout the day then that is another issue which we will discuss when the time comes :bow:
     
  4. 78Suburban

    78Suburban 1/2 ton status

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  5. Mad-Dog

    Mad-Dog 1/2 ton status

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    Call me crazy, what does "ttt" mean ?
     
  6. 78Suburban

    78Suburban 1/2 ton status

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    to the top
    I was wondering if anyone else had any input, because I need all the help I can get. I surely don't know what I'm doing when it comes to motors.. :o
     
  7. Mad-Dog

    Mad-Dog 1/2 ton status

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    Gee, I'm sorry that was'nt clear enough for you..... :crazy:
     
  8. 78Suburban

    78Suburban 1/2 ton status

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    it was clear and I really appreciate the help.. I was just nurotically checking again to see if there was any other tips as to solving the problem. I will try to work on adjusting it soon.
    thanks for the help :bow:
    James
     

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