Low rpm stumble

Discussion in 'Troubleshooting & Diagnosis' started by Justin79maro, Dec 22, 2017.

  1. Justin79maro

    Justin79maro Veteran Member

    104
    1
    Jan 15, 2015
    Bonne terre, missouri
    I gap the plugs at .045
     
  2. Joekool

    Joekool Veteran Member

    5,892
    27
    Feb 4, 2002
    Hollister CA USA
    Drive the car, get it to happen and then shut it off and pull the plugs. 1500-2000 rpm is usually light load/cruise territory and that will fall under the main circuit and transfer slots for proper fuel delivery.

    By pulling the plugs you have a chance to see if one cylinder or a bank of cylinders or most of the cylinders are running too lean or too rich or there's some fouling going on. The main circuit tends to the leanest circuit, so any problems with fuel flow or delivery will tend to show up there first.

    You can also do things like checking float level by pulling the plugs on the side of the bowls and making sure fuel is at the proper level which is just trickling out the bottom of the plug hole. You can also adjust the idle mixture screws and makesure they arent turned too far out or in. The idle mixture screws do impact the main circuit somewhat.
     
  3. Justin79maro

    Justin79maro Veteran Member

    104
    1
    Jan 15, 2015
    Bonne terre, missouri
    I cleaned the carburetor with cleaner then blew out the holes with a air compressor and changed the jets to 69-79. So far it hasn't done it anymore and seems to do fine. My electric fans stopped working so as soon as I get those working I'll drive it more and fill you in on if it truly fixed it or not. Thanks for the awesome reply btw!
     
  4. Cardinal

    Cardinal Veteran Member Lifetime Gold Member

    8,243
    13
    Jun 22, 2003
    Endicott, NY
    Check and verify the following:

    Float height (I'm assuming that there are sight screws on the passenger side off the carb to see the fuel height).

    Fuel pump pressure = it should be 5 to 7 PSI.

    Timing: initial timing should be around 8 degrees. Total timing should be no more than 38 degrees.

    We're ASSUMING that you have a good brass terminal cap and rotor. Those cheap aluminum terminal caps are notorious for causing misfires.

    Spark plugs: the spark plugs need to be a very light tan. IF black, carb is TOO rich. IF white, TOO LEAN!
     
    Last edited: Jan 24, 2018
  5. Justin79maro

    Justin79maro Veteran Member

    104
    1
    Jan 15, 2015
    Bonne terre, missouri
    Check all that and everything was fine. Floored it today and it hesitated and then took off, once I let off it started spitting and sputtering and sounded kinda sorta like a valve foot A little bit and it was like dieseling with 0 power still at low rpms. I'm totally lost so I'm going to do a compression check on it.
     
  6. Justin79maro

    Justin79maro Veteran Member

    104
    1
    Jan 15, 2015
    Bonne terre, missouri
    Pulled the plugs and they all look perfect.
     
  7. 71RS6SPD

    71RS6SPD Veteran Member

    203
    7
    Nov 14, 2016
    Check your timing at 2500 rpm under no/light load - in the garage is fine. It should be way over 30. If it's not, move your vac advance to manifold vac if it's on ported vac now. I would move it anyway as ported vac defeats much of it's purpose. Idle timing should be around the mid 20's with the vac connected properly. Most engines want about 50 degrees advance under light load at mid or higher rpm. Hesitation is usually lean fuel mixture if the timing is good - check the accelerator pump cams/nozzle size.
     
  8. 71RS6SPD

    71RS6SPD Veteran Member

    203
    7
    Nov 14, 2016
    I agree here.. if your plugs read fine - it's not jet size. Timing and/or accelerator pump mods. And move vac advance to manifold vac for sure. I had hesitation and installed primary pump nozzles and changed the primary accelerator pump cams. I needed the red one to extend the pump time.

    I installed a wo2 sensor and it was very helpful with carb tuning. They react very quickly and will show if your secondaries are opening at the correct times. They will also indicate accelerator pump issues and of course jetting.
     
    Last edited: Apr 24, 2018

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