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Carb problem

Discussion in 'The Garage' started by 72THING, Apr 11, 2002.

  1. 72THING

    72THING 1/2 ton status

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    I have a Q-jet on a Performer intake on a mild 355. My timing at idle is set to 10* BTDC. It runs great and the secondaries open fine and it pulls hard to 5000 RPM through 1st and 2nd gear, but when I get into 3rd gear and floor it, it'll stumble and stop pulling hard. But when I let off the gas a little, it seems to start pulling again, although somewhat slowly. Anybody have any ideas why it does fine through 1st and 2nd, but not 3rd? I can't figure it out. Thanks for all help.

    Will
     
  2. BorregoK5

    BorregoK5 1/2 ton status

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    Is your static timing set to 10* with ported vacuum advance connected or disconnected and plugged? Sounds like it is load related due to first and seconds good performance.
     
  3. 72THING

    72THING 1/2 ton status

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    The timing is set to 10* with the vacuum advance disconnected and plugged. I think its load related, too, but I don't know what to do. Thanks.

    Will
     
  4. BorregoK5

    BorregoK5 1/2 ton status

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    Hmmm, could you be starving for fuel? Since its a vacuum secondary carb, perhaps in 1st - 2nd there is not enough load to fully engage the secondaries but in 3rd it opens all the way up and you get a starve condition... ?
     
  5. 72THING

    72THING 1/2 ton status

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    Is there any way that it can be adjusted? I don't know very much about it. Is there a spring or set screw that can be adjusted/changed? Thanks

    Will
     
  6. Blazer79

    Blazer79 1/2 ton status

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    There's a screw that adjusts spring tension on the secondary flaps. If it's too tight they will remain closed and open all of a sudden when vacuum is enough. If it's too loose, they will open too soon, and cause an over rich condition.

    The screw is located on the left side of the secondary valve shaft. There's a tiny allen screw that holds it in place. Make a small adjustment and then try it. Don't go too much at once. Also, don't overtighten it because the spring will jump over the screw (happened to me once) and then you'll have to remove the air horn to put it all back toghether again.

    Can you believe Doug Roe's book on Q-jets doesn't even mention this screw on the whole book?

    I hope this helps.
     
  7. BorregoK5

    BorregoK5 1/2 ton status

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    Could be a fuel delivery problem, check the fuel filter first. If not look for restriction possibilities. Try running with an external inline filter and remove the one in the carb inlet. (if its a q-jet) which fairly restrictive. Could be a fuel pump as well but start simple.
     

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