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Technical flathead idle!

Discussion in 'Traditional Hot Rods' started by rodderbilly, Feb 13, 2014.

  1. Hi all....i need a little help and input....i have in my flathead 2 holley has a choke the other not...carb with choke is on rear....problem is this...engine sometimes backfire ,the front carb sucks a lot of air.....when i put a hand on front carb leaving only a little space for air the car idles.......i change the power valves from 7.5 to 4.5....but engine not idle yet...what could be the problem?
  2. summersshow
    Joined: Mar 3, 2013
    Posts: 899

    from NC

    Air mixture setting... Screw to adjust the amount of air/fuel that goes in. Both carbs need to have the same flow... Theres a tool for checking it called a synchronizer.

    A good starting point on your air mixture screws is all the way in, and back out 2 1/2 turns. move them 1/2 turn from there and adjust one than the other, till theyre good.
  3. ROADSTER1927
    Joined: Feb 14, 2009
    Posts: 3,139


    You did not say if you are running your carbs all at once or progressive. Either way what you need is called a unisyn carb balacer to set them correctly.
  4. thanks summers...i have do this..initially the front carb was without idle screws and plugged with some screws and was sure that putting the idle screws the issue was solved but the engine still no idles and backfires....can be a power valve damaged??

  5. roadster carbs are sinchro not progressive...i used the uni-syn but not good results.....on the rear carb ball stay at middle while in the front stays all the way at top....
  6. now this is what i try looking closely on the front carb....a hole where is the fuel pump!
    Last edited: Feb 13, 2014
  7. 48-760
    Joined: Dec 15, 2009
    Posts: 146

    from OH

    To Sync. the carbs you do it with the idle RPM set screws not the idle mixture adjustment. The accelerator pump is in back of the hole, that may have been drilled as a vent?? I would consider adding the choke butterfly and rod to the Carb that is missing it then lock it in the open position. Once you get them synced then lower the idle on both to the RPM idle desired, repeat idle mixture adjustment for the smooth idle or highest RPM or highest steady vacuum. Recheck the sync and you should be done. Backfire can destroy the power valve.
  8. ronnieroadster
    Joined: Sep 9, 2004
    Posts: 1,074


    To tune two or more carbies you need a unisyn gauge to help set the throttle plates in the proper location so that each carb works together once you have that set the next thing to do is adjust the idel mixture screws by using a vacuum gauge the idea is to adjust each screw to get the highest reading on the gauge at idel.
  9. summersshow
    Joined: Mar 3, 2013
    Posts: 899

    from NC

  10. lowsquire
    Joined: Feb 21, 2002
    Posts: 2,567

    from Austin, TX

    Id bet money your powervalves arent seating correctly and engine vacuum is pulling un metered fuel past the gasket on the powervalve. very common on 94s. That hole is a breather hole, dont worry about it.
  11. idle issue is solved!i open the rear carb and i try 2 air holes closed from some sealant,,i clear the holes and a put a new gasket....but i don't solve the front carb issue .....still asking a lot of air....with uni-syn in front carb the ball stays on top.....on rear carb ball stay all down but if i start to close the choke ball starts to go up.....
  12. Petejoe
    Joined: Nov 27, 2002
    Posts: 12,278

    from Zoar, Ohio

    Get your carbs rebuilt first....
    You are not listening to the direction given here to adjust your carb synchronization. You adjust them with the linkage not the idle screws. Start the engine unhook your linkage to each carb separately, throw on the Unisys and adjust each one of them with the accelerator linkage. Next...adjust your linkage to fit the carbs in the position they are in.
    Do a search here on Unisys.
  13. thanks Pete...this is the right way to do a good job...:)

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