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Technical Dual Stromberg Carb issues

Discussion in 'Traditional Hot Rods' started by hotrodrhp, Dec 29, 2023.

  1. hotrodrhp
    Joined: Sep 19, 2008
    Posts: 455

    hotrodrhp
    Member
    from Wisconsin

    Pulled my Holley 94's off my AV8 build and put on two newly rebuilt Stromberg 97's. Running 43 jets and 69 power valves in both carbs. Car wasn't running all that bad with the Holley 94's but thought I'd improve performance. Intake is an Edmunds atop a rebuilt 42 flathead. Have the carb linkage synchronized, idles fine, starts instantly but have a serious throttle lag from idle to acceleration. In fact it has stalled at times attempting to increase rpm's from idle. Curious part is if I close the chokes the engine revs well from idle. I have other 97 carbs I can install but obviously I am doing something stupid. Help, advice is appreciated. Happy New Year
     
    dana barlow likes this.
  2. jaracer
    Joined: Oct 4, 2008
    Posts: 2,671

    jaracer
    Member

    Are the accelerator pumps working? Have you verified that they are squirting fuel as they should? It is obvious you have a lean off-idle condition. That's what the accelerator pumps are supposed to compensate for.
     
    dana barlow, choptop4 and loudbang like this.
  3. hotrodrhp
    Joined: Sep 19, 2008
    Posts: 455

    hotrodrhp
    Member
    from Wisconsin

    From what I can see both seem to be working. Both acc. pumps are brand new.
     
  4. Pete1
    Joined: Aug 23, 2004
    Posts: 2,261

    Pete1
    Member
    from Wa.

    Try some .045 main jets and a 65 power valve.
     
    WiredSpider likes this.
  5. j ripper
    Joined: Aug 2, 2006
    Posts: 846

    j ripper
    Member
    from napa ca.

    Sounds like an intake leak of some sort. How does the engine react when you adjust the idle air screws?
     
  6. alchemy
    Joined: Sep 27, 2002
    Posts: 21,336

    alchemy
    Member

    Is the vacuum brake hooked up? Where?
     
  7. hotrodrhp
    Joined: Sep 19, 2008
    Posts: 455

    hotrodrhp
    Member
    from Wisconsin

    Idles smoothly at about 800 rpm and responds well to idle adjustment. Gaskets are all new
     
  8. hotrodrhp
    Joined: Sep 19, 2008
    Posts: 455

    hotrodrhp
    Member
    from Wisconsin

    Vacuum line hooked to intake
     
  9. alchemy
    Joined: Sep 27, 2002
    Posts: 21,336

    alchemy
    Member

    Where on the intake? Original was a small port between two ports near the gasket surface at the block.
     
  10. 1947coupe
    Joined: Feb 20, 2014
    Posts: 64

    1947coupe
    Member

    Do you have the accelerator pump rods set on the winter position, I have had the best performance there.
     
    HEATHEN likes this.
  11. hotrodrhp
    Joined: Sep 19, 2008
    Posts: 455

    hotrodrhp
    Member
    from Wisconsin

    May order new accel. pumps, change jetting and power valves. Just don't want to waste time chasing my tail. Maybe install new parts in other carbs and swap them out. This shouldn't be this hard frustrating to say the least.
     
    Adventure galley likes this.
  12. alchemy
    Joined: Sep 27, 2002
    Posts: 21,336

    alchemy
    Member

    Newly rebuilt means the emulsion tubes were removed to clean, and the J tubes blown clear too?
     
    Moriarity likes this.
  13. hotrodrhp
    Joined: Sep 19, 2008
    Posts: 455

    hotrodrhp
    Member
    from Wisconsin

    Thats where it is
     
  14. hotrodrhp
    Joined: Sep 19, 2008
    Posts: 455

    hotrodrhp
    Member
    from Wisconsin

    Completely disassembled all parts cleaned, blown out and mating surfaces cleaned and surfaced. That's the frustrating part because I'm really meticulous to a fault. Probably qualify as having OCD
     
  15. hotrodrhp
    Joined: Sep 19, 2008
    Posts: 455

    hotrodrhp
    Member
    from Wisconsin

    Yes set at winter position
     
  16. Petejoe
    Joined: Nov 27, 2002
    Posts: 12,427

    Petejoe
    Member
    from Zoar, Ohio

    Try Running an “unlit” propane torch on all your gasket surfaces.
    Check to be sure your choke plates aren’t falling partially shut when reving off of idle.
    Exactly what fuel pressure are you running?
    I’ve surely never seen any performance differences between 94’s and 97’s.
    “Carbs are synced” I’m going to ask because I’ve seen many who make mistakes when using a sycronizer. .
     
  17. ydopen
    Joined: Mar 14, 2010
    Posts: 237

    ydopen
    Member

     
  18. AULIZ
    Joined: Oct 28, 2005
    Posts: 1,527

    AULIZ
    Member

    when You are try to find optimal tune up to Your carbs, dont use any vacuum outlest. Plug all. You can open those later if possible.

    If You have basic engine, its better to use progressive linkage.
    Jets = when You drive on road, higher RPM
    Power valve = bethween Idle and Jets. I my engines (free flow engines) different size power valves. Another soft spring, another harder.
    Idle = You can try to adjust carbs so that only another carb (if You have plenum in Your intake) is used idle and another carb is tuned so that it comes delay.

    I have note that for example Thickstun PM-7 intake plenum is so far from carbs, not easy to use progressive linkage or tune so that use Idle only one carb and another comes with later when more trottle.... Normally PM-7 works best when both carbs are open same time. Powervalves, jets can be different size...

    I think Your engine is lost vacuum while when You speed up. Many carbs + basic engine --> vacuum is weak.
    Vacuum is need to keep as good as possible because carbs are working like milker...
    When You have hi RPM problem is gone
    When You have supercharger, hi-compression, long stroke, big bore.... then engine easier tune because engine has more vacuum

    Carb tune is interesting because all engine have different psyche (like woman.... never know what happens next... :) ) and many times engines are work different ways.

    Of course first You need to find out that there is not any leaks. Starter pilot or brakleen ....


    Aulis
     
  19. jaracer
    Joined: Oct 4, 2008
    Posts: 2,671

    jaracer
    Member

    With the engine not running, open the throttle slowly to full throttle. You should see a steady stream come out of both sides of the squirter. Keep watching until the stream quits. It needs to be steady, not dribs and drabs, and a good stream. If both carbs check okay, I would probably be looking for a vacuum leak. However, you stated that it idles well and it really wouldn't with a vacuum leak.

    With the engine running, look down the carbs at idle and see if any fuel is coming out the main venturi. There shouldn't be any, it should be dry in the carb throat. If fuel is coming out at idle, it could be that you have the throttle plates open far enough to start pulling fuel out the venturi. This could point to a vacuum leak.
     
  20. oj
    Joined: Jul 27, 2008
    Posts: 6,540

    oj
    Member

    I
    Idle speed is too high, the carb is designed to supply fuel for about 450rpm, the high idle speed means the throttle blades are tipped too far open and the 'transfer slot' (a small hole in this case) is overly exposed and cannot supply fuel when you accelerate, that little circuit operates BEFORE the accelerator pump to get you up and running, when you disable the transfer slot you have a horrific stumble.
     
    Xman likes this.
  21. flatjack
    Joined: Feb 13, 2007
    Posts: 978

    flatjack
    Member

    Get some power valves with synthetic plungers.
     
  22. hotrodrhp
    Joined: Sep 19, 2008
    Posts: 455

    hotrodrhp
    Member
    from Wisconsin

    Thanks for the continued flathead education all. Looks like my plate will be full the first of the year.
     
  23. winduptoy
    Joined: Feb 19, 2013
    Posts: 3,626

    winduptoy
    ALLIANCE MEMBER

    My experience is that 97's have a cold engine stumble and need some choke until the engine gets to operating temperature. Does your stumble go away after the engine reaches operating temperature
    Then, I seem to go the wrong way on jetting unless I use an AFR to set things up and it seems that flatheads like and run a little better rich....
    good luck
     
  24. hotrodrhp
    Joined: Sep 19, 2008
    Posts: 455

    hotrodrhp
    Member
    from Wisconsin

    Pulled the carbs today. Ordered new accelerator pumps, emulsion tubes and gaskets. See nothing wrong with either but eliminating the variables. Also ordered an electric fuel pump in spite of my stock pump holding 2-1/2 lbs. pressure, again no more variables. Figure it will at least prime the engine after sitting a while. Staying with the 63 power valves and the 43 main jets for now.
    Another question...... Can I leave the existing diaphragm in the stock fuel pump and still use the electric as a supplemental fuel source. Perhaps it's best removed?
     
    Last edited: Jan 7, 2024
  25. oj
    Joined: Jul 27, 2008
    Posts: 6,540

    oj
    Member

    Turn the carbs over and have a look for that hole I spoke of, if it is exposed you'll have a stumble no matter what else you do or change, that hole is drilled to eliminate the stumble, that is its' purpose. take a photo of it and post it.
    The electric fuel pump will push thru the mechanical, no worries.
     
  26. hotrodrhp
    Joined: Sep 19, 2008
    Posts: 455

    hotrodrhp
    Member
    from Wisconsin

    Finally got the new internals for the carbs. Pulled the emulsion tubes and they were coated in crud. New emulsion tube replacements installed. Even though they are new I also replaced the accelerator pumps and check valves. Checked float levels. Staying with the 43 main jets and 69 power valves. Have an assortment of jets coming this week so we shall see. IMG_3031.jpg IMG_3034.jpg IMG_3031.jpg .
     
  27. alchemy
    Joined: Sep 27, 2002
    Posts: 21,336

    alchemy
    Member

    Yikes. I thought you said they were removed and cleaned before?

    Also important is to clean the cavity around where the emulsion tubes fit. A brush is required.
     
    Airborne34 and Moriarity like this.
  28. This shows exactly why you have to remove the emulsion tubes to rebuild a Stromberg 97. Unless they are clean and all the small holes are open, the main venturis will never flow correctly. This is a very common problem with folks who are doing their own at-home rebuilds . . . they neglect to perform this vital step.
     
    caprockfabshop and Petejoe like this.
  29. hotrodrhp
    Joined: Sep 19, 2008
    Posts: 455

    hotrodrhp
    Member
    from Wisconsin

    Carbs were rebuilt but not by me
     
    winduptoy likes this.
  30. alchemy
    Joined: Sep 27, 2002
    Posts: 21,336

    alchemy
    Member

    Sorry I assumed it was you since you described the work and said you are meticulous. Maybe you should “out “ the rebuilder so nobody else gets taken.
     
    Airborne34 and Moriarity like this.

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