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Hot Rods Rejetting Dual Stromberg 97s

Discussion in 'The Hokey Ass Message Board' started by CSPIDY, Feb 5, 2025.

  1. CSPIDY
    Joined: Nov 15, 2020
    Posts: 881

    CSPIDY
    Member

    My 29 roadster has two 97s on an edelbrock slingshot manifold on an 8BA flathead
    Cam is mild but not stock, minor lope
    If I let let it idle for any amount of time it would load up the plugs

    I went to a hotter plugs NGK B-4L and this helped with the plugs loading up
    but
    the plugs are still showing rich with carbon on the plugs

    the engine runs very well, that being said I would like to rejet and return to standard heat range plugs NGK B-6L.
    I am currently cleaning and resealing both carbs, what I found is the primary carb has .048 jets and secondary has .045 jets.
    Being relatively new to the flathead and Stromberg carbs is it normal to have two different size jets and do I reduce each carb by one size or just the primary?

    the way the linkage is setup the secondary doesn’t kick till about 1/4 throttle
    Dave
     
  2. quickchangeV8
    Joined: Dec 7, 2010
    Posts: 585

    quickchangeV8
    Member

    I have a 1929 coupe, 276 59A flathead, two Stromberg 97's from England, Eddie Meyer hi rise intake and running an Isky 400 Jr. cam. I went through the jetting process this past summer. Engine has the NGKB6L plugs as well and was running way too rich. The England 97 carbs come with .045 jets right out of the box. Way too rich you could smell gas and slight popping or backfiring out the exhaust. Replaced the .045 jets with .043 jets . Better but still rich. Replaced the .043 jets with .041 jets. Much better but still a little bit rich. Finally took out the .041 jets and installed .039 jets. Engine runs perfect now. Gas smell is gone and backfiring has stopped.

    I found fuel pressure made a big difference too. I'm running 1.5 pounds fuel pressure. I had the engine at 2.5 pounds fuel pressure. Too much pressure. At 1.5 pounds it is just right.

    All I can suggest is getting a few additional sets of jets and keep lowering them until you have found the sweet spot.
     
    kadillackid, alanp561 and CSPIDY like this.
  3. Petejoe
    Joined: Nov 27, 2002
    Posts: 12,534

    Petejoe
    Member
    from Zoar, Ohio

    I’d change to straight linkage for a dual carb setup. Jets should be the same size.
    Use a carb synchronizer to allow both carbs pull vacuum exactly.
    Unhook your linkage totally. Set each carb individually at idle to a perfect vacuum.
    Reinstall your linkage and adjust the linkage so it fits your carb throttle without moving it one slight bit. After both carbs pull exact vacuum at idle recheck with synchronizer.
     
    Wright & Sons, alanp561 and CSPIDY like this.
  4. CSPIDY
    Joined: Nov 15, 2020
    Posts: 881

    CSPIDY
    Member

    So both carbs should have the same size jets
    That’s what I expected to see
    This car has no choke and starts very easily down to about 60 f.
    I plan on adding a choke.

    maybe the thing to do is start at .044 in both carbs and go from there
     
  5. CSPIDY
    Joined: Nov 15, 2020
    Posts: 881

    CSPIDY
    Member

    Any other comments would be appreciated
     
  6. shorrock
    Joined: Oct 23, 2020
    Posts: 187

    shorrock

    I had run dual 97s on my flathead with high rise Thickstun manifold. Straight linkage, did run very rich, lowered the floats a bit, then used smaller mainjets to get mixture correct on wide open throttle but the tickover needed modified idle jets as well. As none were available I soldered them closed and redrilled smaller. Now the plugs stayed clean, no black soot anymore.
     
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  7. 8ba, with Offy 2 carb, and mild cam. Running .043 currently. Thinking of going down a size.
     
    CSPIDY likes this.
  8. CSPIDY
    Joined: Nov 15, 2020
    Posts: 881

    CSPIDY
    Member

    After reading Larry Kirwans thread about Big 97 Stromberg help and the thread about air filters for the 97s.
    It got me thinking that my air filters may be contributing to the engine running rich

    Im running two Edmonds customs
    IMG_2791.jpeg
    not sure how restrictive they are, but at least maybe replace the inserts with KnN foam filters
    What do yinz guys think
     
  9. theHIGHLANDER
    Joined: Jun 3, 2005
    Posts: 10,576

    theHIGHLANDER
    Member

    They make an overall difference. I final-set the carbs on these oldies after it's on, sometimes it's a huge difference. Copper wool, maybe 2 layers, light oil once or twice a year.

    And I think it looks kool...;)
     
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  10. ALLDONE
    Joined: May 16, 2023
    Posts: 3,066

    ALLDONE
    Member

    the accelerator pump opens the power valve, I un hook it when i'm doing the initial jetting... that and look in the carb and make sure while it's running that it doesn't have the float too high....I don't know what kind of reg you are running ... but you need to have a gauge to see what it's really at...I fought jetting on strombergs only to find they were rich for other reason than the jets
     
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  11. CSPIDY
    Joined: Nov 15, 2020
    Posts: 881

    CSPIDY
    Member

    Good tips
    I’m running 3 psi regulator with a gauge at the manifold on the firewall.
    I will be checking and adjusting the floats prior to first start up.
    The fact that there is no choke and it starts right up tells me it must be rich mixture
    Having .048 jets in the carb and .045 in the secondary most likely made this possible
     
  12. ALLDONE
    Joined: May 16, 2023
    Posts: 3,066

    ALLDONE
    Member

    On the bangers with a single 97, I've found the 39 to be the best starting jet.. so with the flathead, with 2 carbs thats for sure where I would start. I fought one carb for what seems like for ever... I would run it with the carb top off... float perfect... idle perfect... stock tank no fuel pump...ran way too rich on the bottom to cruise range... what I found was the float was bent out of level and it would hit the top of the carb and not let the carb inlet valve shut completely

    I ve also found them to actually run better with out the accelerator pump, which just makes me think that the 97 is too big for a banger.. and 2 97s is way too big.. but like I said... you'll have an easier time getting main jetting correct with out the pump squirt and power valve giving false readings... just roll on the throttle and do plug checks... a grumbly throttle is fat.... I've never had one lean pop.... only on wot.... I don't think 97's atomize the fuel well and they puddle fuel in the manifold... so plug reading test need to be at a constant mid range throttle for a longer distance on a warmed up to temp motor...with AFR meters so cheep, thats really the way one should do it.... I just didn't want to weld the bungs in..
     
  13. ALLDONE
    Joined: May 16, 2023
    Posts: 3,066

    ALLDONE
    Member

    what reg. are you using,.. I've found the mister gasket ones to never read right... the one on my vicky set at 1 will read 7 on the pressure gauge...
     
  14. CSPIDY
    Joined: Nov 15, 2020
    Posts: 881

    CSPIDY
    Member

    It’s a Holley, had to replace the gauge because it read 3 psi in my hand
     
  15. CSPIDY
    Joined: Nov 15, 2020
    Posts: 881

    CSPIDY
    Member

    I like the idea
    I’m looking into putting the copper wool into frog style scoops with wire mesh in the inlet to the carb and another in the inlet of the scoops
     
  16. CSPIDY
    Joined: Nov 15, 2020
    Posts: 881

    CSPIDY
    Member

    IMG_2795.jpeg
     
    theHIGHLANDER likes this.
  17. Jeff34
    Joined: Jun 2, 2015
    Posts: 1,185

    Jeff34
    Member

    I can check to see what my builder is doing with my 2 old 97's on an Eddie Meyer intake. Isky 400jr. cam. It's not done yet, so I don't know if it's going to run rich, but he's done these a few times so I would think he knows what's what.
    Subscribed so I have this information if mine is running rich.
     
  18. CSPIDY
    Joined: Nov 15, 2020
    Posts: 881

    CSPIDY
    Member

    I originally contacted Stromberg technical support
    It took over a week but they finally answered my questions
    His first response to running rich was the use of paper filters
    Very restrictive was his comment
     
  19. theHIGHLANDER
    Joined: Jun 3, 2005
    Posts: 10,576

    theHIGHLANDER
    Member

    While this will flow more than paper, well that's good. But fuck that, I think it looks kool as can be. Very period looking. Good for you bud;)
     
    CSPIDY likes this.
  20. Larry Kirwan
    Joined: Nov 25, 2024
    Posts: 86

    Larry Kirwan
    Member

    3psi is a little high, I run just under 2psi on 3 BIG97's.
     
    Petejoe likes this.
  21. CSPIDY
    Joined: Nov 15, 2020
    Posts: 881

    CSPIDY
    Member

    I adjust my float level with the top of the carbs off and have had no issues so far.
    I’m running original 97s if that makes a difference
     

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