Correct jet sizes for Speed Demon 750 VS

Fuel and Air Systems

  1. fmahannah

    fmahannah Well-Known Member

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    I bought a new speed demon 750 VS and can't seem to get it tuned correctly. It had a quick fuel power valve n it out of the box so I suspected it might have been a return, but Holley said they use QF parts when building them since they are interchangeable.

    But now I see the main jets are 74's? Holley site says in specs they should be 68's. Other sites I found with specs of 70 or 72.

    Does anyone know what size primary and secondary jets should have been in an SPD-750-VS out of the box? Holley has not answered this question for me yet. I want to reset carb to factory settings and then start the tuning process over again.

    Thanks, this forum has been a big help to me with my 63 Polara
     
  2. Shorthorse

    Shorthorse Well-Known Member

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    Demon specs.JPG
     
  3. fmahannah

    fmahannah Well-Known Member

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    Actually Summit is sending me a new carb. Holley tech showed that the one they sent me was from before Holley even acquired demon carbs, had no numbers on it under air cleaner gasket, had a quickfuel vac pod, wrong jets, etc. If this carb is not right I am done with demon but can really use the idle eze feature

    And holley tech says these are the correct specs from their page for that carb, not the ones summit is showing. I am not sure who to believe?

    Barrels 4
    Billet Color Shiny
    Booster Down Leg
    Brand Demon
    CFM 750
    Choke Electric
    Circuit 2
    Emission Code 3
    Finish Shiny
    Fuel Gasoline
    Fuel Inlet Dual
    Fuel System Carbureted
    High Speed Air Bleed 28
    Idle Air Bleed Size 70
    Material Aluminum
    Model 4160 Vacuum Secondary
    Primary Main Jet 68
    Primary Power Valve 6.5
    Primary Pump Nozzle Size 31
    Product Type Carburetor
    Secondaries Vacuum
    Secondary Main Jet 78
    Supercharged Application No
    Throttle Bore 1.688 inch
    Part Number SPD-750-VS
    Barrels 4
    Billet Color Shiny
    Booster Down Leg
    Brand Demon
    CFM 750
    Choke Electric
    Circuit 2
    Emission Code 3
    Finish Shiny
    Fuel Gasoline
    Fuel Inlet Dual
    Fuel System Carbureted
    High Speed Air Bleed 28
    Idle Air Bleed Size 70
    Material Aluminum
    Model 4160 Vacuum Secondary
    Primary Main Jet 68
    Primary Power Valve 6.5
    Primary Pump Nozzle Size 31
    Product Type Carburetor
    Secondaries Vacuum
    Secondary Main Jet 78
    Supercharged Application No
    Throttle Bore 1.688 inch
    Part Number SPD-750-VS
     
  4. Shorthorse

    Shorthorse Well-Known Member

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    You're right, what little info I could find showed 3 different specs. I guess they just changed the base unit over the years. Since your carb is new, I'd go with what Holley says.
    It's been about 12-14 years ago but the original Barry Grant Demon carbs (before Holley) had some quality control issues. I was warned about this by several people after I bought a new one many years ago so I decided to take my new Demon completely apart. Sure enough there were metal shavings, there were drilled holes with metal shards still attached and one of the vacuum ports was clogged with sealer. After addressing those issues the carb preformed flawlessly for years and was still working fine when I sold the car.
    I don't know what has been done about those issues, either by Demon or Holley in the time that has passed. Hopefully, that has been addressed.
     
  5. fmahannah

    fmahannah Well-Known Member

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    Yes I also read about the issues with needing to clean and inspect demons before actually running them. This one was not bad inside but I am pretty sure at least one person had the carb and changed jets and other things before I even got it. It was like it was assembled using spare parts from both Demon and Quickfuel. New carb from Summit is supposed to come tomorrow and hopefully it will have all the right parts inside. Honestly I think the one I got had some internal defect because it would not run right over 50 mph no matter what I changed
     
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