Holley 4150 750...recommendation to go 650

Discussion in 'Troubleshooting & Diagnosis' started by JD Z, Oct 27, 2017.

  1. SRGN

    SRGN Veteran Member

    176
    49
    Feb 20, 2009
    Central NJ
    The issue is that the current carb is mechanical secondary per the list number you posted. If you go with a 750 vacuum secondary, the secondaries will only open as needed vs what you have now that open up as much as you tell them to. I've run an 850 vac secondary on a POS Buick 350, and when tuned properly it actually gained me almost a tenth at the track over a 750. Both were tuned by wideband. I'd just put a 750 vacuum secondary carb on it, and tune as required.
     
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  2. JD Z

    JD Z Veteran Member

    It sounds like I’m hearing:

    - Holley 4150 650 double pumper.
    - Vacuum secondaries.
    - electric choke.

    Not sure what the thoughts were of the previous owner and the mods he did with adding a 750 with mechanical secondaries and no choke. Maybe he was building for race/strip use. The car is a 4spd with tight gears, easy to get RPM from a 373 rear end.

    I’ll use this car over the next 30 years for cruise enjoyment and the feel of exhilaration when letting her rip. Not timed or in competition. What will I miss from a 750?
     
  3. Joekool

    Joekool Veteran Member

    5,829
    14
    Feb 4, 2002
    Hollister CA USA
    A 650 vacuum secondary carb is gonna be easier to tune, give better throttle response and better fuel mileage. You might or might not notice a drop in high rpm driving and if your old carb is tired, you might actually pick up some performance just because you put a new, better carb on.

    The best option though is to go EFI, then you have a computer constantly adjusting and tweaking the fuel for you. Reliability also goes up a thousand times.
     
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  4. JD Z

    JD Z Veteran Member


    Thanks Joekool. That helps and I’ll look into EFI. Not sure what undertaking that is.
     
  5. Lowend

    Lowend Administrator. .a car, a man, a maraca. Staff Member Lifetime Gold Member

    14,117
    171
    Mar 25, 1999
    San Jose, CA, USA
    A 750 isn't huge for the car. Does the carb not have a choke at all, or is the chock just not connected?

    It sounds to me like the car probably needs a dyno tune. Carbs are very rarely right "out of the box"
     
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  6. 80sz

    80sz Veteran Member

    2,153
    143
    Feb 2, 2014
    Buffalo,n.y.
    A 3310 750 would be a great choice. Tuning the advance and base timing first might help out current carb. More initial timing for the cam .
     
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  7. Lowend

    Lowend Administrator. .a car, a man, a maraca. Staff Member Lifetime Gold Member

    14,117
    171
    Mar 25, 1999
    San Jose, CA, USA
  8. JD Z

    JD Z Veteran Member

    Thanks for the electric choke link and insights Lowend. Something to consider as that is the primary problem I see. The tech said the carb was too big and I didn’t have time to visit with him more.

    How do I tell what I have. I found the code 4779-9. I don’t see the choke flap at all on the carb top. It does have a vacuum line from carb to distributor. What does that tell me and what else do I need to know? I’m getting mixed signals on 750 being right for the car. Holley site is suggesting 650.

    End of day, I don’t know how long the carb has been on the car or why they chose this setup. If the shop can update to a new 650 carb for a reasonable cost, what will I be missing out on leaving the 750? Do I ask for specifics on the new carb?
     
  9. Zstar

    Zstar Veteran Member

    870
    166
    Mar 23, 2014
    Ocala, Florida
    I have a similiar setup, m21, 3:73,355. I run a demon 650 mech sec, no choke horn, and have no problems. If you're running an hei, try gapping the plugs at .050. I have found with mine that's ideal. I also run Bosch Platinum plugs.
     
  10. BonzoHansen

    BonzoHansen Administrator Lifetime Gold Member

    ^^yeah, you dont need a choke in Florida lol

    I never had good luck with bosch platinums. I stick with NGKs and AC Delcos

    that tells you it has vacuum advance
     
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