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Carb CFM question

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  • Carb CFM question

    I have an Alura 35 with 2 Marine Power 350’s. My mechanic says the carbs (Holley)are 600 CFM and should be 650. In fairness he is not trying to sell me carbs, his point is the engines will run too lean and suffer. The boat is new to me, low time on rebuilt engines, I would replace the carbs if I’m driving a time bomb.
    is this accurate or a conspiracy theory? Thanks in advance for your advice

  • #2
    Your 600 cfm and perfect for the motors. The stock 350 turning a max of 5000 rpms only needs 430 cfm at WOT, and uses less then that at cruise. A race model (one with many mods) will be in the 600 cfm area.
    94 Formula 27PC Custom 509MPI MEFI3 , B3 XR
    95 Rinker 232 w/ 7.4L Carb 0F425011 B1 0F486471
    07 Seadoo GTX
    Merc Adults Only VP Adults Only
    G-Dad always said "First Liar doesn't stand a chance"


    • #3
      Myself, I prefer the Holley 650 marine carb, not because it is bigger(as AllDodge pointed out, the 600, is already more than enough), but because it is a Spreadbore, and the smaller Primaries will help the bottom end.
      i don’t know what intake manifold your engine has, if it is a spreadbore manifold, it would need an adapter to mount the 600, which is a square bore carb. Most adapters are not very good and can cause mixture/distribution issues. Perhaps that is what your mechanic was referring to


      • #4
        AFR is governed by the jets and venturies.

        AFR under acceleration is also affected by the accelerator pump, nozzle, and power valve

        with a properly setup carb, you dont run lean, and your "mechanic" should know that

        FYI, a 5.7 with a 500cfm carb in my buddies spec racer will put out a lot more power than what is in your boat an turn a much higher RPM

        Properly set up, 600 cfm is more than enough
        Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

        1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - https://forums.iboats.com/forum/owner...988-rogue-2420

        Past Boats
        1970 Wooster Hellion - Merc 9.8
        2002 SeaRay 190BR - 5.0 - A1G2 - "Cheeseheads in Paradise"
        1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

        What’s behind you doesn’t matter.Enzo Ferrari


        • #5
          Thanks guys! It sounds like I’ll sit tight until my next Ethanol issue and then consider 650’s...


          • #6
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            My 5.7 VP came with a Holley 500 CFM 2 bbl. When flow rated like a 4 bbl it flows about 370 CFM, which is just about perfect for a 5 litre turning 5000 rpm, but a few CFM shy of what a 5.7 needs. I have put the Holley Spreadbore 650 on it and gained about 550 - 600 rpm. Adding 10 % more Pitch raised my speed to the upper 60s. I am currently using the 2bbl as it burns a bit less gas. Takes about an hour to swap the carbs, as the intake is a 4bbl with an adapter.
            When the 2bbl is installed, at WOT, the engine is pulling about 4" manifold vacuum, which means underpropping is risky as the manifold vacuum will be even higher if I pull back on the throttle to keep the RPMs within spec. High enough to close the Power Valve leaning out the engine at high speed