Idle tuning for a BBC w/ a Blower
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Idle tuning for a BBC w/ a Blower

  1. #1
    Senior Member cave's Avatar
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    Default Idle tuning for a BBC w/ a Blower

    We finished the build and installed the engine into the boat. Now we're trying to get the engine to idle. Can a blower motor idle at 1100 to 1500?

    We can get the engine to idle at 2500 to 3000 but anything under that and it stalls

    Here is what we working with,
    6-71 blower running -8.3 underdriven on a BBC 515 with 2 Holly 4150 850 Mechanical secondary carbs. No chokes. Power Jets blocked off. Jetted 72 primary 70 secondary. Fuel pressure at 7 lbs. MSD 6al box 6k pill w/ MSD Marine Dist. Timing at idle is set at 20° advanced (ran smoothest at this point), 32 total. When I gun the engine, the timing doesn't move past 32° total.

    Plugs were soot black. Electrode was tan. Insulator base was black.

    Can a blower motor idle at 1100? Am I wasting my time trying to achieve this? I know once I install the pump and get it in the water I'm going to loose a few hundred RPMs but at 2500 to 3k I'm asking for a no wake ticket.
    Takin it home

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    Senior Member pacman's Avatar
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    first thing to do is swap the pulleys ,if your going to underdrive a 671 u sould use it as anchor, #2 set the timing at 30 and lock it in

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    Senior Member cave's Avatar
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    Will do. we talked about this last night with Kyle. That will make it +9.1. Pacman How about a lower Idle. Is that attainable?
    Takin it home

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    Lord of the Drinks Havasu Hangin's Avatar
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    My motors are similar to that build. Mine idle at 1,000RPMs, no problem. Mine are happiest locked at 30 degrees, but I had to put start/retard boxes on, as I was going through starters.

    I think you may have some carb setup issues. Sounds lean? Where are your idle mixture screws at?
    "Warming the globe...16 cylinders at a time."

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    Mine is at about 950 out of gear and 800 or so in gear. I have a outdrive boat so it has to be shiftable.

    My timing is locked out at 30 degrees and I had to use a high tq starter. A 6:71 on 500+cubic inches is not going to do much, but I don't think that is your idle problem. Is it surging?

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    IMO, ur running those carbs very lean, especially with the pv's blocked off...u should be at least 80 squared to start & work from there...set the timing as mentioned above or a little more 32-36*...i see no reason why it wont idle at 1k...also...if u overdrive the blower that much, ur putting alot of strain on parts for longevity...its not a race boat!!!!...just my .02 cents, but, what do i know

    FastRat

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    Senior Member cave's Avatar
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    Thanks for all the suggestions. I started to swap the pulleys but decided to keep it under driven and as an anchor for now HH was right about the carbs being a off. I finally thought about this like I would on my Quads. I jetted up to 79 primary then 78 Secondaries. Then adjusted the idle circuit or the screws on the side of the metering block. I opened them up to 2 turns out then the engine smoothed out. I didnt block out my MSD yet but after I snugged the dist up and started the bitch up she purrred She is stubborn but after a little tinkering it fire right up.

    I know if I keep tinkering I'll get it all figured out. Just atap of the peddle and that engine come alive. I hope its like that in the water

    No surge but she runs real nice where its at.
    Again thanks for all the advice. I'm one happy Mo Fo
    Takin it home

  10. #8
    Senior Member Tony Skagen's Avatar
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    What are your air/fuel screws at? The serge you hear in a blower motor is a rich condition,not that thats bad hell serge is cool turn the screws in for less serge out for more.Good starting point is all the way in and 2 turns out if the carbs are stock and the air bleeds have not been played with! I agree with the locked out at 30*!And I always square jet with no PVs.www.skagenboats.com

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    Senior Member The Violater's Avatar
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    If you have an MSD distributer, You can put a bushing in it for your base Idle timing. I run mine at 15 degrees advance idle and 30 degrees total advance at 3000 rpm's. blower 15% overdrive. Idles at 1000 in neutral, 7 to 8 in gear idling in a v-drive. Starts and idles like an old Chevy Caprice !!!
    / The Few The Proud TheBlown

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    mo balls than $cents$ IMPATIENT 1's Avatar
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    Quote Originally Posted by cave View Post
    Thanks for all the suggestions. I started to swap the pulleys but decided to keep it under driven and as an anchor for now HH was right about the carbs being a off. I finally thought about this like I would on my Quads. I jetted up to 79 primary then 78 Secondaries. Then adjusted the idle circuit or the screws on the side of the metering block. I opened them up to 2 turns out then the engine smoothed out. I didnt block out my MSD yet but after I snugged the dist up and started the bitch up she purrred She is stubborn but after a little tinkering it fire right up.

    I know if I keep tinkering I'll get it all figured out. Just atap of the peddle and that engine come alive. I hope its like that in the water

    No surge but she runs real nice where its at.
    Again thanks for all the advice. I'm one happy Mo Fo
    cave, lock your dizzy before goin much further. if you tune the carb now at idle, then lock the dizzy, its gonna idle high again lock the timing as suggested, there's no reason for a mechanical advance in a blown boat imo.

    Dare to be different, if it turns out great you can claim you planned it that way.

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    Senior Member cheyenne580's Avatar
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    Jimmy with all that work you have in this motor.Don't run it lean fatten this thing up to at least 82's or 84's squared.Are you running a pyro on #7 cylinder?Number 7 and #4 will be the lean cylinder's.Go fat on the fuel mixture.Rather be to fat than to lean and melt it down.One more thing i'd like to see ya run the carb pressure at 8 psi not 7.


    Jim S

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    Lord of the Drinks Havasu Hangin's Avatar
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    Quote Originally Posted by cave View Post
    She is stubborn but after a little tinkering it fire right up.
    Sounds like you have it figured out- congrats.

    If I peddle mine while cranking, I run the risk of it popping back through the carbs. Mine start best by tapping the gas, then cranking it over- there is a small delay (while that gas gets through the blower), but it fires everytime.

    There's a fine line between fat and happy (surging) and lean enough to idle in gear without a surge or dying. Mine even surge differently sometimes depending on the weather...lol.

    Time to get her wet.
    "Warming the globe...16 cylinders at a time."

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  15. #13
    Senior Member cave's Avatar
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    Yup I learn that too. A tap is better than a full pump. Tap it, wait a moment then fire her up. Thanks for the help.
    Takin it home

  16. #14
    senior member turbo wog's Avatar
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    Quote Originally Posted by cheyenne580 View Post
    Jimmy with all that work you have in this motor.Don't run it lean fatten this thing up to at least 82's or 84's squared.Are you running a pyro on #7 cylinder?Number 7 and #4 will be the lean cylinder's.Go fat on the fuel mixture.Rather be to fat than to lean and melt it down.One more thing i'd like to see ya run the carb pressure at 8 psi not 7.


    Jim S
    i agree. new carbs & motor , no idea if it's gonna lean out at wot.get a pyromiter on motor! you only lean out too much ,once! the jets do seem small

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