[Question] Lost 500 RPM in my Jetboat
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Lost 500 RPM in my Jetboat

  1. #1
    BTC cardcarrying member sunkisst's Avatar
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    Default Lost 500 RPM in my Jetboat

    I have always run a Davis Unified Ignition (DUI) HEI distributor in my BBC powered boat. This past winter I installed a Mallory Nitrous 6 box, PN6865M. Yesterday I put it in the water to test and it seems it will only turn 5500 RPM. Last year, 6K. Now, the tachometer was triggered off the negative side of the coil with the DUI unit. Mallory has the trigger coming out of the box on the yellow wire. The installation instructions also read that "This output produces a 12 volt, square wave". Is it a possibility that my tach is just not interpreting this as it did a signal from the neg. side of the coil? Got my head spinning....

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    Senior Member jockorace's Avatar
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    Perhaps your previous setup had an affliction we call......Happy Tach!? Sorry, I just couldn't stop myself. I have no idea what's wrong, cuz I'm old and simple and run a cable driven tach off my v-drive. Jocko

  4. #3
    BTC cardcarrying member sunkisst's Avatar
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    Now, it's unhappy tach.

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    Senior Member Oldsquirt's Avatar
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    After adding the new ignition box, did you recheck base and total timing?
    What did you set the rev limiter to?
    Did you set up the nitrous timing retard? If so, is it possible it is wired in such a way that it is activating even though you aren't triggering the nitrous?

  7. #5
    BTC cardcarrying member sunkisst's Avatar
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    I did recheck base & total. Rev limit is set at 7k. I even activated the retard to check it without the nitrous can open or the fuel pump energized. The retard works perfectly.
    Last edited by sunkisst; 05-15-2016 at 11:04 AM.

  8. #6
    steelcomp was here
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    Do you have any other references as to how the boat is running? GPS maybe? 500 rpm is a lot...should be pretty obvious if the tach is off or you're down on power for some reason.
    If God is your co-pilot, change seats!
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    Urban Cougar Trapper Gopher711's Avatar
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    What's your supply voltage to Ign box ? Battery good ? I've seen MSD 7 boxes play similar games when they were not getting needed voltage supply .
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    It's kind of shooting in the dark, but since the boat was sitting for a time, could it be a dirty carburetor?
    Sitting boats often accumulate dirt around the air bleeds, I'd try some carburetor cleaner particularly on the High speed bleeds, unless the carburetors have been recently cleaned.

  11. #9
    BTC cardcarrying member sunkisst's Avatar
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    Quote Originally Posted by scott foxwell View Post
    Do you have any other references as to how the boat is running? GPS maybe? 500 rpm is a lot...should be pretty obvious if the tach is off or you're down on power for some reason.
    Scott, the boat seems to run as it should. No, I have not checked it with GPS. Gonna do that this Friday. As I was leaving the water yesterday, this came to mind. My bad...

  12. #10
    BTC cardcarrying member sunkisst's Avatar
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    Quote Originally Posted by Gopher711 View Post
    What's your supply voltage to Ign box ? Battery good ? I've seen MSD 7 boxes play similar games when they were not getting needed voltage supply .
    I did not check this. I do however know the charging system on the boat functions as it should. My battery, Interstate 24M-XHD is fully charged. The main power to the box is coming from the "Big" wire. I check the charging system after every start up. Thanks for the input. Anything else?
    Last edited by sunkisst; 05-15-2016 at 04:50 PM.

  13. #11
    BTC cardcarrying member sunkisst's Avatar
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    Quote Originally Posted by jimsplace View Post
    It's kind of shooting in the dark, but since the boat was sitting for a time, could it be a dirty carburetor?
    Sitting boats often accumulate dirt around the air bleeds, I'd try some carburetor cleaner particularly on the High speed bleeds, unless the carburetors have been recently cleaned.
    Clean, I wish this were the issue. Dammit.

  14. #12
    BTC cardcarrying member sunkisst's Avatar
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    Default My "unhappy tach"

    Gentlemen, I thank you for your input this evening. The first thing I am going to do is pull some wiring to my HEI Davis unit and check RPM with a stand-alone inductive tachometer. Then I will compare that with my Snap-On MODIS inductive unit. Who knows what I will find. Again, thanks for your time. Jack. I'm thinking there is a problem in the sig to tach. The engine hits really well when the button is pushed. Picks up 400 RPM on a 100 HP pill. Back to 6k... Wrong answer... That is where I started before I started this thread.
    Last edited by sunkisst; 05-15-2016 at 05:44 PM.

  15. #13
    steelcomp was here
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    Did you try putting the tach back on the - coil wire?
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  16. #14
    BTC cardcarrying member sunkisst's Avatar
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    I haven't tried anything yet.

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