25hp Yamaha 2 strk RPM Question

Discussion in 'Outboard Maintenance' started by fredslann, Oct 14, 2009.

  1. Oct 14, 2009 #1
    fredslann

    fredslann I Love microskiff.com!

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    First post..sorry its a question..I'll get decent post later ont he skiff....

    My motor still has throttle left and will not raise RPMs...it sounds like it may have a over rev limiter...does this engine have one of those? It does it fully loaded a tad less but with me and a full load, I have about a good 6th of twist left on the throttle...

    Thanks
     
    fredslann, Oct 14, 2009
    fredslann, Oct 14, 2009
  3. Oct 14, 2009 #2
    Gramps

    Gramps Living & Dying in 3/4 Time

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    Lots of questions... But first get a tach for the motor www.tinytach.com or post your location, someone may let you borrow theirs.

    What year is the motor? It very well could have a rev limiter. More details please!


    Oh and welcome! Johnny Black on the rocks please!
     
    Gramps, Oct 14, 2009
    Gramps, Oct 14, 2009

  5. Oct 14, 2009 #3
    Brett

    Brett > PRO STAFF <

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    Guessing at rpm's is not allowed!   ;)

    Install a tachometer, find out what's really happening.
    If you are over-revving, you'll probably need a different prop.
    One with more pitch.
     
    Brett, Oct 14, 2009
    Brett, Oct 14, 2009
  6. Oct 14, 2009 #4
    fredslann

    fredslann I Love microskiff.com!

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    Thanks for the replys so far...and just about to order the tach..(TT226R-2XLR I think this is the one).
    It's a 1995...and sittin on a 16' alumaweld. Eastern Nc is where I'm located.

    thanks
     
    fredslann, Oct 14, 2009
    fredslann, Oct 14, 2009
  7. Oct 14, 2009 #5
    tom_in_orl

    tom_in_orl Founder of Microskiff, Member of the Gheenoe Army

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    Welcome to the forum [smiley=beer.gif]

    The good news it that I think you have recognized an issue. The bad news is there is not enough data to figure out what that issue is. The guys are right. A tach is the next step. But there are a few things I can share until it arrives.

    - Yes there is a rev limiter in newer Yamaha 25 2 strokes. You will know it when you hit it. The motor goes from sounding like a smooth running high RPM engine to a skipping sound when you hit the max RPMs allowed by the rev limitter. Its an unmistakable sound. My motor was produced in 2007 and hits it at 6500.

    - If you have a GPS you will can guestimate how close you are to peak performance based upon some know information. A light weight skiff with an efficient bottom should run around 30-31 MPH if you have a good 10 1/4" 12 pitch 3 blade prop and the motor is set at an optimal hight on the transom. If you pitch is less and you may get to 30-31 MPH but with higher RPMs.



    -
     
    tom_in_orl, Oct 14, 2009
    tom_in_orl, Oct 14, 2009
  8. Oct 14, 2009 #6
    fredslann

    fredslann I Love microskiff.com!

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    Yup..that is the sound it's making...
    By GPS its making 25mph WOT...or as WOT as the limiter will allow. Assuming the limiter's values are the same I've got a good bit of bite that needs to be added to the next prop. The hull weigh loaded without me in is close to 480lbs. This is a 1987 alumaweld (now Xpress Boat Co.) that has some V and gets decent lift on plane. I ordred the little Tach this afternoon..

    25 feels pretty good!!! Don't know if I can squeeze out 30 but that'd be ahoot if I could. Pullin the prop tomorrow at the shop to see what the pitch on that one is...I'll post on that and see where that leads next.

    Great site...thanks much for the help so far!

    The Grassrat
     
    fredslann, Oct 14, 2009
    fredslann, Oct 14, 2009
  9. Oct 17, 2009 #7
    fredslann

    fredslann I Love microskiff.com!

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    Took off the prop and its a 9 7/8" X 9..with RPM's hittin the limiter @ 6000....with throttle left over...is it better to add one or two inches of pitch. Seems like I read a rule of thumb of 1" for every 200 RPM. And I think I'd like to hit the 5600 zone on the tach...does that sound about right to ya'll?

    One other performance Q. The cavitation place is just about 1.25 inches below the bottom of the hull...with no jackplate should it be at least level wit the bottom to get a tad more perfomance...It really handles well in turns with no blowout in the current height and I'd have to loose that aspect.

    And recomdation on SS props..
    Thanks again
     
    fredslann, Oct 17, 2009
    fredslann, Oct 17, 2009
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