25 2 stroke idle rpm range / tiny tach

Discussion in 'Outboard Maintenance' started by el9surf, Sep 13, 2012.

  1. el9surf

    el9surf Well-Known Member

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    Just hooked up the tiny tach to my new merc sea pro.
    There are 2 cylinders on this motor just for reference. Max RPM per the manual are 5000-6000
    Idle RPM is not specified in the manual so not sure what I should be expecting.

    At first the tach was set at 360 degrees which = 1 spark per revolution. The wire was hooked up to the bottom spark plug wire. Idle rpm read 4000 - 4100rpms with the ear muffs hooked up on the hose.

    Reset tiny tach to 180 degrees which = 2 sparks per revolution. Wrapped once around both spark plug wires, per the technicians reccomendation at tiny tach. Idle reads 1860 -1900 rpms.

    Still seems like a high reading for idle.

    Any ideas or experience on how low should I set the tiny tach degree setting? 120 degree = 3 sparks per rev
    90deg = 4 sparks per rev
    60=6 sparks per rev
    45 = 8 sparks per rev
     
  2. anytide

    anytide Administrator Staff Member

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    i set mine to idle @ lowest rpm and still run smooth in gear.. :)
     

  3. TheBrazilNut

    TheBrazilNut Guest

    That's what you get they are indeed a POS. I have owned about a dozen and have never had one that read my RPM's correct. Super happy if it works just long enough for prop testing. I have also used a few other less expensive ones with the same results.

    Never owned a Sendek. I have been told a multimeter will display RPM's?
     
  4. DuckNut

    DuckNut Brandon, FL

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    There's 100 posts on here where everyone buy one, tries to set them up to no avail. Then they call the company and send it back to get the right one.

    I realy do not think they are reliable enough to trust for rpm. Nobody has ever complained that the hour meter doesn't work.

    Agree with Brazil = POS
     
  5. el9surf

    el9surf Well-Known Member

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    I just need the rpm function to work long enough to get the right prop.
     
  6. anytide

    anytide Administrator Staff Member

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    brand new motor?
     
  7. el9surf

    el9surf Well-Known Member

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    Yeah brand new got 6 hours on it.
     
  8. el9surf

    el9surf Well-Known Member

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    I guess worst case I have an expensive hour meter.
     
  9. anytide

    anytide Administrator Staff Member

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    yup
    you could post :
    load
    hull
    hp
    rpm
    someone might already know the best prop?
     
  10. el9surf

    el9surf Well-Known Member

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    Bob aka skinnydippin gave me some ideas on props but he has his motor higher up on a jack plate

    Hull is a shadowcast
    motor 25 hp 2 stroke mounted with cav plate in line with roof of tunnel.
    weight - approx 500lbs with fuel, motor, battery, gear, not including anglers.
    rpm's no clue
     
  11. Dillusion

    Dillusion devilray snob

    I have a tiny tach on my merc 15 2s, setting it to the 'correct' setting netted me double RPMs. I had to set it to the next setting down (180 I think?) to get it to work correctly.

    Now it DOES infact read correctly, but only with the cowling off. WIth the cowling on, it traps in some interfearence and reads +-300rpm higher than it should.

    Sucks but at least I can minus 300 from what I'm getting at WOT and know my RPMs lol

    Bob did extensive testing with the 25hp on a SC, I would go with what he says. Even though he had a jack plate and a heavily cupped prop, you will need a similar setup.

    There was a posting by Bob in the shadowcast thread somewhere that he states when mel used the stock prop it got the best performance, but the stock prop wasnt good when jacked up I believe.
     
  12. DuckNut

    DuckNut Brandon, FL

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    If skinny gave you his info then use it. The fact that it is 1-2" higher won't make a noticible difference.

    Let's say you do get the "correct" setting - how will you ever be able to know that it is giving the right reading? They just can't be trusted to be accurate - rather they are semi-reliable to give an approximation.
     
  13. el9surf

    el9surf Well-Known Member

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    Yeah I wasn't sure if the height was going to make a big difference or not.
    I took it out tonight and played with the settings on the tach. After some trial and error I got it figured out.
    It idles in neutral at 2000 rpms. Once I put it in gear at the lowest throttle possible the rpms drop to 980-1000 and max out WOt at 5350 which seems about right.
    The manual says max rpm's 5000-6000

    With an android gps app on my phone max speed topped out around 32mph. Right around 30 you can feel the boat starting to walk side to side tilll I engage the tabs and drop the bow a little.
     
  14. DuckNut

    DuckNut Brandon, FL

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    2000 is half throttle - too high

    Rev your truck to 2000 and slam it in gear - get the same result from your motor.

    Something is not right
     
  15. anytide

    anytide Administrator Staff Member

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    sorry if dumb ? - why you setting idle on new motor ?
     
  16. Dillusion

    Dillusion devilray snob

    My idle RPMs are like 750-800 (but my tinytach reads 900-1100 lol)
     
  17. Creek Runner

    Creek Runner Well-Known Member

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    All motors require an idle adjustment out of the box, if the dealer didn't do it shame on them.

    2000 is way to high you're going to need a LU if you don't fix it.

    Idle out of water in neutral should be about 900rpm
    In the water with back pressure should be about 800rpm, and in gear it should be between 650-800 depending were your stall point is.
     
  18. DuckNut

    DuckNut Brandon, FL

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    Which is it? Sounds like it still is not right.

    I don't believe the Sea Pro is not out of the box. I think it is new to him but used for a season in Canada.

    Save the aggrevation and send it back and use the proceeds to pay someone to get it right for you.
     
  19. el9surf

    el9surf Well-Known Member

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    Thanks for the feedback especially the rpm range from creek runner. I wish the owners manual would specify the whole RPM range rather than just the max rpm. I looked at both the merc and tohatsu manuals and coudln't find anything about the rpm's other than the max. Hopefully I caught this before there was any damage done.

    As much as everyone has mentioned the tiny tachs lack of accuracy it saved me a big headache. Having run a 4 stroke for the past 5 years I really had no sense of what normal was supposed to be going back to a 2 stroke. After the tach was hooked up and it read so high at idle I almost dismissed it as a complete misread on the tachs part. The only thing that had me thinking it could be right was the fact that it topped out on RPM's right where the manual said it should.


    Long story short I took the boat out tonight and adjusted the idle screw. Someone at the factory didn't count their turns correctly during assembly. I had to back it out almost one full turn. I ended up just above the stall point where it didn't want to shake and sputter. Neutral is right around 950 in the water. In gear at the lowest throttle is 850.
     
  20. anytide

    anytide Administrator Staff Member

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    awesome - if its smooth your good :) the lower the rpm before stumble the better for the LU :eek:
     
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