40 tohatsu missing while at full throttle

Discussion in 'Outboard Maintenance' started by James4302, Jun 25, 2011.

  1. James4302

    James4302 Chasin' tails

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    I'm new to the outboards/skiffs I'm in the process of trying to prop my boat and am starting to feel how the boat responds to different things but it is missing while at full throttle granted I know I don't have the right prop on I'm only getting 5200-5400 rpm's but I can still feel the miss I will only miss about 200 rpm slightly like a 1 second intervals... Any suggestions would be greatly appreciated?

    Prop 11" 3 bl 12pitch. Only getting 23.5 @ 5200-5400 on a 4 on the Jack plate before I loose water pressure and then get an alarm for high temp. 16' waterman with tunnel...

    Thanks in advance for all of your input.... James
     
  2. Brett

    Brett > PRO STAFF <

    Are you sure you aren't activating the rev limiter?
    Aerated water in the tunnel will let the prop spin up
    and that will cause the rev limiter to kick in until the prop bites clean again.
     

  3. James4302

    James4302 Chasin' tails

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    I will test it in the am.... I will run the prop in clean water on the JP at 0. On a good day with that prop I have only seen 5400rpm the motor is rated at 5150-5850 just asking- do you think the rev limiter would kick in not getting above 5400? Just askin'?
     
  4. Brett

    Brett > PRO STAFF <

    Interesting thing most don't know about tachs,
    many are not accurate, right out of the box.
    It's recommended they be calibrated before use.

    http://www.se-r.net/electrical/top_speed_speedo.html
     
  5. Gramps

    Gramps Living &amp; Dying in 3/4 Time

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    It sounds like you've got a lot of cup in the prop. What make and material is the prop? I'm currently running a custom cupped 13 pitch and only hitting 5400rpm, I'm talking down hill with the wind to get there but the 13 pitch stocker & an 11 pitch cupped prop ran out to 5800 without ever hitting the rev limiter.

    With that said, I'd have to side with Brett, it sounds like you are hitting the rev limiter. Tiny-Tach's are a good option but $50, maybe a local Tohatsu dealer can hook up his computer & data log for comparison? Otherwise I'd suggest finding another tachometer to verify the operation of the OEM unit.
     
  6. James4302

    James4302 Chasin' tails

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    Ok... I started to look in to this issue... I decided I would start with the spark plugs... Bang!!! I found it... My top plug was missing all of the ceramic all the way down to the threads... 12.00 fix... Man I am relieved...
     
  7. makin moves

    makin moves Well-Known Member

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  8. Gramps

    Gramps Living &amp; Dying in 3/4 Time

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    Very cool. It's always a relief when something on the boat is cheap to fix. ;D So, when we fishin?
     
  9. James4302

    James4302 Chasin' tails

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    Well.... I just thought I had the problem solved after a few months of trying to get it proper out I was talking with the guy I bought it from and he said he was getting low 30's out of that motor and I could only get 25.2 on an incoming tide...

    Well I have done a hillbilly compression test on it where u pull off the plug off the cyl and crank it over with your finger covering the plug hole on the top cyl and super low compression compared to two other ones.  I am thinking that the guy knew it had some issues so he would push it over on someone else... Can anyone ball park me on specifics on what this mint be. 

    Best cast senior blown head gasket???

    Worst case senairo busted ring and lots of scaring and piston damage???

    When is being a boat owner fun??? Ever??? ;D ;D ;D
     
  10. Brett

    Brett > PRO STAFF <

    Get a compression gauge, thumb test is fun,
    but not accurate enough for diagnosis.
     
  11. mmjamp

    mmjamp I Love microskiff.com!

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    If it is a 4stroke motor it could be many things, head gasket, intake valve, exhaust valve, rings, etc. Only way to tell is pull the head. But like mentioned before get a good guage and test it first.