Mercury 40hp 2stroke revs high rpms by itself

Discussion in 'Outboards' started by Joe Smith, May 19, 2021.

  1. Joe Smith
    Joined: May 2021
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    Location: Marietta Ga

    Joe Smith New Member

    My 1982 mercury 40hp 2stroke starts and idles for short time, then the motor revs high rpms by itself and I have to shut key off. Have to prime bulb to start again. Any suggestions what could cause this problem
     
  2. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    Older Mercs that I’ve dealt with have inevitably had carbeurator problems, possibly related to ethanol fuel, and/or fuel evaporating and leaving residue, and exacerbated by low quality aluminum parts corroding.
    I’d suggest going through the entire fuel system.
     
  3. Joe Smith
    Joined: May 2021
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    Location: Marietta Ga

    Joe Smith New Member

    I noticed the motor is not pissing water like it should, could motor getting hot possibly have something to do with it
     
  4. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    Cooling water is vital.
     
  5. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Problem with 40HP Mercury - General Discussion Forum | In-Depth Outdoors https://www.in-depthoutdoors.com/community/forums/topic/ftlgeneral_1123363/

    and there are many more sites that discuss this issue. Probably worth checking water flow from a hose under the cowl to determine if the water pump is working. If it is, then it appears that an obstruction in the telltale line is the issue
    If you pull a hose off, choose one between the impeller pump and the thermostat.

    Re the high rpm

    Do you have to squeeze the bulb before quite a bit before it starts the first time? Do you have to leave the choke on say on its initial start up? When you say it revs high, for how long before you shut it down?

    It almost sounds like you pump the float bowl full, the engine runs the fuel level down, the engine leans out and the rpm increases. You shut it off. Squeeze the bulb, fill the bowl, and start over again
     
  6. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    .....and with shaft seals worn after many years, there may be air leaking into the crankcase, prolonging the "lean running phase".
     
  7. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    baeckmo's suggestion is the usual problem for 2 stroke engines, leading quickly to stuck rings to the cylinder or a burned thru piston dome, all due to high temperatures caused by the lean mixture.

    #1 cause in my opinion.
     
  8. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    As always, be guided by troubleshooting procedures outlined in a good workshop manual for your engine, but it sounds a lot like a sticking needle valve to me. To test that, have someone continue to gently squeeze the primer bulb after the engine starts, if the problem as described does not arise while this is being done, odds are it is a sticking needle valve
     
    Last edited: May 21, 2021

  9. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    The need to re-prime to get it to start again, says fuel starvation, and the sudden increase in revs "all on its own", says it is something causing the bowl to empty of fuel, that could be a dodgy fuel pump, sticking needle valve, or some other impediment along the fuel line.
     
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