Lawnmower idle question

Discussion in 'Electronic Repair' started by spongehead, May 11, 2005.

  1. spongehead

    spongehead Guest

    I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
    idle revvs up and down constantly. Never idles normal. The spring on
    the shaft idle looks good. Any help on what this might be would be
    greatly appreciated.
     
    spongehead, May 11, 2005
    #1
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  2. spongehead

    EL Guest

    First you want to make sure all the phase alignment coils are peaked and the transratiometric detector is properly balanced.
    Then inject a circular wave about 1A P-P into TP31C and listen for feedback around the modulator. If any is found, replace
    F4.

    Eric Law

    "spongehead" <> wrote in message news:...
    >I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
    > idle revvs up and down constantly. Never idles normal. The spring on
    > the shaft idle looks good. Any help on what this might be would be
    > greatly appreciated.
    >
     
    EL, May 11, 2005
    #2
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  3. spongehead

    cnctut Guest

    spongehead wrote:
    > I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
    > idle revvs up and down constantly. Never idles normal. The spring

    on
    > the shaft idle looks good. Any help on what this might be would be
    > greatly appreciated.



    Spongehead--

    Recommend cleaning the air intake sponge first then let us know if the
    problem still exists. I assume you have the 99 dollar Walmart mower
    with no variable throttle.

    Tut
     
    cnctut, May 11, 2005
    #3
  4. spongehead

    Bob Shuman Guest

    I agree that cleaning the air intake and replacing the filter may help. I'd
    also suggest making sure the carburetor intake is clean and the mixture
    adjustment is set properly.

    Also make sure all the linkage spring are in place too.

    If I had to guess, I'd say that the mixture may be too rich (or lean) and
    the engine's governor is keeping it alive.

    Good luck!

    Bob

    "cnctut" <> wrote in message
    news:...
    >
    > spongehead wrote:
    > > I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
    > > idle revvs up and down constantly. Never idles normal. The spring

    > on
    > > the shaft idle looks good. Any help on what this might be would be
    > > greatly appreciated.

    >
    >
    > Spongehead--
    >
    > Recommend cleaning the air intake sponge first then let us know if the
    > problem still exists. I assume you have the 99 dollar Walmart mower
    > with no variable throttle.
    >
    > Tut
    >
     
    Bob Shuman, May 11, 2005
    #4
  5. spongehead

    Ken Weitzel Guest

    spongehead wrote:

    > I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
    > idle revvs up and down constantly. Never idles normal. The spring on
    > the shaft idle looks good. Any help on what this might be would be
    > greatly appreciated.


    Hi...

    Happens that I have one; as well as still have the manual...
    hard to believe 'cause it's about 15 years old :)

    Anyway, still runs perfectly, so I'd be happy to take a couple
    of pics of the springs and carb linkage etc, and mail them to
    you if you like?

    Could also scan the applicable pages of the manual if you
    think it would be helpful.

    Take care.

    Ken
     
    Ken Weitzel, May 11, 2005
    #5
  6. spongehead

    kip Guest

    Carb KIT needed....

    "spongehead" <> wrote in message
    news:...
    >I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
    > idle revvs up and down constantly. Never idles normal. The spring on
    > the shaft idle looks good. Any help on what this might be would be
    > greatly appreciated.
    >
     
    kip, May 11, 2005
    #6
  7. spongehead

    quietguy Guest

    I would guess that the carby needs cleaning, or the fuel filter is clogged

    David

    Bob Shuman wrote:

    > I agree that cleaning the air intake and replacing the filter may help. I'd
    > also suggest making sure the carburetor intake is clean and the mixture
    > adjustment is set properly.
    >
    > Also make sure all the linkage spring are in place too.
    >
    > If I had to guess, I'd say that the mixture may be too rich (or lean) and
    > the engine's governor is keeping it alive.
    >
    > Good luck!
    >
    > Bob
    >
    > "cnctut" <> wrote in message
    > news:...
    > >
    > > spongehead wrote:
    > > > I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
    > > > idle revvs up and down constantly. Never idles normal. The spring

    > > on
    > > > the shaft idle looks good. Any help on what this might be would be
    > > > greatly appreciated.

    > >
    > >
    > > Spongehead--
    > >
    > > Recommend cleaning the air intake sponge first then let us know if the
    > > problem still exists. I assume you have the 99 dollar Walmart mower
    > > with no variable throttle.
    > >
    > > Tut
    > >
     
    quietguy, May 12, 2005
    #7
  8. "spongehead" <> writes:

    > I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
    > idle revvs up and down constantly. Never idles normal. The spring on
    > the shaft idle looks good. Any help on what this might be would be
    > greatly appreciated.


    Depending on the model, there may be an idle adjustment, but as
    others have said, the carb either may need cleaning or a refurb.

    --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
    Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
    +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
    | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

    Note: These links are hopefully temporary until we can sort out the excessive
    traffic on Repairfaq.org.

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name is included in the subject line. Or, you can
    contact me via the Feedback Form in the FAQs.
     
    Sam Goldwasser, May 12, 2005
    #8
  9. spongehead

    Ol' Duffer Guest

    In article <>,
    says...
    > I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
    > idle revvs up and down constantly. Never idles normal. The spring on
    > the shaft idle looks good. Any help on what this might be would be
    > greatly appreciated.


    The speed control servo on these does not have much phase
    margin, and may even have a pole in the right half plane.
    Fuel mixture errors can easily add enough loop delay to
    push the system into oscillation. Accumulation of gum and
    varnish and the tendency of the carburetors's soft metal
    to flow with age and develop leaky gaskets all contribute
    to long-term performance drift.

    In layman's terms; adjust, repair, or replace the carburetor
    as needed. A professional who works on these often enough
    to be familiar should be able to diagnose your specific
    needs in very short order, but will probably expect to be
    paid for his expertise.
     
    Ol' Duffer, May 12, 2005
    #9
  10. spongehead

    Asimov Guest

    "cnctut" bravely wrote to "All" (11 May 05 10:52:14)
    --- on the heady topic of "Re: Lawnmower idle question"

    From the small motor faq:

    " Air vane governor. The air flow provided by the
    flywheel/blower passes by a plate which is coupled to the
    carburetor throttle plate acting against a spring force.

    Flyweight governor. A spinning gear assembly with a pair of
    weights is driven by the camshaft. As the centrifugal force
    of the weights increases, they move a sleeve which presses
    against a lever whose shaft passes through the crankcase.
    This lever then operates the carburetor throttle plate
    against a spring force.
    With both types, increasing the spring force will increase the
    engine speed.
    "

    Hunting is often caused by a defect in the governor system. Check the
    blade (plastic?) of the governor paddle is free to move and that the
    cam to which it connects has the proper spring tension. As the motor
    speeds up the flywheel impellor blows air against the blade and cuts
    the trottle down. When the motor slows down the blade drops back and
    the spring returns the throttle higher.

    Often grass goop gets into the governor paddle axle and may stick or
    seize. You might need to remove the pull cord/motor cover and perhaps
    the carb/fuel-tank to get at it. Thankfully you won't need a puller to
    get the flywheel off. If you ever do, inspect the key pin in the
    shaft, as it must be snug, not worn. This could also cause hunting.

    OTOH, if instead of the fan your motor has a governor linkage coming
    out the bottom of the motor, then the linkage may have slipped.
    Hunting means in this case it may be opening the throttle too much.
    There is also a spring arrangement in this type to the trottle cam.

    A*s*i*m*o*v


    cn> From: "cnctut" <>
    cn> Xref: aeinews sci.electronics.repair:47853

    cn> spongehead wrote:
    > I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
    > idle revvs up and down constantly. Never idles normal. The spring

    on
    > the shaft idle looks good. Any help on what this might be would be
    > greatly appreciated.



    cn> Spongehead--

    cn> Recommend cleaning the air intake sponge first then let us know if the
    cn> problem still exists. I assume you have the 99 dollar Walmart mower
    cn> with no variable throttle.

    cn> Tut


    .... The first rule of intelligent tinkering is to save all the parts.
     
    Asimov, May 12, 2005
    #10
  11. spongehead

    Asimov Guest

    "EL" bravely wrote to "All" (11 May 05 13:52:03)
    --- on the heady topic of "Re: Lawnmower idle question"

    Eric,

    Don't quit your day job to become a stand up comedian.

    A*s*i*m*o*v


    EL> From: "EL" <>
    EL> www.newshosting.com Xref: aeinews sci.electronics.repair:47856

    EL> First you want to make sure all the phase alignment coils are peaked
    EL> and the transratiometric detector is properly balanced. Then inject a
    EL> circular wave about 1A P-P into TP31C and listen for feedback around
    EL> the modulator. If any is found, replace F4.

    .... Don't mess with a man that has a rubber chicken.....
     
    Asimov, May 12, 2005
    #11
  12. spongehead

    spongehead Guest

    Thanks for all the replies, I will be taking the carb off for a good
    cleaning and replace the air filter. I'll let you know how it goes.
    Thanks again!!
     
    spongehead, May 12, 2005
    #12
  13. spongehead

    kip Guest

    Change the needle valve..
    Get a Carb KIT.


    yikes

    "spongehead" <> wrote in message
    news:...
    > Thanks for all the replies, I will be taking the carb off for a good
    > cleaning and replace the air filter. I'll let you know how it goes.
    > Thanks again!!
    >
     
    kip, May 12, 2005
    #13
  14. "kip" <> writes:

    > Change the needle valve..
    > Get a Carb KIT.


    Unless he tightened the needle valve with a pair of Vice-Grips, needle
    valves don't go bad, but may get dirty.

    Clean the carb first.

    --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
    Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
    +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
    | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

    Note: These links are hopefully temporary until we can sort out the excessive
    traffic on Repairfaq.org.

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name is included in the subject line. Or, you can
    contact me via the Feedback Form in the FAQs.
     
    Sam Goldwasser, May 12, 2005
    #14
  15. spongehead

    kip Guest

    Yes But its just easier to put a New one in
    cause the chances of him ruining the valve is pretty good.
    We are not talking big coin here ..
    Just done my Lawnboy ..
    cheers
    kip
    "Sam Goldwasser" <> wrote in message
    news:...
    > "kip" <> writes:
    >
    >> Change the needle valve..
    >> Get a Carb KIT.

    >
    > Unless he tightened the needle valve with a pair of Vice-Grips, needle
    > valves don't go bad, but may get dirty.
    >
    > Clean the carb first.
    >
    > --- sam | Sci.Electronics.Repair FAQ Mirror:
    > http://repairfaq.ece.drexel.edu/
    > Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
    > +Lasers | Sam's Laser FAQ:
    > http://repairfaq.ece.drexel.edu/sam/lasersam.htm
    > | Mirror Sites:
    > http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html
    >
    > Note: These links are hopefully temporary until we can sort out the
    > excessive
    > traffic on Repairfaq.org.
    >
    > Important: Anything sent to the email address in the message header above
    > is
    > ignored unless my full name is included in the subject line. Or, you can
    > contact me via the Feedback Form in the FAQs.
     
    kip, May 12, 2005
    #15
  16. spongehead

    cnctut Guest

    spongehead--

    Take off the air cleaner and run the engine--if the problem corrects
    itself--clean or replace the old foam filter. Make sure the carb is
    securely mounted to the engine.

    Does your mower have an adjustable throttle control on the handle or
    not?
     
    cnctut, May 12, 2005
    #16
  17. spongehead

    spongehead Guest

    Yeah, it has a lever which is never used because its stuck, but can
    adjust it right on the carb itself.
     
    spongehead, May 12, 2005
    #17
  18. spongehead

    spongehead Guest

    I replaced the air filter and even let it run without it, but still
    revvs up and down. Next will remove the carb and I actually have
    another carb from my older lawnmower which I'll swap.
     
    spongehead, May 12, 2005
    #18
  19. spongehead

    cnctut Guest

    Asimov

    Not sure what you meant by "XX bravely wrote" but--Tut "bravely writes"
    more again.

    Briggs makes a 3.5 HP engine matted to a 20 Murray mower sold by
    Walmart for $99 US. My experience is that if the air cleaner (which
    uses a sponge filter) is dirty--which happens frequently--the engine
    rpm will search for neutral idle. Perhaps because the mixture is over
    rich and the engine Air Fuel ratio changes on the "fish hook" pattern
    as engine speed changes--or perhaps because manifold vacuum fluxuates
    excessively causing irratic flapper valve overshoot action--not sure.
    Cleaning the sponge is quick, easy, cheap--and requires no $$.

    The 3.5 Briggs engine is also sold in a throttle free
    configuration--meaning no throttle control for the operator. In this
    case, a small thin bar toward the front of the engine holds the idle
    springs at the proper tension thus preventing the engine from
    unneccessary RPM searching. The bars location is just at the proper
    height to be bent rearward when the mower is pushed under low
    brush--this reduces the idle spring tension and the mower will RPM
    search.

    Tuts suggestions/comments.

    1. Clean the filter sponge and let us know if the problem exists.
    2. Does your mower have an adjustable throttle control on the handle?

    The answer may not be here--but a great starting place--short of carb
    teardown or flywheel removal--wouldn't you agree?

    Tut
     
    cnctut, May 12, 2005
    #19
  20. "Asimov" <> writes:

    > "cnctut" bravely wrote to "All" (11 May 05 10:52:14)
    > --- on the heady topic of "Re: Lawnmower idle question"
    >
    > From the small motor faq:
    >
    > " Air vane governor. The air flow provided by the
    > flywheel/blower passes by a plate which is coupled to the
    > carburetor throttle plate acting against a spring force.
    >
    > Flyweight governor. A spinning gear assembly with a pair of
    > weights is driven by the camshaft. As the centrifugal force
    > of the weights increases, they move a sleeve which presses
    > against a lever whose shaft passes through the crankcase.
    > This lever then operates the carburetor throttle plate
    > against a spring force.
    > With both types, increasing the spring force will increase the
    > engine speed.
    > "
    >
    > Hunting is often caused by a defect in the governor system. Check the
    > blade (plastic?) of the governor paddle is free to move and that the
    > cam to which it connects has the proper spring tension. As the motor
    > speeds up the flywheel impellor blows air against the blade and cuts
    > the trottle down. When the motor slows down the blade drops back and
    > the spring returns the throttle higher.


    One note though: He has an idle speed problem where the governor is
    likely not involved unless it is way misadjuated.

    --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
    Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
    +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
    | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

    Note: These links are hopefully temporary until we can sort out the excessive
    traffic on Repairfaq.org.

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name is included in the subject line. Or, you can
    contact me via the Feedback Form in the FAQs.
     
    Sam Goldwasser, May 12, 2005
    #20
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