Why won't my lawnmower re-start when it's hot?

Discussion in 'The Pub' started by berniesidney, Aug 8, 2012.

  1. berniesidney

    berniesidney Senior Member

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    Starts right up when it's cold... but after mowing for a while... if I have to stop it to empty the bag or refuel it won't re-start until it has cooled off for a couple of hours.

    I checked the spark plug, the air filter, fuel line... it all looks good.

    Any suggestions?
     
  2. hank57

    hank57 Gold Supporting Member

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    Guessing and I don't understand them gas motors al that well but, once the engine is too hot it might be creating the right air/fuel mixture to start or fuel filter might be just plugged a little and the pumping of the fuel gets a kind of negative pressure like suction so no fuel moves into the carb.

    Like I said I don't much but I listen to Car Talk a lot....
     
  3. aynirar27

    aynirar27 ssshhh... it's around the corner Silver Supporting Member

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    any idea what brand of motor it has? Briggs and Stratton's have a reputation for compression issues when hot.
     
  4. Tuberattler

    Tuberattler Member

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    Hot start problems are almost always valve related. As the engine heats the valve grows a few thousands thus not closing 100%. Here's a test next time it's cold pull it over to gage how strong your compression is, then when it's hot and you shut it down now pull it over and gage how strong your compression is.

    My guess is that you have much more compression when cold than when hot... valve problem.

    Solution; make sure valve lash is to spec when cold. This is a fairly technical ordeal but basically on the older style flat head engines you have to remove a small amount from the valve stem end to make sure your valve lash is to spec. You'll have to look up the spec and measure it.

    I'd bet this is your problem.
     
  5. Pedal Dan

    Pedal Dan Member

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    It's called vapor lock. Basically an air bubble from the heat. Put an electric fuel pump on my 6 bbl. Cuda to end it.
     
  6. sunburst79

    sunburst79 Member

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    It's most likely vapor lock or your ignition coil or magneto is going bad. If its vapor lock letting it cool down for 15 minutes to half an hour should let it restart. If its a coil or magneto winding that's getting hot it could take a couple of hours to cool down before it restart. If the mower is a B&S and it's 7-10 years old it quite common for the coil to go.

    I used to use a Velcro cable tie to keep mine running. My latest one has a key start that makes me grin every time I use it.

    I always let it cool down as I'm not really enamoured with accidentally spilling gasoline on a really hot engine.
     
  7. berniesidney

    berniesidney Senior Member

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    It's a Craftsman 7 HP Briggs & Stratton. We've had it for about 10 years... it usually takes a couple hours to cool down before it will re-start.

    How do you test the ignition coil or magneto? Is there a specific resistance they should measure?

    If the valve lash out is out of spec... after its hot and still running wouldn't there be excessive exhaust smoke from oil burning?
     
  8. Tuberattler

    Tuberattler Member

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    Ignition spark can be easily checked with either a spare known good spark plug or you can pull the plug out and turn it over with the plug grounded against the engine. Be careful as it can ignite gas vapors...

    I'd bet it's still the valve lash as it won't make it smoke unless the valve guide is worn out which it'll be out of speck but not enough to do what you're illuding to. Ignition coils HARDLY ever go out.

    BTW I owned an outdoor power equipment shop for almost 10 yrs and was certified Briggs, Onan, Wisconsin and specialized in two cycle engines including ultra lights before I got into Real Estate. :)
     
  9. berniesidney

    berniesidney Senior Member

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    Thanks Tuber... I'll start by checking the clearance on the rocker arms.
     
  10. Travst

    Travst Member

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    This may also be related to the alcohol in the fuel due to its low boiling point. I've had that problem with an old Corvette and solved it by placing a non-metallic shim or riser between the carb and the engine.
     
  11. Tuberattler

    Tuberattler Member

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    If this is an overhead valve engine the valve lash should be adjustable via the rocker arm hold down bolts, refer to an appropriate repair manual for spec's and procedure.

    Since this obviously is an OHV engine you will most likely have a carb with fuel bowl. Make sure you don't have any air leakage between the carb throat and where it mounts to the head. I've seen these setups get a little loose and leak when hot. Also check to make sure your muffler is free flowing as if it has a carbon screen it can get plugged and carbon tends to swell when hot.

    Did you perform the before and after compression strength test?
     
  12. cardamonfrost

    cardamonfrost Member

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    My guesses:

    1) the head is warping when hot causeing a loss of compression
    2) the valve(s) are not seating when the head is warped
    3) your coill is shot
    In either case, check the head bolt torque, may be loose. Do you see black gunk built up around the head gasket? I wouldnt think that it is vapor lock unless it stalls while running, but I could (and probably am) be wrong.

    C
     
  13. BarkingTree

    BarkingTree Member

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    Im voting plug wire or coil..as well. Heat expansion to the ignitor and wire is
    pretty common.
     
  14. jumpnblues

    jumpnblues Member

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    Are you using the choke to start it when it's hot/warm? If so, don't. Try starting it without the choke when it's hot/warm. The engine needs choked only when cold to add more fuel to the mix. In other words it needs "enriching". When the engine is warm it doesn't need enriching and if you use the choke you'll "flood" the carburetor with fuel (make it too rich) and the engine won't start for a few minutes until the excess fuel evaporates.


    Tom
     
  15. Dr. Tweedbucket

    Dr. Tweedbucket Supporting Member

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    :red reported :red
     
  16. Whiskeyrebel

    Whiskeyrebel Member

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    We all change as we age. You just need to adjust and enjoy the time between.

    I never heard any guy call it his " lawnmower" before.
     
  17. art_z

    art_z Supporting Member

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    Put some NOS tubes in it.
     
  18. Tuck

    Tuck Member

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    Before you get too mechanical, check the fuel cap and make sure the vent hole isn't plugged up. This happened to my John Deere rider. Easy fix.
     
  19. Structo

    Structo Member

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    Also after 10 years I'm sure the cooling fins and carberator are all gunked up with grass and residue.

    I would clean the carberator being careful not to damage any springs and pull the cowling off of the engine to clean out the fins.

    When is the last time you changed the spark plug?

    You can tell a lot about an engine by looking at the plug.
    It may be oil fouled or worn out.
     
  20. berniesidney

    berniesidney Senior Member

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