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Bobcat 250 dies under load

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  • Bobcat 250 dies under load

    Im working on a miller bobcat 250 s/n-mc261202r. It has the Kohler power plant. With the machine running in the "RUN" position it dies as soon as you strike an arc. Im pretty stumped by this. I have verified that low and high idle RPMs are correct. Any advice?

    Thanks, Bob.

  • #2
    Dirty carburetor, dirty spark plugs, sticking throttle linkage, bad gas, bad ignition components, bad fuel pump. Usually the first option, about 90% of the time.


    • #3
      I had just cleaned the carburetor in this particular machine about a week ago. I will check the other components you listed. Thanks!


      • #4
        Sometimes carb problems can be subtle, like a bad needle or seat resulting in just a trickle of fuel that will let it idle but instantly drain the bowl under load, or a piece of crud stuck behind the seat or some awkward place that's hard to clean. I haven't done much work on kohlers (I need to do a head gasket set on a command 20, _again_, but the carb is fine!), so I don't remember if there's any specific places to double-check, just making sure that you can blow brake cleaner through every passage.

        The spark voltage needed to fire increases with the pressure in the cylinders, which increases the more the throttle is open, so worn out or fouled spark plugs or other ignition problems can give an engine that idles fine but can't run under load... but carb problems are more common. Cleaning carbs repeatedly has made me a big fan of fuel injection! The vacuum fuel pumps are finicky and don't last as long as electric pumps do, and can be another source of annoying problems.

        I forgot that kohlers tend to have a fuel shutoff solenoid... Make sure that it's receiving steady 12V power, and the unit isn't shutting off due to low oil pressure.


        • #5
          Great advice Bushy! Thanks! I just returned from the jobsite and found that the crank case was way over filled, definitely the cause of my issue (I wanted to ask my question before I drove an hour to look at the machine so I would have an idea of what im looking for). I drained the oil and it was super thin and reeked of gas. So either my float is stuck in my carb or maybe the fuel pump is leaking fuel into the crankcase. Either way I have a new carb on the way and have a new fuel pump on my truck. Do you have any more ideas on how fuel would be entering the crankcase? Thanks again!


          • #6
            The only places the fuel system and the engine intersect are the carb and the fuel pump, so sounds like you have that covered. There's two styles of fuel pump. One style uses vacuum pulses from the crankcase to move a diaphragm, the other is built into the valve cover and mechanically moves the diaphragm, but in either case the diaphragm eventually cracks and fuel leaks into the oil. And, of course, a sticky float, bad needle or seat, or on some carbs bad gaskets, and the carb leaks fuel down the intake when the unit is off and it ends up in the oil.

            While you have everything apart, replace the fuel filter, and also see if any of the lines are looking a little old, there's any leaks where the lines connect to the tank (my bobcat had a rubber grommet that had failed and let water into the tank), etc. Easier to do it all at the same time than to take it apart again later.