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610 mule fouls plugs after about 1 hr run time


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2007 610 mule will foul a pug after 1 hr of run time on clock.  Plug will be black with sut.  Buddy go a 610 and it does same thing. Have had dealer adjust valves twice and still does it.  When i bought mule valves were ticking and I ran it that way for some time and took it in and dealer adjusted valves and that is when problem started.  I rechecked valves and the were tight and I adjusted to .005 and still fouls plug.  I always use no ethenal gas and have gone to a hotter plug.  I have read that this is a common problem with mule.  Any help appreciated.

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No smoke and doesn't smell like the carb is rich exhaust muffler pipe is black.   I ran it out on the road for about 3 miles and pulled plug and it looked pretty good.  Like a lot of guys my mule idles a lot and puttering around the yard a lot so not a lot of wide open throttle.  My friend has a mule just like mine and it does same thing.  When I bought the mule used it had a valve ticking but ran fine.  I ran the mule like this for probably a yr or so and never fouled a plug.  Took it to a dealer and they adjusted the valves and that is when the plug fouling began.  Have had dealer re-adjust since then and same thing.  I went in and checked valves myself they were about .002 and I loosened them up to .005 and am going to see if that helps before I do the carb cleaning adjusting.  Thanks for your replys.

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The valve specks are .0039 to .0059 and dealer told me initially they set them a little tight and I didn't question it.   I hate to hear a valve tick but I may set them at .007 and see what happens.  Funny it didn't foul until dealer tightened them down to around .002.  Again I have read on several sites that the mule is bad to do this and my friends does same thing.  I always run no corn gas and add seafoam or marvil mystery oil.  Thanks for reply

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I'd try the maximum factory spec. Of .0059. Maybe you can lose the tick, and minimize fouling. Maybe even try .0060, or .006. If you don't like the ticking( me either) maybe you can split the difference. You'll probably still have fouling. Maybe that can be addressed by other means. Like leaning the mix, or the hotter plugs. 

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Yea I may set valves to loose .006.  I have a hotter plug and closed gap to ,025 instead of .030.  I have almost an hr. on the clock run time and hasn't fouled yet so we will see.   Mule only  has 350 yrs so not worn out and has been taken care of by me since owning it about 7 yrs. Thanks again for replying.

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  • 10 months later...

Just an update from several months ago.  As noted had carb cleaned and air filter changed and helped a lot but have had plug foul twice in several months.  Battery was weak and finally died so I replace it with a lawn mower batter i already had with a little mod to the hold down device.  Mule runs much much better and has not fouled a plug since, have pulled the plug several times to inspect and no where near as black and suttey as before so I am assuming the weak battery was the cause or at least a big part of it.  What U guys think?   Thanks and b well

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Hi bjones. I have an '04 550.  When I bought it 3 years ago, it had a lawn and garden battery in it and they last a lot longer than the small atv/utv batteries.  I'm not sure the battery has any effect on the firing of the cylinder since it fires off the flywheel magnet and the coil. But ya never know. I don't know if you looked at my thread or not about the overnight compression loss, but as Travis said, sometimes they have a mind of their own...



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