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Thread: My 400ex is running rough, won't stay running

  1. #1
    Join Date
    Aug 2010
    Location
    Houston
    Posts
    25

    My 400ex is running rough, won't stay running

    Guys,

    Just bought a 1999 400ex and looking for some guidance. Overall she seems in good shape, but I've now noticed that it doesn't seem to want to stay running at idle, even when its warm. At first it wouldnt start but after learning the two quick throttle pump trick before starting, I can get her fired up quickly most of the time. After that however, it sputters and wont run smoothly at idle and dies. It seems to run okay when I keep it moving.

    I've posted a video to show you the problem I'm having.

    400ex wont stay running

    The air filter & box are squeaky clean, and the plug was slightly dry carbon fouled, but nothing that looked serious.

    Any ideas on where I should start looking?

    Thanks,

    Brandon
    2008 Honda TRX 700 XX
    1999 Honda TRX 400 EX
    2002 Honda TRX 250 EX

  2. #2
    Join Date
    Jul 2010
    Location
    Missouri
    Posts
    348
    Is your gas gauge turned off? Because I that's the only answer I have.. I meean mine doe's that little gasp for air like one or twice when I first start it up then it stops...
    Honda 400ex, YELLOW!
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  3. #3
    Join Date
    Aug 2008
    Location
    Central NJ
    Posts
    11,186
    Does it idle good once it warms up or does it keep stalling till you are moving?

    All 400ex's are cold blooded, when cold its a pain to get them going and then keep them going for a bit. Could change out to a 40 pilot and see if that helps or adjust your air to fuel screw maybe a half a turn.
    If it's stupid, and it works, it isn't stupid. - Murphy's Law
    2000 426EX,sold
    2004 450R WIP
    -Longtime A.A.D.S. (Accessory Acquisition Disorder Syndrome) sufferer.
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  4. #4
    Join Date
    Aug 2010
    Location
    Houston
    Posts
    25
    Here's a couple of pics of the plug.


    2008 Honda TRX 700 XX
    1999 Honda TRX 400 EX
    2002 Honda TRX 250 EX

  5. #5
    Join Date
    Aug 2010
    Location
    Houston
    Posts
    25
    Originally posted by BenHonda400ex
    Is your gas gauge turned off? Because I that's the only answer I have.. I meean mine doe's that little gasp for air like one or twice when I first start it up then it stops...
    Gas was set to on. It run for a good 5-10 minutes before I took the video, so it should have been completely warm but it was still dying.
    2008 Honda TRX 700 XX
    1999 Honda TRX 400 EX
    2002 Honda TRX 250 EX

  6. #6
    Join Date
    Aug 2009
    Location
    PA
    Posts
    467
    get a new plug, mine did the same thing, plug looked the same as yours (minus all the rust and mud and stuff) put in new plug and it ran perfect. i would also bump the pilot up to a 42 works wonders on these ol girls.
    then, if that dont work clean the carb good and adjust the valves
    2005 400EX
    99-04 plastic, 155 main jet, 42 pilot jet, HMF Sport Series Slip-On, UNI air filter, LSR PB Block off plate, TAG bars, AC nerf bars, 15/39 gearing, CST Ambush rears, stock Ohtsu fronts

    1973 Yamaha DT360A Enduro
    hooker headers/expansion chamber no other pipe

    98% of the teenage population does, will try or has tried smoking pot. If you're one of the 2% who hasn't, copy & paste this into your signature.

  7. #7
    Join Date
    May 2006
    Location
    STL-MO
    Posts
    1,013
    mine does the same thing when its cold, once warm it goes away, it goes away with a 40 pilot and the right setting on the a/f screw
    ol' 2004 400ex

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