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Thread: 400ex popping on deceleration, backfire

  1. 03-05-2010, 10:04 AM #1
    unitednations
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    400ex popping on deceleration, backfire

    I have an 03 400ex that started to pop like crazy on deceleration. Any ideas what this could be? It ran good for like 2 years, and just started to do it. It does it if i let off the gas around half throttle or more. its a constant popping out of the exhaust. I tried adjusting the air fuel screw 1 turn each way, not sure if it changed any. still popped.


    HFM pipe
    Jetted 40 and 162? i cant remember exactly
    air/fuel screw 2-1/2 turns out.
    KN air filter
    Hot-cam
    11:1 compression

    Any ideas would be very helpful! thanks!
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  2. 03-05-2010, 10:07 AM #2
    05440EX
    05440EX is offline Junior Member
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    Popping on decel is usually a result of being rich, or unburned fuel loading up in the exhaust. Popping on acceleration is a lean condition.
    2004 440EX. Pro Taper Bars, CFM airbox with UNI Filter, FMF Powerbomb pipe, ITP Holeshots, DG Bumpers, 14-38 sprockets and other crap I forgot..
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  3. 03-05-2010, 10:13 AM #3
    unitednations
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    could it be a leak in the headers or in the pipe as well?
    I dont know how the carb got more rich. I didn't touch anything. it just started doing it.

    Isnt there a air cut off in the carb or something? do these go bad?

    anyone know of a way to find a leak in the exhaust? a redneck way haha.
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  4. 03-05-2010, 07:24 PM #4
    JOHNDOE83
    JOHNDOE83 is offline Jedi Master
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    No! poping and backfiring on decel means to lean.....

    www.jetsrus.com has the info.....
    400ex hybrid Finished!
    http://www.youtube.com/watch?v=YMDIAw8UskQ
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  5. 03-05-2010, 07:33 PM #5
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    honda400ex2003
    honda400ex2003 is offline Code Red Racing #23
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    turn your f/a screw out a 1/4 turn at a time until it goes away.if you are more than 3.5 turns out you need a bigger pilot. it is a lean condition also. steve
    2019 Maverick X3 XDS Turbo R
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  6. 03-05-2010, 07:38 PM #6
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    rooster300ex
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    if it just started to do it i bet its a exhuast leak. check your header bolts. Samething happend to me. Quad ran fine and then it started to do this. After the header cooled off i wiggled it and sure enough it was loose.
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  7. 03-05-2010, 08:18 PM #7
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    rmcfadden68
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    i had the same problem with my 05 and it turned out to be loose header bolts
    livin on the edge cause there`s no room left in the center.
    05 400ex
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  8. 03-06-2010, 12:53 AM #8
    unitednations
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    yeah i dont think its a jetting problem. it didnt re jet it self. i did just replace the float in the carb tho. maybe i screwed something up in there. idk.. but ill redo the headers. they seem tight but it could leak. No way to really tell. cant put your hand there long to test for air haha
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  9. 03-06-2010, 07:56 AM #9
    05440EX
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    If you turn the A/F screw, don't you want to turn it clockwise (in) to richen it up? Its an air screw and by turning it in you decrease air and increase fuel.
    2004 440EX. Pro Taper Bars, CFM airbox with UNI Filter, FMF Powerbomb pipe, ITP Holeshots, DG Bumpers, 14-38 sprockets and other crap I forgot..
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  10. 03-06-2010, 08:07 AM #10
    JOHNDOE83
    JOHNDOE83 is offline Jedi Master
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    use a process of elimination....

    if its not the headers, if your plug is good, if your gas is good, its your carb....

    something inside like a jet could be cloged not letting enough gas in, clean your carb and if it still pops and backfires....id say get a bigger jet.
    400ex hybrid Finished!
    http://www.youtube.com/watch?v=YMDIAw8UskQ
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