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  • back fire at start up

    Ok guys every once and a while when I start my doo (97xp) I get a back fire it is comming off the rear cylinder, I know this becouse when it happened a few times last season it blew the bellows off the rear RAVE valve. Anyone have an idea of what might cause this?? It does not happen every time it start the motor Just every once and a while. It got frustrating putting the RAVE back together ever time it happened. I have since rebuilt the RAVE with everything new except for the valve itself, new bellows and guides and gaskets, orings ect. and updated the caps to the new white plastic ones. I had melted the old black plastic ones before. Sorry for the long post but i want to get this problem cured.

    Thanks in advance,

    Chirs :D

  • #2
    The two most common causes of this are 1) a lean condition in one or more cylinders, and 2) rotary timing that is off. Since you think this is only in once cylinder, I would start by first checking your carb to the cylinder for correct jetting, screw settings, clogged fuel filter, bad gasket, loose bolts, etc. - to make sure you aren't running lean, or have an air leak. Now is also a good time pickup a couple carb rebuild kits and rebuild them. We sell them for $30 and have online instructions in the tech section for doing so.

    To check the timing, use a degree wheel and at least verify that yours is set correctly. Online instructions for this are also provided in our tech section. If your timing is off, and this problem appeared suddenly, you most likely have stripped your brass rotary gear. I would bet on a carb related problem as the culprit here though.

    [ February 20, 2001: Message edited by: Bryan Glynn - SBT Webmaster ]

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    • #3
      Thanks Bryan, I am installing a new set of carbs this season so I will find out soon enough if it was a fuel problem or not.

      Chris :D

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      • #4
        New carbs will not reveal much, if anything if they are not properly jetted and tuned. I'm with Bryan that it's timing or lean.

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        • #5
          Typical if the internal fuel filter in the rear carb is plugged up. Clean both carbs.
          Bill O'Neal WCM
          <a href="http://www.watercraftmagic.com" target="_blank">www.watercraftmagic.com</a>

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          • #6
            Change your spark plugs

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            • #7
              Just for clarification, if your backfire is ONLY out the exhaust side, not through the intake whatsoever, it can be due to an over-rich condition, as opposed to a lean condition. In that case I would check your carb for correct screw adjustment. This is, of course, after following Joe's advice and changing your spark plugs first.

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              • #8
                It is not the plugs, i check and replace them often and the problem was still there, Since i have removed the old carbs I will pull them appart and check the internal filter, The new carbs I am installing are Novi 44's that are freshly rebuilt so if it was the filter on the old carbs then the problem will be gone if that was what was causing it. Thanks Bill and Bryan again.

                Chris :D

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