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  1. #1

    Seadoo 787 huge backfire out of exhaust!!! KABOOM

    Hey guys I just put a new engine in this 97 seadoo xp 787.

    I was not the one to build the engine but everything looks to be built correctly with wiseco pistons, new crank, and new gaskets and seals, etc

    I try to start it and it backfires through the exhuast and will not start. The backfire is loud and it has done it multiple times. It blew out the rear exhaust hose. Haha my neighbors came over to my shop and thought I got shot or something because it was so loud.

    compression is 150 in both cylinders
    rv timing is right on the money (I checked it multiple times)
    The flywheel key is not sheared
    The mag cup is aligned properly with the flywheel.
    It has great spark
    The carbs are jetted right and clean
    The rv cover o-ring is good and brand new
    the carb gaskets are good and not leaking air
    the crankshaft is not twisted
    the pulser coil, the stator and the ignition coil are in spec. resistance wise
    the exhaust is not blocked
    I unplugged the regulator/rectifier and it still did it
    I tried different plugs

    could the MPEM cause this? I am out of ideas on what it could be!!!

    any ideas what could be wrong? I am kinda stumped

  2. #2
    flyin' the friendly skies airbornexp's Avatar
    Join Date
    Aug 2007
    Mahomet, Il. USA
    your mag cup is off one bolt hole. you need to pull the motor and take the front cover off and see what your timing looks like.

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