hey i dont know enough about these motors to talk, but if you go to polarisproriders.com there are some highly trained guys on there that are always willing to help with ANYTHING polaris related.
however it does sound like its compression to me as well.
Either the valves are toast or your rings are gone.
Low compression can create a lack of vacuum to suck the fuel and air in at low RPMs.
* Spode Freestyler *
Team Spode Racing
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2004 YFZ450