Some caps are bad, some holes are oval, some caps are wrong some gaskets are bad. The only thing I can say is it is one of those. You could overfill and it can come out the breather tube, or that may be off. The cap is a self tightning desighn, finger tight is all that is needed and it will tighten itself. Gas cap is the same. So maybe it was cranked on and is warped and may come back. I also don't fill mine to within 2 inchs of the top. I hear the arctic cat cap works well on ours, maybe try it looser and see first then tear it apart and rebuild it with a new bottle and cap.
Then replace the carb boots and exhaust bolts and maybe rings while you are at it.
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