The only purpose of the flap is to shorten/lengthen the intake runners. And it is not called for until 3600 RPM.

I would start with codes. Your backfire may be cam advance problem, not air intake....if you suspect valves held open, take a compression test.

A Forum Tip for you...........

You will get a better response starting a new thread than piggy-backing an Archieves Post that is 7 years old...

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A Dalton