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Join Date: Jul 2006
Location: Kennett Square, PA
That problem is usually caused by an air leak (per niterider), so you'll want to check a couple of things. Check the boots off of each carburetor and make sure they are snuggly seated against the carbs and up into the airbox. Then check the boots that go from the airbox to the air filter housing units - again, check connections at each end.
I bought a down-and-out bike a few months ago - had worked with PO on trying to find the leak (which was causing some really really scary overrevving) and we were never able to identify it. During the teardown, I discovered that there was a nice split in the airbox itself... so, sometimes those leaks can be a little hard to detect.