So, did you get it figured out frenchman?

I'm guessing your bicycle motor has an air leak at the manifold or where the carb clamps on the manifold....don't over tighten either. Take the manifold off the engine, smear on a THIN layer of any RTV sealant on both sides of the gasket and reinstall.

You guys are cleaning the tanks before you install them on your bikes, right?
That little screen inside the petcock will fill up with crud and clog up in 20 miles if you don't. (that is if your tank is as bad as some have claimed)

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If it ain't broke, and you mess with it long enough, it will be.