Unlike the current fuel injected 660s, the old 5 valve carburettor models shouldn't pop or backfire on the overrun. However, the one I owned back in the late 90s did backfire when I bought it, but that was down to poorly sealing exhaust gaskets at the cylinder head/downpipe joint which allowed air to be drawn into the exhaust on a closed throttle. New gaskets solved this issue.

First thing I'd do is check closely for leaks anywhere in the exhaust system, but particularly close to the cylinder head.

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