Gary---The bi-pass valves are emission control devices that reduce
manifold vacuum when the engine is on overrun. It is said that the lean
mix that can occur (by not limiting vacuum) raises the NOx. Introducing
fuel thru these bi-passes is supposed to balance the air/fuel mix during
these periods when the throttle plates are closed, (or nearly so) The
"popping" could come from a lean mix. If you don't hear this with yours
blocked off, fuel may be getting in thru another source, or engine
vacuum may be down. Performance cam?
Dick
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