The flapper valve is operated by vacuum. An internal combustion engine will create the highest vacuum on hard deceleration with the throttle closed. I think there is some myth out there about the flapper closing at 5,000 RPM. In reality, the higher the RPM in the engine, the lower the vacuum. I did the flapper mod (disconnected and capped of the vacuum hose) to my bike a few weeks ago. What I immediately noticed was smoother deceleration. My belief is that the flapper acts like some sort of rudimentary Jake Brake by starving the engine for air and causing it to decelerate faster while engine braking. To see it actually 'flap' you will need to take off the side covers, find an empty road, and stretch your neck way to the left while rolling on and off the throttle at various speeds.
Press Any Key To Continue.