About the flapper.... You said that it closes by vaccum, but on Steves TL1000S pages, i read this:
The Air Valve aids with throttle response by not allowing too much air to enter the throttle bodies at low engine speeds.
With the engine not running, the Air Valve will be in the closed position.
The Air Valve will open at 4000 RPM. Start engine and allow it to idle.
Look in the left air inlet duct on the bottom of the air cleaner.
Verify the Air Valve is closed by the Actuator Rod being in the down position.
Rev the engine to 4500 RPM.
At 4000 RPM the rod should move upward, opening the Air Valve.
Allow the engine to idle, the valve should close again.
If the valve does not work properly, check vacuum hose routing and connections.
Follow the troubleshooting procedures in the Service Manual, section 4, if needed.
Turn off engine.
Mine is open all the time

When i look inside the airbox, and engine not running my flapper is open. How can that be??