When the engine is running, the air intake valve is lifting up.
If the valve does not operate properly, check the air intake solenoid valve hoses for leak.
If there are no leaks, check the air intake solenoid valve.

Air Intake Solenoid Valve Unit TestSwingarm Removal
Remove:
Rear Wheel (see Rear Wheel Removal in the
Wheels/Tires chapter)
Mud Guard with Rear Brake Hose (see Mud Guard Removal
in the Frame chapter)
Rocker Arm (see Rocker Arm Removal)
Remove:
Brake Hose Clamp Bolt [A]
Unscrew the swingarm pivot shaft nut [A].
Usi ...
Valve Removal
Remove the cylinder head (see Cylinder Head Removal).
Remove the valve lifter and shim.
Mark and record the valve lifter and shim locations so they
can be installed in their original positions.
Using the valve spring compressor assembly, remove the
valve.
Special Tools - Valve ...
Lower Crankcase Assembly
Install the ball bearing [A] and the oil level inspection window
[B] until they are bottomed
Apply soap and water solution to the oil level inspection
window before installation.
Install the new circlip [C].
Apply a non-permanent locking agent to the oil passage
plug [D].
Tighten: ...