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 TestFuel Reserve Switch Inspection
Fill the fuel tank with fuel and close the fuel tank cap.
Remove the front seat (see Front Seat Removal in the
Frame chapter).
Disconnect the fuel pump lead connector [A].
Connect the test light [A] (12 V 3.4 W bulb in a socket
with leads) and the 12 V battery [B] to the fuel pu ...
Engine Sprocket Removal
Remove:
Engine Sprocket Cover Bolts [A]
Engine Sprocket Cover [B]
Flatten out the bended washer [A].
Remove the engine sprocket nut [B] and washer.
NOTE
When loosening the engine sprocket nut, hold the rear
brake on.
Raise the rear wheel off the ground with the stand. ...
Piston Installation
Apply molybdenum disulfide oil solution to the oil ring expander,
and install the oil ring expander [A] in the bottom
piston ring groove so the ends [B] not butt together
Applymolybdenum disulfide oil solution to the oil ring steel
rails, and install the oil ring steel rails, one above th ...