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 TestMuffler Body Installation
Replace the muffler body gasket [A] with a new one.
Install the muffler body gasket to the premuffler chamber
[B] until it is bottomed so that the chamfer side faces premuffler
chamber.
Install the muffler body clamp [C] as shown in the figure.
Insert the projection [D] into the slot [ ...
Air Suction Valve Inspection
Remove the air suction valve (see Air Suction Valve Removal).
Visually inspect the reeds [A] for cracks, folds, warps,
heat damage or other damage.
If there is any doubt as to the condition of the reeds, replace
the air suction valve as an assembly.
Check the reed contact areas [B] of ...
Commutator Cleaning and Inspection
Clean the metallic debris off the between commutator
segments [A].
NOTE
Do not use emery or sand paper on the commutator.
Check the commutator for damage or abnormal wear.
Replace the starter motor with a new one if there is any
damage or wear.
Visually inspect the commutato ...