Replace the gasket if it is damaged.
If they are clogged, remove the tank and drain it, and then blow the breather free with compressed air.

NOTICE
Do not apply compressed air to the air vent holes [D] in the tank cap. This could cause damage and clogging of the labyrinth in the cap.
Fuel Tank Installation
Fuel Tank CleaningCamshaft Chain Tensioner Installation
Replace the O-ring [A] with a new one.
Apply grease to the new O-ring.
Release the stopper [B] and push the push rod [C] into the
interior of the tensioner body [D] so that the position of the
push rod is 3 or 4 notches [E] as shown in the figure.
Install the tensioner body so that the ...
Oxygen Sensor Installation (Equipped Models)
NOTICE
Never drop the oxygen sensor [A] especially on a
hard surface. Such a shock to the unit can damage
it. Do not touch the sensing part [B] to prevent oil
contact. Oil contamination from hands can reduce
sensor performance.
Tighten:
Torque - Oxygen Sensor: 25 N·m (2.5 kgf·m, 18 ...
Brake Line Bleeding
The brake fluid has a very low compression coefficient so
that almost all the movement of the brake lever or pedal
is transmitted directly to the caliper for braking action. Air,
however, is easily compressed. When air enters the brake
lines, brake lever or pedal movement will be partially used
...