The mixture adjustment screws are preset at the factory and permanently capped. They are not set for optimal performance, but set to run slightly lean to ensure the emission requirements are met. They only affect the bike significantly during idle. They can be adjusted by removing the gas tank to get access, drilling the caps and prizing them off, and then adjusting with a little screw driver. The screws are in the little vertical towers dead center on the carb just ahead of where the carbs go into the rubber boot of the engine side manifold. The caps do not need to be replaced. When drilling the caps be very careful not to let your drill plunge through deep enough to destroy the head of the soft adjustment screw, this is a common error. On top of this the screws are often seized, and attempt to adjust them destroys the screw head. But if you insist, here is how to adjust them.
Set as follows, for each cylinder. When the bike is running on one cylinder only, back it off on the idle adjustment screw to where it is running as slow as it will run reliably. The turn out the air/mixture screw out (anti-clockwise) till the motor starts to rich stumble (run badly and shake). Then turn it in (clockwise) counting the turns until the motor starts to lean roll (run badly and shake). You then have two choices:
(1) Turn the screw back half the number of turns counted so you are half way between rich and lean.
(2) Turn the screw back one quarter anti-clockwise turn from lean roll. This is my preferred method. Turn the cylinder up to about 700 RPM, and repeat above adjustment to the air/mix screw.