G'day nut, sorry to take so long to reply. The air/fuel mixture screw and idle speed screw don't affect your full throttle running, that sounds like you may be starving for fuel at high engine speed. For tis I would guess to check you vacuum line going to your petcock, check that the inline filter is clear and the screen in the petcock is clear.
To adjust the idle mixture screw, screw it right in, do not force it, just gently to the end of it's travel, then screw it out 1 full turn. From there adjust it until you get your highest idle speed and then turn it out 1/4 turn, once you have done that you can adjust the idle speed to your desired setting using the idle speed screw which is the larger of the two. If you increase the airflow through the engine, you will need to increase the fuel flow to suit. If you have reduced the air flow through taping up your airbox mod, you may have closed it off too much and may be flooding at full throttle.
