You can lock them to almost any position and the bike will run. Wide open tends to make the midrange a little weak but will go almost as normal at the top.
Half open is better. Good midrange with very little lost at the top.
You have to remember that the power valve on an NSR n(mc18/21) isn't fully open until 12,000rpm. Most make peak power below this figure anyway so having the powervalve wound fully open doesn't always give the best peak power anyway.
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If I have to take the carbs off once more...