Hi Barie,
The power jet is located under an aluminum screw inside the float bowl. See picture of TA22 carb attached which is similar. (Fully dismantled).
I don't know which way the valve works, however the basic idea is air is allowed into the carb to either enrich or weaken the mixture depending on throttle and rpm. See section 20-4 for operation description and 20-32 for exploded diagram. (pages 264, 265 and 292.)
Download manual here: http://www.midnightfantasy.com/cyber/bike-pics/Honda_MC18_NSR250/manual/manual.html
Regards Ian