ok the needles are for fuel. the Low needle will flow ALL FUEL for the carb, but only handles the RPM range from Idle to about 1/3rd throatle. then the H needle will flow additional fuel (over what the low needle is supplying for the lower rpms) for 1/3rd throatle to top rpm.
stock settings for a stock motor/exhaust combe are 1 1/4 turns out from closed on the Low needle and 1 1/2 turns out from closed on the High Needle. modded motors or non stock exhaust pipes will require a slightly different needle setting. also note these are Stock Settings, and even on a stock motor, will need to be lightly adjusted for best running performance.
one last thing to keep in mind, if you adjust the low needle you ll have to make a small adjustment to the H needle in most cases. but if you adjust only the High needle, you wont need to adjust the low needle (cuz the low gives the motor ALL Fuel flow needs, and the high just adds to the low needle flow for the upper rpm range).