they can be tuned just like a jetted carb but not by just changing a jet. you would need to change the entire needle just to richen or lean the mid range. top end could be close on the needle and that where the power jet comes into play. the needle acts as pilot also. set the slide all the way shut and get it started with the needle only. let it warm up and raise the slide 1/4 of the carb bore. if it runs away its lean. richen the needle. once you get it where it holds for about 5 seconds it should be good. set your slide height. start with powerjet all the way open and start leaning it out until the bike pulls clean. and start doing plug chops. if you get to where the powerjet is shut all the way off than you will need to get a leaner needle for top end. its also best to make sure the throttle lets the slide shut all the way down and all the way open. if it doesnt. than dont run it. the first number on the needle is for top end. the second number is for midrange. the xl is just the generation of the needle. the higher the number the richer it is. a 5-0xl needle is richer than a 5-1xl in the midrange. A 4-0xl needle is leaner on top end than a 5-0xl needle.