It depends. Someone with some experience can do a good job at home, tho' a dyno with an analyzer is always better. If the bike hasn't been modded beyond a pipe or can you're probably on the wrong track messing with the main. Most bikes have a main jet that's plenty rich enough. It's the low-speed circuits that are lean, because the EPA tests are done at small throttle openings. I've had good luck with the Factory jet kits, following their recommendations for the screw, pilot and needle, and leaving the main alone.