When do these need to be changed? I see lammy and old Vespa owners change them as as often as main jets are done but rarely ped tuners bother? Is it unnecessary or overlooked?
Needles aren't so much of a problem as they work off the main jet. The pilot jet is only changed for idle and when you let off. Usually changed when the mixture screw adjustment isnt enough, for example my Mixture screw was richened as much as possible, and it was just about okay, so by going up 2 sizes on the pilot jet I could wind the screw back out to about the middle and retain some room for adjustment.
The thing is i know what the mixture screw does and everything but i just dont know how to tell when you have it right for your bike???
Warm the bike up, and turn the screw out almost all the way. Then start to screw it in 1/2 a turn at a time, until the revs drop.
Yeah, and then usually another 1/4 turn after that. Usually when you have an unsteady idle that's kinda high, it means the mixture is lean. So either there is an air leak or you just need to turn the mixture screw in a little bit.
Take a look at this document, it gives a great instructions on how to set up your carb http://www.tomosforum.nl/uploads/Pim/Tomos_Streetmate_A55_-_Information_and_Tuning_Manual.pdf
