Needle valve ass question

hello,

the specs for my carb say i need a 3.0 needle valve (throttle valve as ive seen it called), if i run 4.0 what will be the difference?

ls there a 'rule of thumb' to ajust using a different pilot jet etc?

thanks

A 4.0 is leaner than a 3.0. The reason it is leaner is more air passage across the pilot jet venturi with LESS throttle opening. I always start with the recommended pilot jet and if I have to give the engine more throttle to keep running at idle my throttle is too rich. Opening the throttle more to keep the bike running at idle and off idle is then allowing more air to pass through the carb but also starts pulling fuel from main circuit also making it more rich!
I try to tune my bikes so it will start easy and idle well with the least amount of slide opening. If I have to screw idle screw in more and dont get response from idle air/fuel screw, I'll lean my slide more. It takes time but you want to be able to have good response with the idle air/fuel screw. This keeps all the carbs circuits working like they are designed to, somewhat independent of each other with some overlap. I always jet my bike for WOT high load and then clean up the idle circuit. Never lean the main to clean up the idle. This is just the way I do it and there could be better ways and please chime in.