Jetting Basics

Remember that all carb tuning happens AFTER the engine is fully warmed up

Starter jet sets the choke richness. Reduce this jet size if possible to allow the bike to run/warm-up on the choke for a minute before taking it off the choke.

Idle adjuster screw is for setting the richness at idle (slide fully closed) and is a very fine adjustment. The engine idle speed should be set as low as possible to do this. Turning it in should increase engine speed (leaner) and out will lower the engine speed (richer).

Idle jet size adjusts the fuel output for cracked slide to about 1/8-1/4. Too rich will blubber (4-stroke) at a slightly cracked to 1/8 throttle position while riding.

The slide cut-away affects this range too (up to about 1/2 throttle). Most of the time you can get around changing this because of the wide overlap between the idle jet size and needle position.

Needle clip position adjusts 1/4 to 3/4 mid ranges. Lowering the clip on the needle - raises the needle position and richens the mix. The needle taper and needle jet can also be changed, but most people don't bother. This is generally to fine tune for a specific purpose, like track racing for certain turns and stuff. Most Pedders just ride WOT when possible.

Main Jet size affects mixture from about 3/4 to WOT.

This is very basic list and they of the phases/circuits overlap. But for the common guy trying tune his bike, this is a good starting point.

Reference Dellotro A-Guide 1