they do do something. it fills in the flat power spots when you are on and off the throttle. the more you are on and off the throttle the more it works. and if you have a pj34 carb you will really notice a difference. and i have never heard anything about it leaning the mixture.
2001 Blaster: CP's Extreme Trail Porting w/ Head Mod, Hot Rods Crank Assembly, Bored 80 over w/ Wiseco Pro Lite Piston, LRD Adjustable Exhaust, K&N Filter & Outerwear, Rifled Stock Carb, Self Ported Boysen Rad Valves, Trenga "Cool" Reed Spacer, Oil Injection B/O Kit, and VCA Air Scoops