i have a 4mill,T5s,stock carbs,toomey 2-1 air filter,stock timing,19cc domes(175 psi),V3s, no porting,useing the spacer plate.i run a 15/38 for trail riding. but if you plan on going to a 38 you may wanna get a lonestar hub that uses a honda sprocket to a yamaha axle. reason is because when you go that small on on a yamaha the sprocket is so thin and easy to blow apart. and if you do plan on using you a yamaha sprocket you will have to grind down the hub so the chain won't hit it.