You need the proper combination of cams, carb, exhaust, and intake to get the full use out of those big bore kits.
My KFX has a 434 (athena) kit, stage 2 cams, high comp piston (14.25:1), FCR40 carb, K&N Filter, and HMF Carbon exhaust. I have no problems keeping up with a buddies shee thats bored .40 over... Of course he'll kill me on top end, but who beats their quad off the rev-limiter constantly? Probably have a little over $1000 into the motor, pushes around 45hp at a local dyno shop. :cool: