KIPS powervalve

I have a 2005 GT250 (carb'd) that runs fine up to redline (10,000) when warm but will not go past 6000rpm when cold, bouncing between 6200 and 5500rpm, meaning I am limited to 80kph on the m'way on my way home. Also it only gets 300km out of a tank when my bro has a GT250R that is a year older and gets over 400km from the same size tank.

I guess that the fuel is due to poor tuning but would it be responsible for the valve bouncing, or might it be something more sinister, ...