its running weak for some reson,there is a shortage of fuel and when you open the throttle it gives it a big gulp of air but not enough fuel to match it.the problem could be one of a few things
damaged air filter hose or no secured to the carb properly
water in the fuel
blocked main jet as it ticks over but wont take throttle
leaking vaccum air pipe as this will cause fuel shortage
damaged reed valve petal or a leaking crankseal left or right
if you have had the carb off and it was ok and had no water in it firstly i would check the reed valve petal for damage and then the crank seals.let me know what you find
adam bike doctor(of milton keynes motorcycle centre)
December 2010