Before buying valves check you revs, its probably just under revving, is it a direct drive pump? if so it should be doing 3400rpm, if it has a gearbox on then around 3190rpm... if it had a gearbox on before and you have changed the pump to a direct drive then it will be doing around 200 rpm less which would equate to a drop in flow.
Check that your throttle lever nut (10mm between air filter and exhaust) hasn't come loose, if so, nick it up slightly. also turn the revs to full and then watch for the lever springing back.
If the revs need turning up then you can do this via the posi head screw located just under the air filter, it will have a spring around it, just unwind in half-turn(ish) incriments until the desired rpm is achieved.
unsure of rpm?digital rev counters are about a tenner off ebay
