Ola'!
Have you tried changing the batteries of the Transmitter / Charger? Perhaps you could use a set of Duracell or, even better, fully charged NiMh / NiCads and see if there is a significant change in flight timings. You should at least get a 4 min flight time under normal circumstances.
If this doesn't improve things then you need to check out the battery inside the Heli. The blades have nothing to do with this, leave them alone. Bigger blades will put an extra load on your motor and other electronics.
Tchau!