Gilera Coguar 125cc: engine over-revs?

Whilst waiting to renew my CBT i left my coguar sat idle in the garage under a bike cover for a few months. Upon riding it again I have noticed a distinct lack of acceleration; when I open the throttle rapidly, rather than provide a burst of acceleration the engine simply appears to over-rev. This also appeared to be the cause of my speedometer breaking whilst I was riding home.
Can anyone provide an explanation for this and perhaps a possible fix that does not involve visiting a mechanic, as money is quite tight at the moment.