Is it pinking? I would suggest checking the ignition timing, which is easily done - do a search on this forum for instructions on how to go about this. Poor ignition timing can affect performance signifigantly.
I wouldn't run with out the air filter, the air intake is quite low down and in damp conditions I would be concerned with water getting in. - As well as dust, insects, grit which all damage the engine. As a compromise, you can get so called 'performance filters', or try trimming down the existing one.
Have you checked the throttle cable as John suggested?