The stg 1 filter will allow more air into the engine so to get it back to the way you like it you need to put some more fuel in. You can change your CO read out (look at kevs signature below his post about mods) this will change the fuelling on the bike slightly but nothing great.

Next would be to either buy a kev fuel mod (about �40 if i remeber correct) or a Power commander (from �300 or so up to �500 including time on the dyno, fuel etc etc). Both have plenty of information on the site about.

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Plough on