derbi terra 125 lack of power?

My derbi terra 125 has recently started playing up. It judders/lags when I get half way through a gear? Like it gets to a point and cannot gain more speed so I have to change up a gear early. Im new to biking so don't kno what it maybe but I think it might be related to a dirty carb? Any suggestions/help is greatly appreciated. Thank you in advance people.