get your hands on a plug spanner its the easyest way to tell how its running and there like £5 along with a new spark plug (iridium is my choice)
sounds like you need to go up a main jet but if you go up a main jet you wont know if its correct unless you checkyour plug afterwards i cant emphasise how important the mixture is on a 2t you get it wrong it will cost you more than a plug spanner and a couple of plugs.
cheers
ken