sounds like a fouled plug. go get yourself a new br8es plug, maybe a couple.
take the spark plug cap off(the wire thats right at the top of the motor, just under the gas tank) and get a spark plug socket and take it out. if you dont have a spark plug socket, you'll need one because first of all, you cant get a wrench in there, and secondly, this wont be the first time you do this.
oh by the way, gas tank doesnt need to come off, you should be able to get the plug out with a socket between there. carefull with the new plug. they will break easily if you drop it then your back to the same problem.