Motorcycle Questions? Ask a Mechanic.
Hi and welcome to JustAnswer,Thank you for the question and your patience. Should you still need assistance for information purposes only...Initially and if have not been tried; inspect the battery & ground lines and/or check/clean the battery cables + terminals, connectors and wiring as a preliminary consideration. Have the battery externally charged and then LOAD TESTed (even if new and just for the purpose of eliminating it as a suspect).
Still, "B" would be for battery while "M" would be for the starter motor (relay). Please click HERE to reference the starting circuit wiring diagram.