DNA 180 problem with starting?

Hello everyone, after that i did not use the bike for around 5 months now, I went to start it so that it will charge a bit but i found out that the battery was nearly flat. I decided to charge the battery without taking it off. After an hour of charging, I turned on the bike leaving the battery on the charge. The bike worked but I noticed some slight black smoke come out of the silencer. Then I turned it off and left it to charge. The day after I went to start the bike. The battey is ok as the engine is turning but it is not starting. Backfire is coming out of the silencer. Any idea of what gone wrong? Let me add that spark is available when engine is turning.

Thanks for all your comments!