Turn the gas petcock off. Disconnect the hoses. Flip open the seat and you'll see the rear tang of the tank resting in a rubber fitting. The fitting should have a rubber tab on it for you to grab and pull back while lifting the rear of the tank. Once free of the rubber fitting the whole tank gets moved back a couple inches, then up on off.
The starter issue could be the switch on the handlebar. If brake fluid from a leaky master go into it, the plastic of the internal switch housing disintegrates. This can make the headlight fail to operate also as this circuit is interrupted during starting. Or, the solenoid under the left side cover could be faulty, or, the wires to it aren't happilly connected, or the neutral and clutch switches or their wiring is faulty or... You're going to need a wire diagram to chase the problem. You can also direct wire 12 volts across the two small post terminals of the solenoid to see if it clicks and engages the starter. Be sure it's in neutral first.
One possible reason for it to not run at idle is clogged slow jets in the carbs. Has the bike been sitting? Is the inside of the fuel tank and the fuel clean and free of contamination?
Are you using the choke to start it? Will it idle after it warms up?
