Like he said, most likely is your battery. However, when the hold in coil is out on the starter solenoid it will do the same thing that you are just describing. In that case you need a new starter motor or you could try to rebuild your solenoid. The starter motor and is very easy to swap out. First, disconnect your battery. Do not skip this step. Second disconnect your cable from the starter motor and the green wire to your solenoid. Third, remove your primary cover (follow the step-by-step instructions in your manual)
Fourth, remove the compensating sprocket nut and clutch hub lock ring. Your primary chain should be loose by now from when you removed the primary cover.
Fifth, remove the two starter mounting bolts, should be 5/16 Allen bolts and remove the starter.
You can bench test the battery by using a known good battery hooking up positive to the starter copper post and the battery positive post then negative to the starter case, I usually clamp the jumper cable to the mounting bolt ear. Then get a jumper wire, 18 gauge or bigger and touch one end to the copper starter post(or the battery (+)post and the other side momentarily touch the spade connector on the solenoid ( where the green wire connects to) hold the starter firmly because it will jump. The starter will either push the jack shaft out and spin (then the starter is good )or the jackshaft will click in and out rapidly and then you'll know it's the solenoid hold in coil.
But it's probably just the battery.