Neighbor's DS 650 will start and run , as long as you hold the start button . As soon as you let go of the start button it dies ???

Battery was so dead it killed the bike , we had a charger connected to the battery to start the bike ..

Check the carb too, it may have varnished clogging the jets...

I would look into the key switch... getting power when in the 'start', but loses it in the 'run' position.. or just sell it to me..

John