1. Cause it is old and needs replacing.
2. 16952-MBY-003 SCREEN SET 0 $11.45 ronayers.com
3. If you have the screen out, isn't the tank empty? If you put it back together without it, junk will get in the carbs and cause damage.
But, I would open the Petcock with the tank sitting on a 5 gallon bucket and open the filler cap to let it run into the bucket.
I have a couple extra screen sets in inventory so I don't have to wait on ronayers or Hondaline.
Good luck.
Hay

Jimmyt