My YS engine used to start very easy, but now it's hard to turn over and floods all the time.

If you've had the engine a while, it might be time to flush out the muffler. As time goes on, oil will accumulate in the muffler and can even run into your engine, causing it to hydrolock.  Just pour some alcohol into the muffler, plug the outlets and give it a good shake. Drain it for a while then reinstall it.

This is true of any two stroke engine, but it's more prevalent in the YS engines.