As MinnFlyer stated, two strokes will run backwards, but usually only at very low throttle openings. Overprimed two strokes also like to kick back hard while starting if the engine is cold. All the extra liquid fuel in the cylinder serves to raise the compression ratio which causes the fuel-air mixture to ignite way to early. A hot two stroke that is overprimed simply won't fire.
To make sense of this, first you have to know that the fuel and air doesn't know or care whether it is in the cylinder of a two or four stroke engine. A lean mixture fires earlier than a rich mixture does, that's why four strokes kick off the prop when the needle is too lean. We actually use the fuel air mixture to fine tune the ignition timing of a glow engine.
So why does a cold flooded engine kick back? Because in a cold engine, the actual air/fuel vapor mixture is actually a bit lean, liquid fuel does nothing but take up space in the cylinder. When the engine is hot, the fuel vaporizes making the air/fuel vapor mix too rich to ignite with the engine's compression ratio.

If the idle is too rich, the engine will slowly load up while it is idling until it quits. A restart is the same as starting an over-primed engine and can cause it to loosen the prop.

Once the high speed needle and the low speed needle is properly adjusted, the engine should idle for an entire minute and transition cleanly to full power when the throttle is opened. An engine so adjusted is also so easy to start that you can throw your electric starter away.