So I have the spacer, it isn't cracked or anything. When I bought the ATV the PO had removed the carb and put it back without the spacer. No big deal but I don't know how the spacer goes on.

The carb has an O ring on the manifold side, then the spacer has an O ring on one side and is flat on the other. How does it go on? With the spacer O ring facing the manifold or facing the carb?

I also see where some of these sell with a gasket. I'm assuming the gasket goes on the flat side of the spacer, but if both the carb and spacer have O rings is it needed for this particular setup?

 

The O ring on the spacer goes to the engine side so, the carb O ring fits against the "blank" side of the spacer. These spacers are to stop heat from traveling through to the carb from the engine.

Presumably there are some models that don't have a carb O ring and would need a gasket instead, yours won't.