Heat riser restricts exhaust flow from manifold when engine is cold.
Is supposed to help heat the choke spring quicker.
When you say "broke"...I'm assuming it's "broke" in the open position.This could/would cause the choke to stay on longer than necessary,resulting in a "rich" condition after the engine is at operating temp.
You don't state as to what engine/carb combo you have...
Changing it depends on how crusty your exhaust is!