Board Thread:Questions and Answers/@comment-179.177.217.243-20151121014207/@comment-28533556-20160524104833

You can get time bomb enchantment only by enchanting your weapon with a medium recipe.

Also i don't recommend you change your poisoning enchantment, because it's better than precision. You are lucky to get poisoning because it's a bit rare, so no need to get your precision back.