After 5ml of liquid, it's likely that the coil in the head is gunked up and needs to be cleaned and dry burned. Your wicks are no longer able to move fluid to the heating surface, and the burnt taste is the taste of the gunk on the coil being burned.
Wash the coil head in piping hot water for a few minutes. Some people use vodka for cleaning, too. After it's clean you'll need to dry burn it to finish the cleaning job. There are several good YouTube videos on this process.
If all else fails, the coil head will need to be replaced or the coil itself rebuilt.