510 cartridges contain a coil inside and cotton to absorb the cannabinoid contents so they can burn evenly over the coils. When you have refilled your 510 carts too many times, the cotton in the coil will eventually build up too much grime and burn off cannabinoid concentrate and start to taste burnt and nasty. Because of this, it is advised that you replace your 510 carts with new ones once they have been depleted once. Another common cause of a 510 cartridge tasting burnt is dry hitting the vape. Dry hitting refers to taking pulls off of an empty or almost empty 510 cart. Because there is no oil inside the vaporizer, the coil will begin to scorch the cotton inside, causing a burnt taste every time you use it. This is a common thing to encounter once your 510 cartridges are empty or almost empty, and you attempt to pull on them. Unfortunately, if your 510 cart begins to taste burnt, you will need to replace it, as the taste will not likely go away.