I had this happen to four 510 attys. 2 of them started 4 days after using them and the other 2 were brand new with that burnt taste. After rinsing in rubbing alcohol, peroxide, boiling in water for 5 minutes, changing cartridges, direct dripping with an empty cart, blowing them out with an air compressor, and even dry burning them 4 or 5 times in a row the taste was still there every bit as bad as before. As a last resort I vaped them without inhaling for 9 days and they finally cleared up. I would have guessed that it was just the factory primer except that the first two worked for 4 full days before producing that extremely horrible burnt taste. So whatever the cause, if you just use them, although you have to waste a crap load of e liquid, it will eventually eliminate the burnt taste/smell.