A burnt taste is normally caused due to the wicking material used in the coil beginning to 'dry burn'. To avoid this, ensure that you leave the tank, with the coil fitted and filled with e-liquid, to sit for 15-20 minutes before your first use.
Always make sure that your tank is well topped up and that there is sufficient e-liquid for the wicks to be fully saturated, and ensure that the adjustable airflow is opened up far enough to allow enough air to pass across the coil.
Finally, monitor the performance of the coil. Coils will typically last for around 5 fills before the wicking material begins to clog up. At this point, flavour and vapour production will being to decline and leaks can occur, eventually leading to a burnt taste. At this point, you should replace your coil.