If it's tasting 'burnt', not enough liquid is getting to the coil. Since you changed the coil and are still getting the burnt taste, it's probably one of two things. One, you got two bad heads in a row (unlikely). Two, the liquid is not wicking fast enough to the head. Did you change to a liquid with a higher proportion of VG (which makes it more viscous so it doesn't wick as well)? Are you taking longer inhales?
Couple of things to try. Empty the tank and fill it up with a lower VG liquid. Inhale several times without hitting the button then take a few gentle, short inhales. Continue using it for an hour or so but keep your inhales relatively short. Is it still tasting burnt?
If so, you may have burnt your coil too badly for it to recover. Put in a new coil and repeat the process with a lower VG liquid and the 'primer' puffs (without hitting the button). You want to make sure that the wick in the coil is saturated. I'm guessing it should be ok. If not, post again.