The one thing that I can think of is it might be the type of ejuice you are using. With some of my cheaper juices I get a burnt taste with my protank filled all the way but fine when half also. I switched to a higher quality juice and have no issues at all. The other thing I could suggest is to replace or clean your coil and try it again. A dirty coil will cause the same issue sometimes.
the juice i am using is 50/50 sour granny from maple leaf vapes if that makes a difference? this is the second coil that has done this to me i just replecaed the coil last night...so i guess i will try another one and see how it goes thanks