I had the same experience, in particular a frequent burnt taste despite the carto appearing to wick well. In fact I wrote a short editorial of single coil vs dual coil in my blog. I have a personal theory:
View attachment 187764 cutaway diagram of a cartomizer
Using the above graphic as an aid, imagine another coil (the 2nd of a dual coil) about midway up the shaft of the carto. As we vape the suction pulls juice from both the tank and the top of the filler in the carto. Over time, unless you top off the carto frequently (who does that?), the top and possibly the mid portion of the filler is going to become dry from the suction. That upper coil is going to be exposed to dry filler and can burn it and the wick sock it is enclosed in, resulting in a burnt taste.
You wouldn't ordinarily have that happen with a single coil carto because you have the one lone coil down by the punched hole which is always bathed in juice.
That's just the amateur scientist in me...
View attachment 187782View attachment 187784View attachment 187785View attachment 187787
dissected dual coil cartomizer