So after a week or so of troubleshooting, I think I've come to this conclusion about the burning taste I get from my 510.
I first thought it must have been from the polyfill fibers getting stuck in the atomizer and melting, but I'm not noticing any charred fibers. Even when the occasional fiber does get stuck and I have to pull it out of the atomizer after removing the cart, I haven't really noticed any of the fibers actually melted or charred, so I think burning plastic is out.
What I *have* noticed, though, is that the burning taste usually occurs with either a cart that's becoming dry, or when I switch to a new cart. I think what's happening is, the atomizer basically has a "sponge" holding liquid around the coils to be heated, but it needs a constant flow of liquid to work properly. When a cart is going dry the flow of new liquid in is being reduced and when a new cart is put on, it takes a little while for the flow of liquid to start up again.
In that window of time where the flow of new liquid coming in is weak or broken, the liquid already in the atomizer gets heated up too much and *that* is probably what's burning. And it's probably also why it's hard to get rid of the burning taste because, once you've got burnt liquid stuck in the atomizer, it's going to mix with and taint the new liquid coming in making all subsequent draws taste burnt.
This could also occur if the cart is both too full of filling or not full enough of filling. With too much filling, you wouldn't get enough liquid being drawn up through the compacted mess to create a stable flow, with too little, you wouldn't be making any contact with the atomizer. Also, with the right amount, but pushed too far down in the cart, you wouldn't be making good contact either.
The right amount of filling seems to be just enough to fill the cart without being too compacted, and then pulled all the way to the top of the cart, so that when you put it on the atomizer, the atomizer makes full contact and squashes it back down in the cart very slightly.
And my solution on consistent burning taste now is to pull the atomizer out, blow out all the liquid in it out of it from the battery end, run it under water from both ends, blow it out from both ends, then drip a drop or two of new liquid on the atomizer and connect a new cart. So far, the consistent burning has gone away.