I know the last post here is a couple weeks old but, I'm going through the same problem. Nasty, bitter, burnt tasting hits. I am using a 510, by the way. It started a number of days ago. I've been using two of my three attys for about three weeks. The third is relatively new by comparison. It's about a week old with not much millage on it yet, but well past burning off the factory primer. I am getting the same problem with all three.
I've been pretty gentle with my attys in terms of cleaning them. I've used only isopropyl alcohol and hot water, canned air to blow them out and that's it. I've been using the PTB mod (pyramid tea bag) and cleaning my cart filler every night, mainly so they don't leak all over the place in their case. That was working fine until the bad taste came about, then cleaning did nothing.
Over the last couple of days, I noticed that way I was using my PVs might be causing the problem. I was a heavy smoker, heavy in terms of dragging and the speed at which I smoked my stoges. Before any one of my other friends had finished their smokes I was already on my second or third, burned down to the filter.
I am really starting to think that I'm getting this burnt taste because I'm not letting the wick soak up anything. I'm taking too many hits, too close together and not letting the atty rest. I've heard that 510s get a little hotter than other models so that doesn't help when I'm chain vaping.
When I first started vaping, I was very conscientious about how I was hitting my PV. Taking only one to three hits and then letting the atty rest. I think I started, unconsciously, hitting my 510 more like I would a stoge. You can't 'smoke' an e-cig like a cigarette. I think that's why the vapor started tasting bad as of late.
Today, I again started to make a conscientious effort to hit my PV like a PV, not like a smoke. So far.. So far, no more burnt tasting vapor. Here's my advice;
First off, make sure you keep your wick wet. When you drip, give it a chance to prime, let the juice soak into the bridge. Don't hit the thing non-stop and over heat it. If you chain vape try switching between attys, as others in this thread have suggested, in order to let them rest and have the wicks soak up more juice.
I know this burnt tasting vapor may have many different causes but, this is just my personal experience. With my luck, I'll wake up tomorrow and the nasty taste will back.. hopefully not though. Good luck and I hope this post helps someone.