Let me try to help you out here, being new to vaping and made a post a couple weeks here prior, I ran in the same problem as you did or still do. Let me start off with, the Q2 0.4 ohm coils, are meant for 40-80 W but best at 55-65W, however I found that with my high VG juices I can't go above 35/40 with that coil or it will taste either burned, and/or the tank starts leaking etc. My juices I am currently using are 80VG and 20 PG @ 3mg nic, but my kit came with the Q2 - 0.4 and the 0.6 ohm coil. But I was using juices prior to that, that were only 70/30 and I would get constantly a burnt taste when exhaling, needed to run that coil at around 58-62 for it to work, which you can't do with your mod if it's a 50W.
The first thing I did with my 0.4 was a hot water bath for 2 hours, this is what the guys recommended in my store, having the same experience as you did/do. Then after that, I rinsed them under ice cold water and closed the air holes. Blew in it around 5 or 6 times, wrapped it nice and neatly into a paper towel and let it dry like that over night. Next day I unwrapped the coil, primed it (I just read you only put juice in the side holes of your coils, put a huge dab on the top of the cotton in the middle, don't run it over the wire itself tho), filled the tank and placed it on my mod. I got the Smok AL85, not that it matters but I'll include it. Then I took about 4 or 5 dry hits, of course not pressing the firing button, let it stand for 15 minutes. After that, I turned my mod all the way down to 25W, and yes I am aware that the coil doesn't do anything at that wattage, but the reason I did that is the following, at this low wattage press the fire button but don't take a hit. Hold it for 1 second (if your mod shows that but I think most SMOK mod's do), blow out the vape/steam that comes out of the drip tip. Rinse and repeat another 3 or 4 times.
Now let it sit for another 5 minutes, and then take a hit. Again you probably getting nothing out of it, since the coil is not meant to be fired at that low wattage, but it primes the coil very well and apparently that line of coils seem to only work that way. Now go up by 3 aka 28 W, take a hit or two, up another 3, till you end up at the recommended starting wattage of 40 W. Take a couple hits from there, then go up by 5 till you reach the point were you like the vape and not have that burnt taste anymore.
I switched after a week to the 0.6 ohm ones, because I wasn't vaping at this high of wattage anyways, and my juices had the most flavor at around 30/35W. A week ago I switched to the x4 coils for the baby tank, after having tried the M2 coils but I didn't liked them much. Have to say so, that was the best decision I've ever made because the flavor is just amazing as well as the cloud production and my juices work very well with them.
Give it a try and see if my way gets you any better results.
Edit: just remembered, the last pack of Q2's 0.4s I purchased did have a factory defect, simply way of fixing that, I unscrewed the bottom pin of the coil, and took a look inside of it. The little coil/wire whatever you want to call it, was not correctly adjusted to the edge underneath the ohm ring. I took a screw driver and just pushed it underneath, screwed the pin back in and followed the method above. Maybe that's your issue as well, take a look and see how it goes. I normally also prep all my coils with little holes, since they last longer that way, but with the 0.4 you can't do that. It will always end up either leaking or getting a metal/burnt taste/flavor. Try it as stated above but without poking the holes in it, or give some other coils a shot. Another thing would be the juice itself, it's always hit or miss, some brands just have this nasty aftertaste, that can be sometimes mistaken as burnt, or if you have flavors like coffee/chocolate, I found after having tried over 250 juices, at my local store, 20 out of them tasted burned in general, or like burned popcorn, it could be something simple just like that. Might want to try different brands, or even different flavor profiles. Hope it helps.