It seems like you are not burning your cotton but getting dry hits witch may taste like burning and if they are bad enough they WILL end up burning your cotton and messing it up until you re-wick it. If it comes and goes you are getting an occasional dry hits. To fix this do three things:
1. Make sure that your cotton is not rolled up super tight. It needs to be fluffy on the tails, the fluff that you lay over the deck should be about as tall as the terminal posts.
2. Make sure the bit of cotton that is actually inside your coil wraps isn't overly tight. If you have to twist at it to much to force it through its to much. I feed mine by twisting on it as I pull it through but once its through I'm able to pull it back and forth with some resistance. This is the trickiest part and you'll get it with practice. I can't be so tight that just pulling on it will move the coil around but it can't be so loose that you can just pull the cotton out without moving the coil. There needs to be some resistance to it enough so that you can pull on the cotton gently and that it feels tight but not stuck to the coil wraps.
3. Lay the cotton tail puffs over the entire deck, don't be afraid of covering the juice channels, just make sure they don't get plugged shut when you screw the chamber in. There HAS to be cotton over the juice channels to catch the juice as it tries to enter the chamber. If you don't you'll keep vaping until the cotton dries out a little and you get a dry it with burned flavor. A properly laid cotton tail will keep the entire cotton wick nice and juicy at all times.
Also note that blackened cotton does not necessarily mean burnt cotton. My cotton wicks look black and tarnished after a few days of use. With some flavorings it looks encrusted in black gunk, with some its only right around the coils. This does NOT matter one bit. If it tastes good, keep vaping on it. I only change my wick when I notice that he flavor is off, usually about 5 - 6 tank fills with the juices I've used the most or if I change flavors of course.
Finally you don't need a syringe to empty our your tank if you need to work with the coils. The usual plastic nipple tip on most bottles seals right up on the screw hole of the fill port. I just take out my screw, keep the device with the fill port facing up, grab my juice bottle and squeeze the air out of it, press it against the hole, flip the device while keeping pressure between the bottle and the fill port and start letting the bottle expand sucking your juice out of the tank. Usually 2 or 3 tries of this will get 95% of my juice out of the tank. Then I set the tank over some paper towels, wipe the base, chamber and chimney with some paper towels, open it up, fix it, put it back together and put my juice back in.