What Dave said is correct, when the juices vaporize they tend to leave a darkish remnant that seeps back into the cart. Also over time the wick on the bridge if not cleaned thoroughly will contain this darkish Burnt fluid. Struggled with this atomizer for the last month or two I've discovered what works best for me is to use the clearest PG only liquid available for vaping, when cleaning; blowout residual juices then take a shotglass 1/2 full of purified water and 70% isopropyl alc. (or you could use a food grade 3% hydrogen peroxide and using a syringe or pipete drip a few drops first on the bridge wick, wait a few seconds and blow it out, repeat again until water is almost clear. Then drip a few drops in again but wet burn it, the liquid will gurgle and evaporate to some degree then blowout, repeat and burn until water evaporates and you can see the coil start to glow. Important - when the liquid is completely evaporated then care is necessary so you don't "open or pop" the coil. Do this by slowly pulsing the battery button and monitoring the amount of smoke being produced. You don't need to bring the coil to red hot, what is important is that as you pulse you ramp up the heat being built up slowly and if you monitor the reflection of the coil on the inside of the atomizer the following things should occur. 1 the smoke qty should subside,(you'll have to blow out the smoke to properly monitor the coil color) 2. the coil should turn orange faster. Once these 2 things occur continue to pulse power to the atty but ramp down the frequency and on time you do it so that it cools gradually - Electronic failures usually occur due to large quick swings between states. At this point you can add a few drops some purified water via pipette or syringe and flush specifically aiming for the coil the best you can. Blow out from the battery connector side. Do a minimal dry burn add ejuice to the wick and then take a dozen hits but don't inhale until the taste of the ejuice is normal. Don't want to be inhaling any alcohol. These are my favorite atomizers when they work and lately before this last cleaning method haven't had one last over a week, though I am a heavy vaporer. FYI - I don't recommend the denture cleaning method with this type of atomizer it may work with others but I ruined 3 (done at the same time) using this method. Ultrasonics is debatable there appears to be wick fragmentation left in the liquid afterwards. Tried a boiling alcohol ultrsonic method with a long soak time (3 hours) that left alot of carbon and coloring but the atty after being dried and prepped worked great for a day then the coil opened. Good Luck hope this helps.
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