The yellow brown substance is probably burnt castor oil most motors that run castor will start to turn brown it's nothing to worry about. As far as the mixture setting I only adjust mine for the first flight, I never have to mess with them the rest of the day. As far as your dead sticking all the time you should set your mixture a little rich on the ground it will lean out in the air for 2 reasons. One the prop unloads going through the air and two as the tank runs out it will also lean out so if you are running at peak rpm on the ground you are leaning out too much in the air and that's why your are dead sticking. Make sure you have a real good smoke trail coming from the exhaust as you fly over the runway no matter what you should always see smoke if you don't land immediately or you take a chance of burning up your engine.