Off the top of my head, it sounds like it may be an airleak. If it's only when it's warm, it sounds like something is opening up and letting air in, giving you a higher rpm, lean condition. It's a new saw you said? Hopefully not leaking seals, but rather something simple like an impulse line or cylinder/case seal. Have you adjusted the carb at all since you've bought it? It will surely need to be adjusted after it gets broken in, and worked a little, which by 50 tanks it certainly is. If it's idling fast, and you can't slow it down enough with the LA (idle) screw, it sounds like a generally lean condition, and not a problem with your idle screw. Cleaning the exhaust and air filter would make it run leaner as you let more air through it. You can pull those limiter caps pretty easily. There's a tang on them that will line up with the groove in the carb itself. You can do it with a sheetmetal screw, but that can be a hassle, and a good way to strip it out if you're new to the procedure. You can go to your Stihl dealership and ask for a limiter cap tool. He knows what it is. P/N 5910 890 4500. He can sell you one for like $6. It's LH threaded stud that threads into the caps and allows you to pull them out without destroying them. The manual says you can't reuse them, but you can if you're careful with them. When it's hot, and you have the idle problem, how does it run in the cut, under load? What mix ratio are you running? Soot and smoke sounds like too much oil, not necessarily too much fuel.
Jeff
