I wonder if some brands/models just have carbon build up in the ports worse than others--nothing to base this on other than the equip I've maintained. One of my guys has run Kawasaki trimmers for several years--no guards, long line, run hard 5 & 6 days/week--almost no problems except stripped slines on shaft, and plugged exhaust ports by the bunches--I mean to the point they don't run (not one single carb problem). Runs various oil brands--not equip mfg. brands.
Another guy runs Shindaiwa 270's (about worn out)--run into the ground. No guards, long line, run hard, fuel cans with no caps--just run and abused. Have carb problems, but no exhaust port problems. This guy does run Shindaiwa oil.
It's got to be either the design of the equip., or the oil, or both!