I have an approximate 6 yr old BR550 which gets a lot of use. A fine machine that always starts and never quits once on your back. However, two weeks ago, it did just quit. Problem is a hung up exhaust valve due to carbon buildup. Stihl sells a decarbonizing liquid for these BR blowers which is an admission these 4-mix engines have a tendency to carbon up in the cylinder. Many on the Stihl forums suggest running Seafoam, MM Oil or some other cleaning agent in these engines to prevent this from happening. I'm working on freeing up the valve now but may have to tear into it to resolve the problem. If I can't get it loose today, I'm buying a new blower and saving the old one for a winter project. My new buy will be another Stihl, probably a BR600 but I'll be more diligent with maintenance this time.
Glen