I own several pieces of Stihl equipment, all of them are very picky about stale fuel. If you are running gas that has been in a gas can for more than a month I would suggest trying some fresh 89 octane or higher. I was going crazy with my chain saw and Trimmer cleaning carbs and changing plugs until I found out it was just some stale fuel. I verified this by trying it in my other Stihl trimmer and it developed the same symptoms (poor running above idle) A little sea foam and fresh mid grade gas and all is well again.
I hope this helps somebody.
Edit; The new Stihl oil has a fuel stabilizer in it to help with this known issue and the newer owners manuals mention it as well.
