My trimmer has no power and struggles even under a small load. Also it does not want to rev up even with WOT. After about a minute of basically sitting there and idling at WOT it picks up and runs faster but the minute you apply a load it drops down again. Now here is what i checked so far. Plug is good and properly gaped. Valves operate as they should and lash is correct. Air intake filter and exhaust spark arrestor have been removed for testing, but both are clean. Any idea what the problem could be?

Looks like the problem has been resolved. Turns out it was a carburetor problem of some sort. I almost gave up and was going to take the trimmer to Stihl and let them figure out what the problem is. Then I remembered that I had a small bottle of Mechanic in a Bottle sitting on my shelf and I said to myself, give it a shot before you take it to Stihl. I put a tablespoon of this stuff in the gas tank and started the trimmer and it was limping along just like it did before, but all of a sudden after about 5 minutes or so it showed signs of running a little faster and the rest is history. After doing about an hour's worse of trimming by the end of that time my trimmer was running like a bat straight out of hell. Frankly I couldn't believe it. That potion actually did what the bottle said it would do, clean up everything inside, dissolve gum, soften flapper valves etc. I will check it again in a couple of days, but I am pretty sure a dirty carburetor was the problem. That wonder potion is still in the carburetor now and working.