I'm having a lot of trouble with my 4-mix Stihl's (FS90, FS90R). The first one I bought ran good, so I bought a second, I recently had a thread detailing my troubles with it but suffice to say it was a turd and the dealer took it back and gave me another. Well, now both trimmers are running poorly and I'm frustrated. Both have good power, idle well, and accellerate well but there's the problem, it's that there's a "gap" in the throttle....namely in between low-idle and rev. If you've got the head turning fairly good, about 10% throttle and you blip it or go to full power, it zips right up to speed. If, however, you've let off the throttle enough that it's down to no spin on the head, and you attempt to gas it up, it bogs, dies, or takes 2 good seconds to get up to speed. Both my FS90's are doing this same thing. Has anyone else run into this? My dealer has looked both units over, and assures me this is common with these trimmers and it's not a big deal. Well, fair enough if that's true but I've owned 10 trimmers of all makes, including Stihls, and none did this. My old FS45 baby trimmer spins from dead idle to full power in less than a second. I'm running good gas, with Stihl Ultra. Both saws are new, but for my own tests I removed the screens to make sure that's not an issue. The carbs are adjusted optimally, or in other words as good as they can be. If this is just how the 4-mix Stihls like to run, I can't say I'm too happy....but if it's not systemic with the motors then I'll be leaning on the dealer a little harder to make them right.