Ok, this is a weed whacker, not a chainsaw but, it's an interesting problem. The fs250 is a weedwhacker on 'rhoids and great to use when you have a large area that needs attention. The problem was when it started up for the first time this year it wouldn't even run at half throttle, idled fine but when the throttle was opened it would die. The carb is a weird thing with a snorkle on the fuel pump cover that goes up into the air filter box and appears to suck air into the venturi. There is no low or high adjustment, just a single screw, marked "-Ld+" that looks like an idle speed adjuster, but isn't. It has very fine left handed threads and a blunt end that seems to cover a big hole in the bottom of the venturi, a little behind the choke. The hole looks like its there to let fuel into the venturi, the screw to meter it. Anyway, I spent hours fooling with the carb but nothing worked. This afternoon I took the carb off again so I could take it to the dealer to match with a rebuild kit. I started twisting the little screw, saw it was LH threaded and took it all the way out and then screwed it back in with just a little of the hole covered. It started and kicked butt. Anyone know the skinny on these carbs? It's a stihl branded zama. I've found several mentions of them using google but none that figured out that the "idle speed" screw was a fuel metering needle.