I fought the same problem on a FS85 off and on for about 3 weeks and finally found the problem. I though it was a carb problem and that it was starving for fuel, but after cleaning the carb a half dozen times and checking the crankcase seals I finally removed the insulator block that sits behind the carb. At the bottom of the cylinder, behind the insulator block there is a small hole that was clogged up. After running a piece of wire through the hole and opening it back up, I reassembled the unit and it has run like a brand new one since then. Hope this helps you. It was a very educational experience for me. Now I'll check that first when I get a machine in that won't idle good but has good top end performance. Good luck!