Like others have said, the first thing would be adjust the low speed screw. The next thing I would do is pull the carb and disassemble it. Are the diaphragm's pliable without holes? Stiff diaphragm's will really make a saw act up.(ethanol) I would then take contact spray and shoot it in the low speed circuit and watch for it to squirt out of the idle discharge holes near the throttle plate. You will have to hold the throttle open to see this. There usually are 3 or 4 holes in this area. When the throttle is fully closed, only one of these holes are visible, the other holes are there so when the throttle plate starts to open, more fuel can come out. If those holes are clear, I would adjust the fuel meter arm. SLIGHTLY (.02" or .5mm at a time) bend the arm so more of it sticks up for the diaphragm to hit it. It's like raising the fuel level in a carb by adjusting the float level. It will richen everything but idle. When I was working on jet ski's and the 650's first came out, every one had to have the fuel arm adjusted richer to fix the part throttle operation.