That's a membrane carburetor... I am surprised the folks here aren't on the case yet.
You'll need to set the metering arm: if you have the Tillotson HS carburetor you just need a flat ruler and set the arm flat with the carburetor bowl, if you have the Walbro WJ you'd better get a Walbro metering adjustment tool. It will make your life a whole lot easier.
Just order a setting gage for both types of carbs. Now for the time till I have then I will work on some ts420 and ts400. I think he has about 6 saws.
Lester