Having one of those days, rebuilt 2 carbs to try on my MS390 and they both did the same thing so I'm guessing I have an air leak. Tried putting a p/c on the 361 which isn't an easy job either, liked to never got the piston inside the cylinder, wrist pin clips were difficult to get seated, not a big fan of working on either of these but eventually I hope to get them both going.
Just started the 361 a few minutes ago, it had a few more problems, found a hole in the fuel line so ordered a new one. I may have to check the 390 for fuel line also, should have done that when it was apart but I got in a hurry..
Edit: Checked the fuel line on the 390 and it was fine. Changed it and filter anyway since I had them handly. It started and still wouldn't idle down so it looks like a tear down is in it's future if I don't use it for a door stop.