Hi Rob, diaphragm carburetors are a source of a fair amount of frustration to me as well as many other people. However we seem to be stuck with them on machines that have to operate upside-down and on various extreme angles.

Here is the Owner Manual for your machine:
http://www.stihlusa.com/WebContent/CMSFileLibrary/instructionmanuals/FS80_85_Manual.pdf

Here is the workshop manual:
http://www.worldoftrainz.com/Downlo...4137%20Components%20Service%20Manual.pdf

If you focus on the service manual, starting on page 44, hopefully the solution is in there somewhere.

I'm having issues at the moment with a Walbro WA74 carburetor, and finding it is a long process, mainly because they seem to have a large number of design variants and haven't documented them individually. However with a bit of luck you'll do better, just because Stihl have that detailed manual on your specific machine.

Please come back after checking through the manual - if you haven't found it by checking for an assembly error, we can start a bit-by-bit process.