Groucho, according to a pdf fed 2 instruction manual I downloaded from butkus, when you take the back off, asnd look underneath the camera, there are four screws which are used to when altering the speed of the curtains:
"FED-2 Instruction manual
– 6 –
You’ll need to do the following: (look at the picture) open the bottom, you’ll see 4 bolts.
You can regulate the first curtain (it opens the shutter) with the bolts 1 and 2. The bolts 3 and 4 are for the second curtain (it closes the shutter). The bolt 1 and 3 are for fixing, the bolt 2 and 4 are for regulation of the curtains. Take off the lens, put the shutter speed in position “B”, cock a lever and press a shutter button. Look at which of the curtains moves slowly.
For example, the second curtain (it closes the shutter) moves slowly. In that case you need to unscrew the bolt 3, and turn around one or twice the bolt 4 (anticlockwise).
For example two, the second curtain (it closes the shutter) moves quickly. In that case you need to unscrew the bolt 3, and turn the bolt 4 (clockwise) once or twice. This method is used for all Leica type cameras, Zorki and FED"
'Bolts' 1 and 3 are the ones with the flanges, 'bolt' 1 is nearest the front of the camera.
I don't know about the front curtain.
I'd post the pic but I don't know how to get a pic from a pdf.