To change the belt on a post world war 2 Kirby vacuum, you have to deal with a sometimes confusing device called the belt lifter. You also have to deal with this when removing the nozzle to use the hose, carpet shampooer, or other attachment. To use another head(nozzle) or hose on the Kirby motor unit, or change the belt, you have to remove the head from the rest of the vacuum. WARNING! Always unplug any electrical appliance before working on.
To remove the nozzle, first lift up the headlight. Then turn the belt lifter (the thing on the front that can turn about 3/4 of a turn) counter clockwise as far as it will go. This should pick the belt up off of the motor shaft. On many Kirbys some arrows saying belt off will line up. Some brown models In the early 70s had a window in the belt lifter that would say belt off. Now you can unlatch the latch on the main body that holds the nozzle. This is right in the center over the top of the nozzle. You can now take the nozzle off. If you look inside the hole in the back, you should see that the steel hook inside is at the top, and the belt is hooked to it. This is how it should look when you are ready to put the nozzle back on. If you are putting the hose or another head on, I recommend turning the belt lifter back to the belt on position, so that you aren't stretching the belt.
If you want to use the hose, hook the bottom of the hose end behind the little shaft under the motor shaft on the main unit, tilt it up, and latch it at the top. If the little shaft is not centered (equal amount of shaft sticking out on each side), tap it gently until it is centered.
Belts stretch out and slip, especially in thick carpet. The genuine Kirby belt for machines made since 1981 has a tread on the inside, to help reduce slippage. Also machines made since the mid 80s have had a green LED on top of the edge of the nozzle that lights up when the brush is turning at a good speed. If you have an older machine (or your green light mechanism is broken), we recommend changing belts routinely every 3 to 6 months, as they will stretch. If your light is lit up when you're using the machine at the correct height adjustment, then your belt is adequate.
With the nozzle off the machine, as described earlier, to change the belt, turn the belt lifter to belt on if you haven't all ready. Then remove the bottom plate. On machines before the G (G series are Generation 3, 4, 5, 6, ultimate G, Ultimate G Diamond, and Sentria) series (which came out in late 1989) the brush will be attached to the bottom plate. Carefully pry one end out, remove the belt, slip a new belt over the brush, and reinsert brush into bottom plate. The screw heads should face the bottom so you can adjust them. On G series kirbys, the brush is in the nozzle rather than the bottom plate. There is a number 1, 2 or 3 facing out, there are also 1, 2, or 3 notches on the square end. The same number should be facing out on both sides when you reinstall. Cut off any hair. Put bottom plate back(some had latches, some just push in). Push the brush up. Put a straight edge across the bottom. The bristles, when pointing down, should hang down past the straight edge the thickness of a coin. On a G series machine 1 puts the least bristle down. As the brush wears and not enough bristle hangs down, you put the 2s facing the bottom, if it still isn't enough, the 3s. On the older models, you adjust the screws until a coin thickness of bristle hangs down below the bottom plate.
IF you took the whole brush out to clean it more thoroughly, which end goes where is important. The bristles angle so that, as the brush turns from front to back, the dirt will go towards the center. One end is bigger than the other so it should be impossible to put it in wrong. BUT, if someone removed the ends to clean the bearings and reversed them when they put them on, then the brush won't go right. Also, on the pre G series brushes, the large end can be forced into the small slot, although it tears up the end. Also, on machines after the mid 80s, there is a magnet at one end of the brush that induces a current in a coil of wire that lights a little green LED on the nozzle. If the magnet isn't at the end where the coil & LED are, somethings wrong. The ends can be removed and switched to make it right. IF you decide to remove the ends to clean the bearings, do it one end at a time and replace it before removing the other end.
Then, when you are ready to put it back on, turn the belt lifter to the belt off position, making sure it has grabbed the belt (you can see it through the hole in the back). Then hook the nozzle over the shaft (making sure shaft is centered, see paragraph 3) under the motor pulley on the main unit, tilt it back, and latch it, just as you unlatched it. Then turn the belt lifter to the belt on position. This should drop the belt onto the motor shaft, and it should be ready to use.
The rug shampooer and the old handy butler attachments also have a belt lifter and hook and unhook the same way from the front of the machine. Leaving the belt up in the air on the hook will stretch it.