To perfectly balance the Kiyor a total of 5,548 pounds of lead would be required, which works out to a solid block measuring about 1 x 1 x 2 amos (18 x 18 x 36 inches). To balance the Kiyor in the afternoon (when it was empty) would require only 3,356 pounds of lead, or a block measuring about 1 x 1 x 1.25 amos (18 x 18 x 23 inches). When I first made my model I had simply guessed at the size of the counterweight but, after looking at the numbers above, it turns out that the block of lead I used is actually quite close to 3,356 pounds. So while the Muchni does offer some mechanical advantage in the gearing and pulleys, the Kohen raising the Kiyor still has to put some muscle power into it.
Here is an updated version of the animation showing the Kiyor being raised from the well of water.