Open the door on the back of the mantel clock and look inside toward the top right of the brass movement. You should see a silver “arm” coming out of a clear box in this area. This part is called “the platform escapement”. This is the main time keeping part of the clock.
(**before continuing, make sure your clock is fully wound on all three winding areas.**)
Changing the speed of time keeping is accomplished by ever so slightly moving the time adjustment lever. (See figure 1). To slow the clock, move the lever down towards the “-” (minus). To speed the clock up, move the lever up towards the “+” (plus). A small change will make a quantum change in the speed of the clock.
Check the clock 24 hours later and make another slight change in the arm up or down and continue as needed until the clock is keeping good time. Once this is set, you should not ever need to change it for many years.