A number of factors can affect the timekeeping of a clock (temperature, balance etc.). No matter how well a clock may have been overhauled, final adjustment is almost always necessary.
The longer the pendulum, the slower the clock will run.
Most pendulums have a small rating nut under or in the bob, and small adjustments should be made as necessary.
Do not alter the pendulum more than once in 24 hours and remember that spring driven clocks may run slower as the spring unwinds. Keep a note of the amount of adjustment made and the resulting affect.
Some clocks, particularly heavy French ones, have pendulum suspension systems that allow regulation without moving the case. A watch or double-ended key will fit the arbor protruding through the dial near the numeral 12. Generally, rotation of the arbor in a clockwise direction shortens the effective length of the pendulum, thus making the clock run faster.
Never move a clock with the pendulum attached, as this will put it out of beat, damage the delicate spring on which the pendulum is suspended and may cause expensive damage to the escapement.