on the pendulum should be a small knurled adjustment wheel.
if the clock is gaining adjust the wheel anti clockwise
when viewed from the back of the clock.
if the clock is losing adjust the wheel clockwise
suggest you mark the wheel with felt pen then turn the wheel 1 complete turn in the direction needed, you then have to time the clock over 12/24 hrs does it still gain/lose ? you then repeat the adjustment again and time it again you WILL get it right eventually.
wirecutterman, August 2011
