On many electromechanicals the transformer has a HIGH and NORMAL (or LOW) tap.
This was designed for those areas where the line voltage, which should be
110 VAC, is low--such as 105 VAC. The HIGH tap is intended to compensate for
low line voltage. If your line voltage is high or normal you should be
using the NORMAL (or LOW) tap.
Switching to the HIGH tap to improve bumper and flipper action may be just
what the machine needs. It is not likely to cause problems with burning out
coils unless your line voltage is over 140 VAC. The effect you will notice
by switching to the HIGH tap is that all the bumpers and flippers will be
slightly stronger. Sometimes this is enough to make the game fun again. I
don't recommend doing this unless the game is dog slow and you've tried all
the other standard stuff, like cleaning, and replacing broken and worn coil
sleeves, bushings, links, and so on.