The coil is probably OK. As a primative test, I do the same thing as you, just shorting wires by hand and observing a small, weak spark. That generally indicates a good coil. Ignition coils depend on a very sharp turn off voltage to generate higher voltages. In addition to using a proper battery, using driver electronics that generate a proper excitation signal will produce the high voltage you want. Also, longer excitation "on" times produce a higher voltage, up to a point where the driver transistor switches modes on it's own.