Briggs and stratton 8000 watts generator, the output of the 120 is only 102. How do I adjust the output?

Voltage is adjusted by increasing rpm of engine. Use meter with Hz setting and try to adjust to 60 cycles and check voltage to be at about 115-120. If engine is turning 3600rpm/60 cycles and voltage is very low, then problem is most likely with brushes/ commutator rings, or voltage regulator. Some voltage regulators have an adjustment screw to bring voltage up/down (adjust to proper rpm/cycles first). Good luck