HB,
Is this at idle or at road speed?
If it's only happening at idle then it's a matter of Ohm's Law. The voltage is dropping due to the large current flow to the lights, which also changes the current flow on the gas guage. The same thing happens with the blinkers...at idle they flash slow and as soon as the engine speeds up they blink faster.
If it happens at road speed your generator is not charging and Ohm's Law still applies.
Mike B
