There shouldn't be any issues running the OEM 12V bulbs. I would check your battery and charging circuit. If the battery is old (and possibly low on electrolyte) it will neither hold or take a good charge from the system.

If the stator or reg/reg is failing, that will mean the battery is either not receiving the correct charge, or the system may spike, causing damage to the battery (and possibly even the PGM-IV).

Messing about with 6V bulbs could cost you a lot in the (no so) long run!
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Andy.
NSR-WORLD.COM

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