The bus multiplier is set by the presence or absence of several resistors. They are ultramini resistors soldered onto the CPU card.
First, you hit up Motorola's site and download the pdf spec sheet for your CPU ... probably a 7410. The sheet identifies the resistors by name, and lists what multipliers you get from each combination of resistors.
Then you need to find where the multiplier resistors are on your CPU card. xlr8yourmac has a picture guide for fiddling the multipliers on the original Sawtooth CPU cards, on those cards the multiplier resistors are on the underside of the CPU card. I don't know if they are in the same place on a QS card.
I'd advise you to ask in the Mac Mod forum, if you plan to fix the multiplier. It will take some very fine soldering work.