If the 991 is like other Yaesu rigs, you will need to inject a voltage into the ALC port on the back using a battery and a pot. See W8JI's excellent page on this very topic at http://www.w8ji.com/amplitude_modulation.htm

You may also have to set the carrier in that mode using a menu item in a manner simlilarly as the 897.

It seems a LOT of solid state rigs do this (ie downward modulate).

GOTO to the Foxtango Group on Yahoo for more inflammation.

Philip KA4KOE