If you insist in your way, then you need put an update statement in the after_update event of the combo box to update other information to the master table.
the update statement may look like the following:
Code:
private sub combo1_after_update()
'if combo1 is text
currentdb.execute "update mastertable inner join referencetable on mastertable.key=referencetable.key set mastertable.field1=referencetable.field1, mastertable.field2=referencetable.field2, mastertable.field3=referencetable.field3 where referencetable.key='" & combo1 & "'"
end sub