Hi,
Feeds generally have been used for syndicated content, like blogs or new articles and those kind of feeds usually have publication dates on each item in the feed. In the settings of Feed Manager there is a setting for "Show posts up to this many days old" and this is typically used to filter out old posts so we only see the latest posts from a given feed.
The problem is that with a feed from a search, there is really no publication date in the feed so the only way to make that feed work is to change the setting "Show Posts up to this many days old" to 0 so that is does not filter by date at all.
Note, this only works as of mojoPortal 2.3.3.6 as I made it possible to use this kind of feed recently when someone else was trying to use a yahoo search feed.
Hope it helps,
Joe