This seems like a slightly broken option, I followed the manual on wordpress codex and related recommended manual, and each time item_wrap did nothing, my menu source code was
<?php wp_nav_menu( array( 'theme_location' => 'primary','items_wrap' => '<ul id="%1$s" class="sf-menu %2$s">%3$s</ul>' ) ); ?>
What didn’t work, and after a lot of hair pulling, I switched to this
<?php wp_nav_menu( array( 'items_wrap' => '<ul class="sf-menu %2$s">%3$s</ul>' ) ); ?>
What happens is both basically identical, so if it doesn't work the first time, don't give up, throw it back and keep trying!