functions.php

<?php
/*
 * Custom Function File
 *
 * Place your custom function, filter and hooks here.
 *
 */
/*
This functions display a custom teaser, by finding the teaser content using preg_split. It then appends a custom more link and returns the filtered content which then will show up in your RSS feed.
*/
function ffm_rss_teaser( $content ){
  $teaser = preg_split( '/<\/span>/', $content );
  $content = $teaser[0];
  return $content;
}
add_filter( 'the_content_feed' ,'ffm_rss_teaser' );