Hi, I'm pretty new to Drupal (just done my first site conversion), though with a 30-year career of coding behind me! I hope I'm posting this in the right place, and not contravening etiquette.

In trying to set up RSS (Drupal 7.41), I found that the config option "Titles plus teaser" doesn't work. There are a few old discussions on this, but nothing that provides a clear answer.

With some digging I found that the RSS rendering routine only looks for one value in the RSS settings. If it's 'title' then it outputs just titles, if it's any other value it uses the RSS display mode to generate the output. If that mode is not enabled it falls back to the Default display mode. Having discovered that, it was perfectly easy to configure the RSS stream the way I wanted.

This is perfectly logical behaviour, it's just the config options that are misleading. There should be just two options: "Titles only" and "RSS display mode". The fix is in core, but trivial, here's a patch (don't seem to be able to attach a file here):

--- modules/system/system.admin-orig.inc	2015-10-21 19:47:00.000000000 +0100
+++ modules/system/system.admin.inc	2016-01-15 19:44:12.000000000 +0000
@@ -1906,7 +1906,7 @@
     '#type' => 'select',
     '#title' => t('Feed content'),
     '#default_value' => variable_get('feed_item_length', 'fulltext'),
-    '#options' => array('title' => t('Titles only'), 'teaser' => t('Titles plus teaser'), 'fulltext' => t('Full text')),
+    '#options' => array('title' => t('Titles only'), 'rss' => t('RSS display mode')),
     '#description' => t('Global setting for the default display of content items in each feed.')
   ); 

HTH
Rick Jones