Try adding excerpt_separator

_config.yml

@@ -69,6 +69,8 @@ kramdown:
     # Use existing pygments syntax highlighting css
     css_class: 'highlight'
 
+excerpt_separator: "<!--more-->"
+
 # Set the Sass partials directory, as we're using @imports
 sass:
   style: :expanded # You might prefer to minify using :compressed