How can I get a longer automatic excerpt in Jekyll?

The following gives a 75 words automatic excerpt:

{{ post.content | strip_html | truncatewords:75 }}

Actually <!-- more --> is the answer. At least, it works well now.

1. Add this line to your _config.yml:

   excerpt_separator: "<!-- more -->"

2. Then add this to each post in your site:

   <!-- more -->

Hope it will help.