I may have found a fix. I’m using wp 4.0 and AutoChimp 2.15. Got to line 769 of the file autochimp.php and replace
$postContent = AC_TrimExcerpt( $post->post_content );
with
$postContent = wp_trim_words( $post->post_content, 55, '<p>Read the post <a href="'. get_permalink() .'">here</a></p>' );