Filter pre_get_posts
:
add_filter( 'pre_get_posts', 'wpse_98213_add_post_types_to_tax_query' );
/**
* Let WP search for custom post types on taxonomy archives.
*
* @wp-hook pre_get_posts
* @param object $query
* @return object
*/
function wpse_98213_add_post_types_to_tax_query( $query )
{
if ( ! is_main_query() or ! is_tax( 'your_taxonomy_name' ) )
return $query;
$query->set( 'post_type', array ( 'portfolio', 'post' ) );
return $query;
}
More examples in our tag pre-get-posts.