WP_Query Filters

I can not figure out how to apply a filter to a custom query.

For example:

<?php
wp_reset_query();
$published_query_string = "author=39&post_status=publish";
$publishedQuery = new WP_Query( $published_query_string );
while ( $publishedQuery->have_posts() ) : $publishedQuery->the_post();
	do_some_stuff();
endwhile;
?>

<?php
function filter_where_custom() {
	$where .= " AND (wpostmeta_type.meta_key = 'foo')
		AND (wpostmeta_type.meta_value = 'bar')";
	return $where;
}
?>

<?php
add_filter('posts_where', 'filter_where_custom');
?>

This will apply the filter to the standard query, but not to this custom one created with new WP_Query.

I feel like I may need an apply_filters() hook somewhere… help?

(sorry for the double post)