function bd_change_product_query( $q ){
$meta_query = $q->get( 'meta_query' );
$meta_query[] = array(
'key' => 'custom_acf_key',
'value' => 'custom_acf_value',
'compare' => '='
);
$q->set( 'meta_query', $meta_query );
}
add_action( 'woocommerce_product_query', 'bd_change_product_query' )