Main feed not showing custom posts (wp 3.1)

Is it possible to modify the type of posts shown by main wordpress’s feed?

to access the feed, I use in the header template:

<link rel="alternate" type="application/rss+xml" title="RSS 2.0" href="<?php echo get_bloginfo_rss('rss2_url'); ?>" />

I tried both adding filters and actions but nthing changed the posts in the generated feed; my post type is lbotika_ads but wp outputs only normal posts.

so far I use

add_filter( 'pre_get_posts', 'my_get_posts' );
function my_get_posts( $query ) {

	if ( is_feed() )
		$query->set( 'post_type', array( 'lbotika_ads' ) );

	return $query;
}

any hint?