trackback teaser (excerpt?) truncation / unclosed tag

The webmaster of an external site told me that the trackbacks my drupal site sent to his posts did not close the <em> tag. (The <em> then formats the comments following my trackback, affecting his site content.)

A fix should put </em> between the end of the teaser excerpt and the [Read More] link to the node.

The teaser text in the node database entry has the text properly close tagged. It seems that the trackback truncates the teaser text at 255 and doesn't close:

from trackback.module:
"
function trackback_exit() {
global $trackback_node;

if ($trackback_node) {
//gather together data from node fields and format as a ping message
$edit = $_POST['edit'];
$params = array('title' => $trackback_node->title, 'excerpt' =>
truncate_utf8(check_output($trackback_node->teaser), 255), 'blog_name' => variable_get('site_name', ''), 'url' => url('node/'. $trackback_node->nid, NULL, NULL, 1) );
"

I tried these experiments to patch the problem:

truncate_utf8(check_output($trackback_node->teaser), 240).'dork' inserts "dork."

truncate_utf8(check_output($trackback_node->teaser), 240).'</em>' doesn't do anything...