set_item for custom configs

[eluser]Unknown[/eluser]
Is there a way to set the value of a custom config. I can retrieve a custom config but I can't find a way to set it.

Code:

echo $this->config->item('base_url') . '<br>';
echo $this->config->item('base_url', 'myconfig') . '<br>';

$this->config->load('myconfig', TRUE);
echo $this->config->item('base_url') . '<br>';
echo $this->config->item('base_url', 'myconfig') . '<br>';

// This sets the base config
$this->config->set_item('base_url', 'new_url');
// How do I set the myconfig setting???
$this->config->set_item('base_url', 'new_myconfig_url', "myconfig");

echo $this->config->item('base_url') . '<br>';
echo $this->config->item('base_url', 'myconfig') . '<br>';