i erased the <?php get_sidebar(); ?> from single-portfolio.php and then i added to style.css
.single-portfolio #content { width:100%; max-width: 980px; float:none; }
problem solved 😉
thank you all for the infoz
@bled05 i have the same problem too. Can you please tell me about the modification you did at single-portfolio.php? adding the code Devin proposed, did the problem solved?