Open a HTTP port on Amazon EC2

In EC2 console, look for the column "Security Group" of your instance. Then go to option "Security Groups" above "NETWORK & SECURITY" in navigation menu on left. It will show up the security group you saw before. Click on it, it will show a split screen where the one is above has 2 tabs: Details and Inbound. Go to Inbound, in port range input 8181 and source the ip you want to allow incoming access to instance. Don't forget to apply changes.

As per Agat, double check the firewall settings on the server itself. I had the same issue with port 8090 and then I realised I needed to open it in the Windows firewall.