error 414 uri too long

Per the W3C definition of the 414 status code:

The server is refusing to service the request because the Request-URI is longer than the server is willing to interpret. This rare condition is only likely to occur when a client has improperly converted a POST request to a GET request with long query information, when the client has descended into a URI "black hole" of redirection (e.g., a redirected URI prefix that points to a suffix of itself), or when the server is under attack by a client attempting to exploit security holes present in some servers using fixed-length buffers for reading or manipulating the Request-URI.

Additionally, $this->input->post() is used to retrieve data which has been posted to the server, not to post data from a form.

You're probably going to have to show us some code to get any real help with this. Your description of the problem implies that you may have some problems with your view and controller as well as your JavaScript to handle the AJAX post.

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide