2-legged OAuth implementation in C#

I'm trying to implement a two-legged OAuth authentication, so I can get to create a User on a website. I managed to craft this ruby code that performs that task, but I need to convert it to C#.

I'm getting a 401 error when making the last POST in C#, but the code is practically the same. I'm getting crazy here. Can someone jump in and help me know what's wrong with my code?

Ruby

consumer = OAuth::Consumer.new(KEY, SECRET, :http_method => :post)
# response = consumer.request(:get, "#{SITE}/oauth/request_token.json") 
# request_token_hash = JSON.parse(response.body) 
# puts request_token_hash
access_token = OAuth::AccessToken.new consumer
user = { 
     "user[name]" => 'John' 
}
response = access_token.post("#{SITE}/users.json", user) 
user_hash = JSON.parse(response.body)
puts user_hash

string consumerKey = "KEY";
            string consumerSecret = "SECRET";
            Uri uri = new Uri("#{SITE}/oauth/request_token.json");

            var oAuth = new OAuthBase(); // this class generates signatures
            string nonce = oAuth.GenerateNonce();
            string timeStamp = oAuth.GenerateTimeStamp();
            string outURL;
            string queryString;

            string sig = oAuth.GenerateSignature(
                uri,
                consumerKey,
                consumerSecret,
                string.Empty,
                string.Empty,
                "POST",
                timeStamp,
                nonce,
                OAuthBase.SignatureTypes.HMACSHA1,
                out outURL,
                out queryString);


            sig = HttpUtility.UrlEncode(sig);

            var sb = new StringBuilder(uri.ToString());
            sb.AppendFormat("?oauth_consumer_key={0}&", consumerKey);
            sb.AppendFormat("oauth_nonce={0}&", nonce);
            sb.AppendFormat("oauth_timestamp={0}&", timeStamp);
            sb.AppendFormat("oauth_signature_method={0}&", "HMAC-SHA1");
            sb.AppendFormat("oauth_version={0}&", "1.0");
            sb.AppendFormat("oauth_signature={0}", sig);

            var tokenRequest = (HttpWebRequest)WebRequest.Create(sb.ToString());
            tokenRequest.Method = "POST";
            var response = tokenRequest.GetResponse().GetResponseStream();
            if (response != null)
            {
                var responseReader = new StreamReader(response);
                var requestToken = JsonConvert.DeserializeObject<RequestToken>(responseReader.ReadToEnd());
                System.Diagnostics.Debug.WriteLine("REQUEST TOKEN: " + requestToken.token.oauth_token);

                // EVERYTHING IS OK UNTIL HERE

                // Creating user
                nonce = oAuth.GenerateNonce();
                timeStamp = oAuth.GenerateTimeStamp();

                var usersUri = new Uri("#{SITE}/users.json");

                // Generate signature
                string userSig = oAuth.GenerateSignature(
                userUri,
                consumerKey,
                consumerSecret,
                requestToken.token.oauth_token,
                requestToken.token.oauth_token_secret,
                "POST",
                timeStamp,
                nonce,
                OAuthBase.SignatureTypes.HMACSHA1,
                out outURL,
                out queryString);

                userSig = HttpUtility.UrlEncode(userSig);

                // Generate request URL
                sb = new StringBuilder(userUri.ToString());
                sb.AppendFormat("?oauth_consumer_key={0}&", consumerKey);
                sb.AppendFormat("oauth_nonce={0}&", nonce);
                sb.AppendFormat("oauth_timestamp={0}&", timeStamp);
                sb.AppendFormat("oauth_signature_method={0}&", "HMAC-SHA1");
                sb.AppendFormat("oauth_version={0}&", "1.0");
                sb.AppendFormat("oauth_token={0}&", requestToken.token.oauth_token);
                sb.AppendFormat("oauth_signature={0}&", userSig);
                sb.Append("user[name]=John");


                // Prepare web request...
                var myRequest = (HttpWebRequest)WebRequest.Create(sb.ToString());
                myRequest.Method = "POST";

                // Get response and read it
                var reader = new StreamReader(myRequest.GetResponse().GetResponseStream()); // THROWS AN UNAUTHORIZED EXCEPTION (401 Status Error)
                System.Diagnostics.Debug.WriteLine("RESPONSE USER: " + reader.ReadToEnd());

Thanks,