Utf8 string braking one the way to web service function - web-services

Question

When passing utf8 data to a web service, the data breaks, and it gets to the function with weird symbols after every chunk of text, always in the same location.
This problem does not happen always.
When adding a character that changes the length of the string -the problem disappears.
This problem happens only when sending utf8 characters. When sending English letters, the problem does not happen
example:
this is the string I send (as regular string, not as XML)
string string_to_send="<API><METADATA id="METADATA"><SITE>sapir</SITE>
<SESSION_ID>52CA5BF6-472B</SESSION_ID><READER_ID></READER_ID>
<LANG_ID>HEB</LANG_ID> <LANG_UI_ID>HEB</LANG_UI_ID><ITEM>
<ITEM_ID>1234</ITEM_ID><update><FIELD lif ="SH2" collector_lif="0"
collector_val="0" old_val="N" new_val="N"/><FIELD lif="TA" collector_lif="0"
collector_val="0" old_val=""
new_val="טטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטט
טטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטט
טטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטטט"/>
</update></ITEM><IDEASTATE></IDEASTATE></METADATA></API>";
This is how I send the data to the function in the web service:
ideaCatApi.idea_edit_item(string_to_send);
I have service reference to the web service:
this is the configuration in the web config
<system.serviceModel>
<bindings>
<basicHttpBinding>
<binding name="idea_api_cat" />
</basicHttpBinding>
</bindings>
<client>
<endpoint
address="http://my_ip/ws/services/idea_api_cat_service"
binding="basicHttpBinding" bindingConfiguration="idea_api_cat"
contract="API_cat_idea.idea_api_cat_service"
name="idea_api_cat_service" />
</client>
</system.serviceModel>
I run this code from my local machine - Windows 10
to the server - Windows Server 2012 R2
I tried checking if the web service function brakes the text, but its developer said he gets it with the weird symbols
I expect the string to either always brake if there is an encoding problem,
I want to get to a state that the string will get to the web service function with no weird symbols
Am I missing headers?
I tried encoding the c# string but it did not help
Any other idea what can cause the problem?
I tried adding content-type header, but it did not help. So now my code looks like this:
using (new OperationContextScope(ideaCatApi.InnerChannel))
{
HttpRequestMessageProperty requestMessage = new HttpRequestMessageProperty();
requestMessage.Headers["Content-Type"] = "text/html;charset=utf-8";
OperationContext.Current.OutgoingMessageProperties[HttpRequestMessageProperty.Name] = requestMessage;
xmlString = ideaCatApi.idea_edit_item(sb.ToString());
response = parseResult(xmlString);
}