.net wcf web service 400 bad request

I am stuck.  I am trying to make a wcf web service on my sharepont 2010 site.  I created my web service using a cksdev visual studio plugin that creates the file for you.  I had a more complex sample, but it wouldn't work, so I created a new one straight out of the box that returns a "hello world".

I am calling it from jQuery and keep getting a 400 bad request response that I can see in fiddler.

If I try an incorrect url, I get a file not found error, so I know that the web service exists.  I can't step into code and have no idea how to diagnose this.

my svc.cs file
    [BasicHttpBindingServiceMetadataExchangeEndpoint]
    [AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Required)]
    public class WCFService1 : IWCFService1
    {
        // To test this service, use the Visual Studio WCF Test client
        // set the endpoint to http://<Your server name>/_vti_bin/Fmg.Extranet.ContextSelector.UI/WCFService1.svc/mex
        public string HelloWorld()
        {
            return "Hello World from WCF and SharePoint 2010";
        }
    }


My .svc file
<%@ ServiceHost Language="C#" Debug="true"
    Service="MYASSEMBLYNAME.WCFService1, $SharePoint.Project.AssemblyFullName$"  
    CodeBehind="WCFService1.svc.cs"
    Factory="Microsoft.SharePoint.Client.Services.MultipleBaseAddressBasicHttpBindingServiceHostFactory, Microsoft.SharePoint.Client.ServerRuntime, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>


My IWCFService1.cs file
    [ServiceContract]
    public interface IWCFService1
    {
        [OperationContract]
        string HelloWorld();
    }



My jQuery
                $.ajax({
                    url: "http://affiliatedfmonline/_vti_bin/Fmg.Extranet.ContextSelector.UI/WCFService1.svc/HelloWorld",
                    dataType: "json",
                    error: function (XMLHttpRequest, textStatus, errorThrown) {
                        alert(errorThrown);
                    },
                    success: function (data, textStatus, jqXHR) {
                        alert("success");
                        response($.map(data, function (item) {
                            return {
                                label: item.Name,
                                value: item.ID,
                                desc: item.Location
                            }
                        }));
                    }
                });

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