cant get capybara-webkit to work

I am using capybara to do integration testing on my rails 3 app.

When i add :js => true to a scenario my tests work with selenium but when i change to use the webkit driver for js i get errors for each like this :

Capybara::Driver::Webkit::WebkitInvalidResponseError: Unable to load URL: http://www.example.dev:7171/user_sessions

My add is a multi domain app so in my spec_helper i set port : Capybara.server_port = 7171

And in a background block i set the app_host like this : Capybara.app_host = "http://#{subdomain}.example.dev:7171"

If i then add this to my spec_helper rspec.configure block i then get the errors above :

config.before(:each) do
   DatabaseCleaner.start
   Capybara.run_server = false
   Capybara.javascript_driver = :webkit
   Capybara.default_selector = :css
   Capybara.server_port = 7171 
end

Can anyone help with this? I am also using spork for auto testing.