You need to use 'loadData' of combobox, for the options to be set to it.
Declare combobox as
<input class="easyui-combobox" id="FormatoCarta" name="FormatoCarta" data-options="valueField:'id',textField:'name'">
<script>
var loadData = [];
</script>
@foreach (var item in @Model.FormatiCarta)
{
String selected = String.Empty;
if (@Model.Defaults != null)
{
if (item.Id.Equals(@Model.Defaults.IDFormatoCarta))
{
selectedId = item.Id;
}
}
<script>
loadData.push({"id":item.Id,"name":item.name});
</script>
}
<script>
$('#FormatoCarta').combobox({data:loadData});
$('#FormatoCarta').combobox('setValue', selectedId );
</script>