SharePoint Fast Search2010 Incremental Crawl BCS

1. You can add these properties to your list methodInstance in bdc file and deploy again. 

   <MethodInstance Type="Finder" ReturnParameterName="DocumentReadList" Default="true" Name="DocumentReadList" DefaultDisplayName="ProductDocument Read List">          <Properties>        <Property Name="LastModifiedTimeStampField" Type="System.String">YourModifiedDateField</Property>   </Properties>   </MethodInstances>

   For IdEnumerator set __BdcLastModifiedTimestamp Property
   

   <Properties> <Property Name="__BdcLastModifiedTimestamp" Type="System.String">LastModifiedOn</Property> </Properties>

1. You can add these properties to your list method of entity using PowerShell:
   $entity = Get-SPBusinessDataCatalogMetadataObject  –BdcObjectType “Entity” –Name “EntityName” –Namespace “ModelNameSpace” -ServiceContext adminurl
   $methodinstance = $entity.MethodInstances | where {$_.Name –eq “ReadListName”}
   $methodinstance.Properties.Add("LastModifiedTimeStampField", YourModifiedDateField)
   $methodinstance.Update()

You can check sample of:

HOW TO: Create a Searchable SharePoint 2010 BDC .NET Assembly Connector Which Reads From A Flat File