Tom, a couple of thoughts.
One, the picture you posted to demonstrate that people have used angled feed tubes seems to have caused some confusion. People are assuming that is a picture of your project.
Two, have you experimented with your airflow by restricting the feed tube opening? Seems to me it would be a really fast test to see whether or not you're getting too much air into the system, which might contribute to lower temperatures and prevent the full "rocket" effect.
Other than that suggestion, I've got nuttin'
