diff -r e27684eed3b6 Lib/email/generator.py
--- a/Lib/email/generator.py	Tue Nov 26 19:24:01 2013 -0600
+++ b/Lib/email/generator.py	Wed Nov 27 16:35:03 2013 +0800
@@ -14,6 +14,7 @@
 
 from io import StringIO, BytesIO
 from email._policybase import compat32
+from email import message_from_bytes
 from email.header import Header
 from email.utils import _has_surrogates
 import email.charset as _charset
@@ -225,9 +226,16 @@
         if _has_surrogates(msg._payload):
             charset = msg.get_param('charset')
             if charset is not None:
-                del msg['content-transfer-encoding']
-                msg.set_payload(payload, charset)
-                payload = msg.get_payload()
+                out = BytesIO()
+                BytesGenerator(out).flatten(msg)
+                copy_msg = message_from_bytes(out.getvalue())
+                try:
+                    # Downcast 8bit data to 7bit data
+                    del msg['content-transfer-encoding']
+                    msg.set_payload(payload, charset)
+                    payload = msg.get_payload()
+                finally:
+                    msg = copy_msg
         if self._mangle_from_:
             payload = fcre.sub('>From ', payload)
         self._write_lines(payload)
diff -r e27684eed3b6 Lib/test/test_email/test_email.py
--- a/Lib/test/test_email/test_email.py	Tue Nov 26 19:24:01 2013 -0600
+++ b/Lib/test/test_email/test_email.py	Wed Nov 27 16:35:03 2013 +0800
@@ -3743,6 +3743,16 @@
         email.generator.Generator(out).flatten(msg)
         self.assertEqual(out.getvalue(), self.non_latin_bin_msg_as7bit_wrapped)
 
+    def string_generator_should_not_mutate_message_when_handling_8bit(self):
+        msg = email.message_from_bytes(self.non_latin_bin_msg)
+        out = BytesIO()
+        BytesGenerator(out).flatten(msg)
+        orig_value = out.getvalue()
+        Generator(StringIO()).flatten(msg)
+        out = BytesIO()
+        BytesGenerator(out).flatten(msg)
+        self.assertEqual(out.getvalue(), orig_value)
+
     def test_bytes_generator_with_unix_from(self):
         # The unixfrom contains a current date, so we can't check it
         # literally.  Just make sure the first word is 'From' and the