diff -r e0a90b1c4cdf Lib/email/generator.py
--- a/Lib/email/generator.py	Fri Feb 07 15:04:26 2014 -0500
+++ b/Lib/email/generator.py	Sat Feb 08 11:37:47 2014 +0800
@@ -62,6 +62,8 @@
         self._mangle_from_ = mangle_from_
         self.maxheaderlen = maxheaderlen
         self.policy = policy
+        self.replace_cte_after_flatten = False
+        self.orig_cte = None
 
     def write(self, s):
         # Just delegate to the file object
@@ -113,6 +115,9 @@
         finally:
             self.policy = old_gen_policy
             msg.policy = old_msg_policy
+            if self.replace_cte_after_flatten:
+                msg.replace_header('content-transfer-encoding',
+                                   self.orig_cte)
 
     def clone(self, fp):
         """Clone this generator with the exact same options."""
@@ -222,12 +227,21 @@
             return
         if not isinstance(payload, str):
             raise TypeError('string payload expected: %s' % type(payload))
+        self.replace_cte_after_flatten = False
         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()
+                orig_payload = msg._payload
+                self.orig_cte = msg['content-transfer-encoding']
+                try:
+                    del msg['content-transfer-encoding']
+                    msg.set_payload(payload, charset)
+                    payload = msg.get_payload()
+                finally:
+                    cte = msg['content-transfer-encoding']
+                    msg._payload = orig_payload
+                    if self.orig_cte == '8bit' and cte == 'base64':
+                        self.replace_cte_after_flatten = True
         if self._mangle_from_:
             payload = fcre.sub('>From ', payload)
         self._write_lines(payload)
diff -r e0a90b1c4cdf Lib/test/test_email/test_email.py
--- a/Lib/test/test_email/test_email.py	Fri Feb 07 15:04:26 2014 -0500
+++ b/Lib/test/test_email/test_email.py	Sat Feb 08 11:37:47 2014 +0800
@@ -3782,6 +3782,16 @@
         email.generator.Generator(out).flatten(msg)
         self.assertEqual(out.getvalue(), self.non_latin_bin_msg_as7bit_wrapped)
 
+    def test_str_generator_should_not_mutate_msg_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) # Should not mutate 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