diff -r 28c04e954bb6 Lib/email/encoders.py
--- a/Lib/email/encoders.py	Tue Oct 29 22:25:55 2013 -0400
+++ b/Lib/email/encoders.py	Wed Oct 30 22:56:58 2013 +0800
@@ -49,7 +49,7 @@
 
 def encode_7or8bit(msg):
     """Set the Content-Transfer-Encoding header to 7bit or 8bit."""
-    orig = msg.get_payload(decode=True)
+    orig = msg._payload
     if orig is None:
         # There's no payload.  For backwards compatibility we use 7bit
         msg['Content-Transfer-Encoding'] = '7bit'
diff -r 28c04e954bb6 Lib/email/generator.py
--- a/Lib/email/generator.py	Tue Oct 29 22:25:55 2013 -0400
+++ b/Lib/email/generator.py	Wed Oct 30 22:56:58 2013 +0800
@@ -392,7 +392,11 @@
     _encoded_EMPTY = b''
 
     def write(self, s):
-        self._fp.write(s.encode('ascii', 'surrogateescape'))
+        try:
+            self._fp.write(s.encode('ascii', 'surrogateescape'))
+        except UnicodeEncodeError:
+            # Writing 8bit data to bytes container.
+            self._fp.write(s.encode('raw-unicode-escape'))
 
     def _new_buffer(self):
         return BytesIO()
diff -r 28c04e954bb6 Lib/email/message.py
--- a/Lib/email/message.py	Tue Oct 29 22:25:55 2013 -0400
+++ b/Lib/email/message.py	Wed Oct 30 22:56:58 2013 +0800
@@ -302,7 +302,10 @@
         set_charset() for details.
         """
         if isinstance(payload, bytes):
-            payload = payload.decode('ascii', 'surrogateescape')
+            if charset is not None:
+                payload = payload.decode(str(charset))
+            else:
+                payload = payload.decode('ascii', 'surrogateescape')
         self._payload = payload
         if charset is not None:
             self.set_charset(charset)
diff -r 28c04e954bb6 Lib/test/test_email/test_email.py
--- a/Lib/test/test_email/test_email.py	Tue Oct 29 22:25:55 2013 -0400
+++ b/Lib/test/test_email/test_email.py	Wed Oct 30 22:56:58 2013 +0800
@@ -702,6 +702,26 @@
             w4kgdGVzdGFiYwo=
             """))
 
+    def test_set_payload_with_8bit_data_and_charset_none(self):
+        str_data = 'АБВ'
+        bytes_data = str_data.encode('utf-8')
+        cs = Charset('utf-8')
+        cs.body_encoding = None
+        msg = Message()
+        for data in (str_data, bytes_data):
+            msg.set_payload(data, cs)
+            self.assertEqual(msg.as_string(), textwrap.dedent("""\
+                MIME-Version: 1.0
+                Content-Type: text/plain; charset="utf-8"
+                Content-Transfer-Encoding: 8bit
+
+                АБВ"""))
+            self.assertEqual(msg.as_bytes(),
+                    (b'MIME-Version: 1.0\n'
+                     b'Content-Type: text/plain; charset="utf-8"\n'
+                     b'Content-Transfer-Encoding: 8bit\n\n'
+                     b'\\u0410\\u0411\\u0412'))
+
 
 # Test the email.encoders module
 class TestEncoders(unittest.TestCase):