diff -r 03a55e207720 Lib/email/charset.py
--- a/Lib/email/charset.py	Fri Nov 22 20:17:24 2013 -0500
+++ b/Lib/email/charset.py	Sat Nov 23 22:50:45 2013 +0800
@@ -386,7 +386,8 @@
         string using the ascii codec produces the correct string version
         of the content.
         """
-        # 7bit/8bit encodings return the string unchanged (module conversions)
+        if not string:
+            return string
         if self.body_encoding is BASE64:
             if isinstance(string, str):
                 string = string.encode(self.output_charset)
@@ -398,13 +399,9 @@
             # character set, then, we must turn it into pseudo bytes via the
             # latin1 charset, which will encode any byte as a single code point
             # between 0 and 255, which is what body_encode is expecting.
-            #
-            # Note that this clause doesn't handle the case of a _payload that
-            # is already bytes.  It never did, and the semantics of _payload
-            # being bytes has never been nailed down, so fixing that is a
-            # longer term TODO.
             if isinstance(string, str):
-                string = string.encode(self.output_charset).decode('latin1')
+                string = string.encode(self.output_charset)
+            string = string.decode('latin1')
             return email.quoprimime.body_encode(string)
         else:
             if isinstance(string, str):
diff -r 03a55e207720 Lib/email/generator.py
--- a/Lib/email/generator.py	Fri Nov 22 20:17:24 2013 -0500
+++ b/Lib/email/generator.py	Sat Nov 23 22:50:45 2013 +0800
@@ -223,7 +223,12 @@
         if not isinstance(payload, str):
             raise TypeError('string payload expected: %s' % type(payload))
         if _has_surrogates(msg._payload):
-            charset = msg.get_param('charset')
+            cs_object = msg.get_charset()
+            # Use original charset is we disable base64 encoding
+            if cs_object is not None and cs_object.body_encoding is None:
+                charset = cs_object
+            else:
+                charset = msg.get_param('charset')
             if charset is not None:
                 del msg['content-transfer-encoding']
                 msg.set_payload(payload, charset)
diff -r 03a55e207720 Lib/email/message.py
--- a/Lib/email/message.py	Fri Nov 22 20:17:24 2013 -0500
+++ b/Lib/email/message.py	Sat Nov 23 22:50:45 2013 +0800
@@ -275,9 +275,16 @@
         Optional charset sets the message's default character set.  See
         set_charset() for details.
         """
-        if isinstance(payload, bytes):
-            payload = payload.decode('ascii', 'surrogateescape')
-        self._payload = payload
+        if hasattr(payload, 'encode'):
+            if charset is None:
+                # We should check for ASCII-only here, but we can't do that
+                # for backward compatibility reasons.  Fixed in 3.4.
+                self._payload = payload
+                return
+            if not isinstance(charset, Charset):
+                charset = Charset(charset)
+            payload = payload.encode(charset.output_charset)
+        self._payload = payload.decode('ascii', 'surrogateescape')
         if charset is not None:
             self.set_charset(charset)
 
@@ -316,7 +323,15 @@
             try:
                 cte(self)
             except TypeError:
-                self._payload = charset.body_encode(self._payload)
+                # This if is for backward compatibility and will be removed
+                # in 3.4 when the ascii check is added to set_payload.
+                payload = self._payload
+                if payload:
+                    try:
+                        payload = payload.encode('ascii', 'surrogateescape')
+                    except UnicodeError:
+                        payload = payload.encode(charset.output_charset)
+                self._payload = charset.body_encode(payload)
                 self.add_header('Content-Transfer-Encoding', cte)
 
     def get_charset(self):
diff -r 03a55e207720 Lib/test/test_email/test_email.py
--- a/Lib/test/test_email/test_email.py	Fri Nov 22 20:17:24 2013 -0500
+++ b/Lib/test/test_email/test_email.py	Sat Nov 23 22:50:45 2013 +0800
@@ -92,6 +92,49 @@
         msg.set_payload('This is a string payload', charset)
         self.assertEqual(msg.get_charset().input_charset, 'iso-8859-1')
 
+    def test_set_payload_with_8bit_data_and_charset(self):
+        data = b'\xd0\x90\xd0\x91\xd0\x92'
+        charset = Charset('utf-8')
+        msg = Message()
+        msg.set_payload(data, charset)
+        self.assertEqual(msg['content-transfer-encoding'], 'base64')
+        self.assertEqual(msg.get_payload(decode=True), data)
+        self.assertEqual(msg.get_payload(), '0JDQkdCS\n')
+
+    def test_set_payload_with_non_ascii_and_charset_body_encoding_none(self):
+        data = b'\xd0\x90\xd0\x91\xd0\x92'
+        charset = Charset('utf-8')
+        charset.body_encoding = None # Disable base64 encoding
+        msg = Message()
+        msg.set_payload(data.decode('utf-8'), charset)
+        self.assertEqual(msg['content-transfer-encoding'], '8bit')
+        self.assertEqual(msg.get_payload(decode=True), data)
+
+    def test_set_payload_with_8bit_data_and_charset_body_encoding_none(self):
+        data = b'\xd0\x90\xd0\x91\xd0\x92'
+        charset = Charset('utf-8')
+        charset.body_encoding = None # Disable base64 encoding
+        msg = Message()
+        msg.set_payload(data, charset)
+        self.assertEqual(msg['content-transfer-encoding'], '8bit')
+        self.assertEqual(msg.get_payload(decode=True), data)
+
+    def test_bytes_generator_after_calling_as_string(self):
+        data = b'\xd0\x90\xd0\x91\xd0\x92'.decode('utf-8')
+        cs = Charset('utf-8')
+        cs.body_encoding = None # Disable base64 encoding
+        msg = Message()
+        msg.set_payload(data, cs)
+        msg.as_string() # It will call set_payload implicitly!
+        fp = BytesIO()
+        g = BytesGenerator(fp)
+        g.flatten(msg)
+        self.assertEqual(fp.getvalue(),
+                (b'MIME-Version: 1.0\n'
+                 b'Content-Type: text/plain; charset="utf-8"\n'
+                 b'Content-Transfer-Encoding: 8bit\n\n'
+                 b'\xd0\x90\xd0\x91\xd0\x92'))
+
     def test_get_charsets(self):
         eq = self.assertEqual