I've seen faulty magneto cam ring housings that caused quite a deviation from 180 degree spark timing. You could have the timing right for the rear cylinder and retarded for the front. A lean mixture for the front cylinder can make a even more marked rise in temperature. Uneven fuel distribution, or as mentioned, an air leak, could be the culprit. But the most like cause is as mentioned by Trevor, perhaps the rear is not doing anything at all but pumping air. After a few minutes running, you should not be able to touch either cylinder if they are firing.
-Doug