Exhaust manifold and pipe turning cherry red

[sonny30coupe]

One of the members in the club tonight called me and asked about a problem he had this evening with his car. He backed it out of the garage to let it run for a few minutes. After about 5 minutes he lifted the hood and noticed that the bottom of the exhaust manifold and first few inches of the tail pipe were cherry red. The things that came to mind to me were timing and running too lean. Maybe a sticking exhaust valve that is not closing completely. Any ideas on what may be causing this? He had not had the problem previous to this that he had noticed. He stated that the motor was sounding good and running well. Thanks in advance for any help. Sonny