Glowing Header

[CWizard]

I checked the timing today on my 418W, which involved running the engine at various rpms for a few minutes. It ran at 1000 rpm for 5-6 minutes, then up to 2000 rpm for maybe 3-4 minutes. I got data at those points and was preparing to up the speed to 2500 rpm when I noticed the drivers side header was glowing red. Passenger side was not glowing. So I shut it down and grabbed my infrared thermometer. Driver's side header was 500F or higher (my gun only goes to 500) and passenger header was about 370F. Timing results: 22 degrees BTDC at 1000 rpm and 30 degrees at 2000. I stopped at 2000, so I don't have a current number for the total advance. Last time I checked it was about 34 degrees. So I don't think I have a retarded timing issue, unless the timing marks have suddenly gone bonkers (motor has 9000 miles on it). I've been fighting an overly rich idle mixture lately. Could a rich mixture explain the single hot header? Help!
Other info: Holley 4150 street HP on a Victor Jr intake. Hydraulic lifters were recently adjusted due to the addition of longer pushrods (valve geometry was wrong). No other changes, except the aforementioned idle mixture tinkering. Thanks.
Jerry