I know that advancing the timing will cause the motor to run leaner so yes, retarding the timing would make it run richer.
Advancing the timing doesn't make it run leaner, but it does increase the "in cylinder" temperature by burning a larger portion of the air/fuel mixture, thereby producing more BTU's of heat..
The air/fuel ratio is what it is. Retarding the timing will lower the amount of mixture burned in the cylinder, reducing heat in the chamber (safer for the pistons), and increasing the temp in the exhaust header (harder on exhaust valves). Advancing the timing does the opposite. It lowers the exhaust gas temp but raises the "in cylinder" temp, making it more likely to melt a piston on a long endurance run. I've seen some that were lured into thinking they had a safe tune because the exhaust gas temps were low, and then went out and melted the pistons. You must be able to read a spark plug to "know" what is going on in the cylinder even when the EGT's are low.
Drag race tunes typically run a lot of timing to burn as much mixture as possible in the chamber which makes more power. An endurance tune, for instance a "One Mile" Viper race, or an offshore boat, must run less timing in order to prevent melting pistons. This raises EGT's, and is why we also run Inconel valves and Berrylium Copper valves seats on the exhaust for endurance apps.
When you richen the mixture it's different. Cylinder temps and exhaust temps go hand in hand with an air/fuel ratio change. Temps in both the cylinder and the exhaust both go down when the mixture is richened. When you lean it, they both go up. Different than timing effects.
