I've got a '76 Triumph TR7 and I was wondering this after I got its carbs tuned after its restoration. As many of you may know, old engines are initially not capable of running as lean as modern engines, due to less (local)cooling capacity, and older and maybe softer valves and valve seats. My manual originally prescribes a (idle )CO% of 2.5-4.5% which equals to approx. lambda 0.87-0.93 or 12.8-13.6 AFR.
The point is, when I rebuild the engine, I put hardened valves and valve seats in it so I could use unleaded fuel. And those modern valves and seats may be more heat resistent than the oldies. Also, modern coolant and engine oil may have higher cooling capacity and better heat transfer. Lastly, modern fuel is different from fuel from those days.
If I sum up those things, I would say one can run this engine leaner than prescribed because the engine benefits from modern parts and fluids. However, I'm not sure if it's true in practice, and how much leaner you can go before you burn your valves or other problems might occur. Does anybody have experience in these matters? Tips, info are welcome.