I think the question was 'why does the lean mixture burn hotter?'
Although I don't have the words to explain it, I know that the heat produced is proportional to the amount of each part of the mixture as it is with any chemical reaction. You could probably figure it out if you got into the physical/chemical properties of air & fuel. 'Stoich' is the perfect combination of air and fuel where both are completely consumed in an internal combustion engine. It's the point of efficiency.
Well, I've typed the following paragraph several times and still can't get it straight, so it no longer exists.
In additional thought on running too rich for too long: gasoline in the oil pan doesn't lubricate bearings very well.
I can't believe PKRWUD hasn't been on here already with the nitty gritty on A/F ratios and the combustion process. Chemical reactions just aren't my mug of beer. I'm still waiting for class to begin
