30 years experience of messing about with bike engines (or a small part of it ):
We all run slightly rich (or wet, as Hans said). It's well known that leaning the mixture slightly gets more power, but it's a fine line between more power & blowing holes through pistons, hence running slightly on the rich side for a safety margin (we haven't got the luxury of FI & finely metered A/F mixtures, it's just a basic Amal carb that's set up to cope with all conditions (running at sea level one day, maybe high up in mountains the following day). I once set the cruise mixture slightly weak (lean) as an experiment after listening to somebody extolling the virtues of lean burn engines. While cruising at 80mph the fuel economy went from about 50mpg to a reliable 80mpg(!!!!!!), low end running & cruising was OK, but hard acceleration & high speed running was impossible due to lack of power & severe pinging (very severe ). Additionally, the engine was running much, much hotter than normal. Normally I can touch the timing cover & it's hot. While doing that run I touched the timing cover & left a layer of skin festering away on it. Why did I touch it? Because the oil had got so hot that at 5000rpm it was thin enough for oil pressure to drop enough for the oil warning light to come on (that isn't a reassuring sight when 800 miles from home in a foreign country, you don't speak the lingo & don't have any spares to do an engine rebuild at the side of the road ). Additionally, we all run chrome exhaust pipes & the people who have set their mixtures lean (erroneously) find that the pipes blue from the head right down until the curve under the engine. Normally they blue for about 1/3 this distance.
So, my experience of running lean is that the motor runs hotter. As the original poster asked, why is this? Assuming max combustion temp is at the stoic ratio, the combustion temp when running lean will be less. My guess is pre-ignition may be responsible (if it's very "mild" then the chances of hearing it over the din of the exhausts is minimal), or it could be due to lean mixtures burning slower. I've forgotten most of this stuff, so could be making some completely wrong assumptions. Assuming that pre-ignition isn't occuring, a slower burning mixture could still be burning as it exits the exhaust valve. This will cause massive heat build up in the head, heat that will then get carried into the rest of the engine by conduction & the oil. Years back a mate decided that many Triumphs have rattly valve gear as the valve clearances were too large (nothing to do with backlash in the gears, worn out followers, etc). He read up on cam design & found out that the clearance was to allow for expansion of head, cylinder, pushrods, etc, so had the bright idea of getting the engine hot & then setting the clearances to zero (shame he didn't read up a bit further ). A test run resulted in an engine that felt a bit "heavy" and power was down. That night we took a ride out to a pub in the middle of nowhere & once we got out into the dark unlit country roads it was obvious to me that his exhaust pipes were glowing red hot (which looked pretty ) proving that burning gasses passing the exhaust valve cause massive overheating (which is common knowledge as the seats burn if clearances are set tight). Additionally, severely lean mixtures cause spitting back through the carbs. I'm guessing that the slower burn means that during the overlap period combusion hasn't completed so some of the fresh mixture coming in is burnt. While running this will just get spat out of the exhaust (causing heating of the valve, seat & exhaust port), but during slow running, eg cranking the engine to start it, there's nothing to stop it igniting the fuel in the inlet tract. And during normal running speeds is it possible that the spent charge that's still burning during the overlap period can cause severe detonation as it's igniting the charge that's been inducted?
Lean mixtures burn slower as there's less fuel molecules floating about. This means that the flame front is hindered due to small volumes where there's nothing to burn (a simplistic way of looking at it). I don't think detonation is involved in any of the above as the engines all had the pistons & chambers smoothed & polished so that carbon doesn't build up & hot spots are reduced. This is even less of a problem now as modern unleaded burns cleaner & carbon build up is a tiny fraction of what it used to be when running leaded fuel. Also, setting the mixture correctly makes the engines run fine. If there were hot spots causing detonation, then wouldn't they still cause detonation with the correct mixture?
Running rich also caused engine temps to climb. This was something to do with the large amount of carbon building up & an increase in CR, though I haven't a clue about the physics of it (the carbon would insulate the engine internals so conduction would be reduced, maybe it's due to hotter gasses passing by the exhaust valve again? Or maybe the flame front is slowed again due to not enough O2....).
All the above relates to air cooled engines, so symtoms may be very different with water cooled ones where heat is removed more quickly from the head?
Due to experience we all err on the side of slightly rich, especially now we have to use unleaded. Common experience has shown that lean mixtures can result in holed pistons (I've got an entire shelf full of pistons out of engines that have done this). My question is the same as the original poster..... why? These are street engines, not race engines. Occasional leaning out of the mixture won't cause a catastrophic failure (proven time & again by the amount of times we run out of gas without causing any damage ), but continuous running while lean will.
Something else I've found with wildly wrong mixtures is that a very rich mixture results in impressive backfires & sheets of flame out of open pipes when kicking an engine over. I'm guessing that the rich mixture resists burning due to the excess of fuel & only fully burns once it's "thinned out" by passing into the exhaust. A very lean mixture can cause bad kicking back. I can't work this one out at all. If a lean mixture is slow burning then why is it pushing down on the piston before it reaches TDC & reversing the rotational direction of the engine? Could it be that it's so slow burning that it ignites the charge that's coming in & this then fully ignites on the compression stroke? Having a modified 850cc twin kick back hurts like hell & has enough force to bend a steel insert in the sole of a boot.
So, I know what the results of lean/rich mixtures are, but not why, it's all guesswork. If somebody knows the details then please post (also if anybody knows where to buy a pair of Carolina style engineer boots (without rinky-dinky buckles or HD written all over them) please let me know. Damn Commandos, that thing cost me a decent pair of boots, a badly bruised foot & a pair of new 32mm carbs! ).
When an engine is running rich there is extra fuel in the combustion space to absorb some of the heat of that combustion. Being a vaporized liquid, this extra fuel can actually soak up quite a bit of heat. With no oxygen to oxidize it that is all the left over fuel has to do, soak up heat.
On the other side of that is the lean mixture. At very lean mixtures the combustion temperature will go down slightly because there is left over air in the combustion space. Being a gas, this left over air can not absorb as much heat as fuel does, but it does soak up a little.
Now, we must add another player to this picture. Elevated pressure is the product of combustion in a confined space. High pressure pushes things around. It will push the piston down in the cylinder. It will also push more heat through the surrounding metal surfaces than would otherwise flow through them in the absence of that pressure. This is why the engine runs hotter when you drive it hard.
With this being said, we need to think about the load duty on the engine. At part throttle cruising speeds the engine is under a light load. The cylinders are intaking their combustion mixtures from an intake manifold that is under a good vacuum. This means that the density in the cylinders is not very high. The combustion pressure (yield) of such a charge is not going to be high at all. Nor does it need to be. Pushing a C3 down the road even with it's brick like aerodynamics only takes a small fraction of the horse power these engines can produce.
Now, let us look at what happens when you go to WOT. At WOT the cylinders get a much denser charge. This means that the resulting pressure and temperature will be much higher. With temperature AND pressure working on them the internal engine parts don't stand much of a chance. The temperature and pressure act as a "shaped charge" on the top of the piston. The temperature flow into the top of the piston increases under the force of the pressure thereby softening the material. The pressure then blows a hole through it.
We make the mixture rich at WOT to absorb some of the heat of combustion. This also decreases the pressure a little and helps the engine live longer. It all has to do with the load duty on the engine.
"Lean" is an oxidizer rich fuel mixture. "Lean" mixtures cause cause extremly high burn temperaures. In an engine, a lean mixure will cause burnt valves. You don't want your engine to run lean.
For liquid rocket engines, the turn off sequence purposedly runs the engine in a rich mixture before turn off. At the approach of turn-off, the oxidizer is reduced in flow to produce a fuel rich mixure. Rocket engine turn-off with a fuel rich mixture gurantees that turn-off will ocur with out an explosion. An oxiderzer rich mixture at turn-off could very well explode.
Wow, alot to think about! Everything from "lean is hot, duh!" to personal experience, to very good physical explanations. Probably many of the explanations have some hand in the reality of it. So far, I like the idea that vaporizing the excess liquid fuel soaks up alot of the heat. And of course the fuel that is used must be vaporized too (nothing liquid burns). I will have to compare the numbers of the heat of vaporization and the heat released in combustion. If they are on the same order of magnitude, that might just be a definitive answer to the question...
I appreciate all the responses, nobody should feel bad for not being sure! The professor of this class (now head engine engineer of a NASCAR Cup team) told us it took him a year to dig up the answer to this question when he learned in grad school that adiabatic flame temperature was lower in any imperfectly mixed combustion. As often they do, theory and experience seemed to contradict and its interesting to find out why.