First, let's set the baseline — Page 90, Lesson 120

I want to walk you through what happens when an air/fuel mixture goes weak — that is, when there's more air and less fuel than the chemically correct ratio. This is a critical concept for you as a pilot, because running weak is how you save fuel, but it comes with real dangers. First, let's set the baseline. The chemically correct ratio — the stoichiometric mixture — is about 15 parts air to 1 part fuel, written as 15 : 1. That's the ratio where all the fuel and all the oxygen burn completely. Now, a weak mixture is anything weaker than that — more air, less fuel. Here's the key physics. A weak mixture burns at lower temperatures, yes, but it also burns at a slower rate. Why? Because there's a greater proportion of nitrogen in the cylinder. Nitrogen is inert — it doesn't burn — so it acts as a diluent, slowing the flame front down. Now, the consequences. Because combustion is slower, power output decreases as you weaken the mixture. But here's the interesting part: because the burning is cooler, the engine becomes more efficient. So the fall in power is proportionally less than the decrease in fuel consumption. That means the Specific Fuel Consumption — SFC, the fuel used per unit of power produced — actually decreases as you weaken the mixture below 15 : 1. That's the whole reason we lean the mixture: better fuel economy. For economical cruising at moderate power, you might use air/fuel ratios as weak as 18 : 1. But note this: at that ratio, you need an advance in ignition timing — the spark has to fire earlier — to allow for that slower rate of combustion. The flame takes longer to travel, so you have to start it sooner. Now, the dangers. This is where I want your full attention, because these are failure conditions you need to recognise. First, with extremely weak mixtures, the gases may still be burning when the exhaust valve opens. The exhaust valve is exposed to those high temperatures, and that can cause the valve to crack or distort. That's a mechanical failure you don't want. Second, and this is a nasty one — as the inlet valve opens, the heat of the exhaust gases is still so high that it can ignite the fresh mixture in the induction system. That causes 'popping back' through the induction manifold. That's a backfire — the flame travels backwards through the intake. Third, this slow burning also causes overheating. Here's the mechanism: a certain amount of the heat is not converted into work by expansion. In other words, some of the heat energy that should be pushing the piston down is instead just wasted as heat, and that heat has to be dissipated by the cooling system. So the engine runs hot. So, the bottom line — and this is the practical takeaway — the mixture requirement is dependent upon engine speed and power output. You can't just pick one ratio and stick with it. The correct mixture changes with what the engine is doing. And there's a typical air/fuel mixture curve that shows this relationship — that's Figure 7.1. Let me just recap the chain for you. Weak mixture → slower burn → less power, but better SFC down to a point. Push it too weak → exhaust valve damage, popping back, and overheating. So the art of leaning is finding that sweet spot where you get the fuel economy without crossing into the danger zone. That's what the mixture curve is all about.