Detonation is likely to occur in the cylinders:

Detonation, or abnormal combustion, is a condition where the air-fuel mixture ignites prematurely in the combustion chamber. This phenomenon is particularly likely when the mixture is weak and the cylinder head temperature is high. A weak mixture can lead to an increase in cylinder temperatures as the engine struggles to maintain operation with insufficient fuel to effectively cool the combustion process.

When combined with high cylinder head temperatures, which can occur due to various operational stresses and inefficiencies, the conditions are ripe for detonation. The heat causes the remaining fuel-air mixture to ignite uncontrollably, leading to the rapid rise in pressure and temperature typical in detonation.

In contrast, an over-rich mixture at idle power usually results in smoother combustion because excess fuel can absorb heat and cool the combustion chamber. Similarly, a rich mixture at high power settings typically leads to stable combustion rather than detonation, as there is enough fuel to lower combustion temperatures. Very low engine speeds might restrict airflow but generally don’t promote the extreme conditions necessary for detonation to occur. Hence, the scenario of weak mixtures paired with high temperatures is particularly conducive to detonation.