When will a weak spark show up?

[NDT]

Does anyone know this?
More likely to misfire at part throttle or full throttle?

[Auntieroll]

In normal running the misfire due to a weak spark would usually occur when the cylinder pressure is at
its higher levels, ie at or around the RPM that maximum torque is developed.

[kev b]

A lean mixture is harder to ignite.

[Auntieroll]

kev b said:

A lean mixture is harder to ignite.

True, but I was careful to specify "normal" (ie not lean mixture) running conditions.

[kev b]

Maybe I should have said "leaner".

What sort of engine are we talking about? A carb motor will have richer and leaner mixtures even when the carb is adjusted correctly.
Injected engines have much better mixture control and electronic ignition as well.

A weak spark will show up when cylinder pressures are high and mixture is lean as this is the hardest mixture to ignite.