Good day.
Running at medium speed, like 40mph, on direct gear, perhaps on 'overdirect', engine hesitates when mildly climbing as throttle is not pushed to force third gear.

Otherwise, runs very well
Had the same symptom about a year ago; cured replacing one damaged spark plug wire. How to detect which of the 8 wires to replace this time ?

And the explanation part... Why a 'pushed' high gear causes the spark to arc/miss ? In other words, why a faulty spark wire shows up its 'leak' when the engine is exerted at low rpm ?

 

A quick scan should show which cylinder is missing, but why not buy a set at this point?

 

The higher the cylinder pressure the more voltage is required for the spark to jump the electrode gap. When an engine is running under very little load very little air/fuel is able to make it in to the cylinders because the throttle plate is relatively closed. This means the voltage doesn't have to "push" it's way through a lot of air to make it across the electrode gap to ignite the fuel. As engine load increases you have to open the throttle more to get the same RPM, so more air is able to make it's way in to the cylinder in the same amount of time which makes it more difficult for the plug to fire. If the wiring (or plug) is in bad shape it won't be able to deliver the voltage needed to ignite the plug and a misfire occurs.

If you push the gas pedal down enough to get a kickdown in to 3rd the engine RPM increases and even though you are opening the throttle plate more the increased engine speed means less air is able to get in to the cylinder in the same amount of time and therefore the spark plug need less voltage to fire. The engine is running faster, but under less load- so to speak.

 

Because I have seven new ones in its box ?

Thanks for the explanation, greisinb !