You may indeed have other problems (what are those needles, and are they the right match for your engine), but a weak spark will cause missing under load, or a complete failure to fire.
To answer one of your questions, revving the car without load is not the same as driving up a long hill under high load.
The coil only rises to the minimum amount of volts it needs to jump the spark plug gap. It rises and drops in a sharktooth pattern. It will only rise to what it needs before it breaks over, and then it will drop away dramatically. If there's a fault in the ignition system (sparkplug condition/gap, leads, cap, rotorbutton damage), then the voltage may never rise high enough.
When you have a car under no load, or very light load, the cylinder pressure is quite low as the butterflies are shut and cylinder pressure is low. At low pressures, a spark finds it quite easy to break over the spark plug gap as break over voltage is proportional to the density of the air it is trying to traverse. The lower the air density, the lower the voltage required to break over the gap.
Once you open the butterfly, then large amounts of air flood in, and the cylinder pressure (air density) rises dramatically under compression. The break over voltage for the spark cab vary by thousands of volts, which in a poorly performing ignition system it may not be able to attain due to faults.
Also, if there is a problem with the components due to dirt or moisture causing tracking, then the voltage will rise until it can find another path to ground. it may choose to pass through the dizzy cap, or the rotor button, the leads or the plugs to the engine block. Spark don't care. Spark goes down the easiest path. As it does so, it creates carbon, carbon is a great conductor, so the situation only gets worse in the case of dizzy caps and rotor buttons.
With respect to your SU, you can tune just about any engine with just about any needle at idle. Its just that once you open the butterfly under load, and the damper piston begins to rise, then any result is possible and all of them are wrong without the right needle. The air/fuel mixture is going to be anything but correct. And much the same thing goes with the needle. Under load is not the same as not under load. For a given rev as a comparison, you will need different amounts of butterfly opening. This changes the volume of air consumed by the engine, which alters the height the SU piston will rise to. Hence the needle is very important.
_________________ SooperDooperMiniCooperExpertEngineering
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