Thought I would provide a bit of information from what I have learned and observed over the years regarding ignition condensers. Hope it helps explain and not confuse the issue further.
A condenser (which will now be referred to as a capacitor ) in a conventional points ignition system has a challenging task as it is subjected to voltage spikes and oscillations as the contact set (points) opens and voltage level in the primary ignition coil windings starts to decay. A capacitor may be viewed as a very small battery that can be charged and discharged very rapidly when the circuit it is connected to reacts with it. A capacitor may also be described as a “status quo” component in that it will tend to try and maintain the voltage level of the circuit it is connected to as other forces try and change that voltage level – it will do this by charging or discharging as it reacts to the voltage level in a circuit. I will use a conventional 12 volt battery operated ignition system as an example. In a conventional battery ignition system the voltage level at the primary coil winding may be anywhere from -100 volts to +200 volts depending on the status of the contact set and the magnetic field inside the coil. When the contact set is closed the capacitor leads may be viewed as almost shorted since the two leads are connected across the contact set. Once the points begin to open current can no longer flow to the coil winding easily through them and the condensers first job is to absorb any current flow to minimize arcing until the air gap between the contacts is such that they no longer provide a current path that the primary voltage current can jump through the air. The voltage level inside the capacitor will rise rapidly from the near zero volt level it was at as it charges to its maximum capacity value in Microfarads. While this is occurring the magnetic field inside the coil primary winding is collapsing since there is no longer a current flow through the contacts of the point set. The majority of the energy from the collapsing magnetic field is induced into a high voltage current flow in the secondary coil winding creating a spark at the spark plug. However the condensers other purpose is to make the collapse of the primary field in the coil happen quickly as the left over oscillations in the primary winding occur. Attached is a scope waveform captured during a firing cycle on a 12 volt battery powered ignition coil primary as the points open on a Wisconsin TRA-10D small engine. As you can see the oscillations have voltage peaks starting at close to 200 volts and then they rapidly decline until they magnetic field is totally collapsed and the points close again. Each square represents a voltage level of 50 volts and a time factor of 500 uS (1 uS equals one millionth of a second). As can be seen in the waveform the capacitor is charging and discharging approximately 5 times per division of 500 uS (millionth of a second) or once every 100 uS if the number of oscillations are counted per grid division of time. At the initial voltage spike of almost 200 volts positive and then dropping to 100 volts negative a capacitor would need the ability to charge and discharge at a rate of 300 volts / 100 uS. If we divide both these numbers by 30 then the capacitor would require a spec of 10 volts / 3.4 uS or better for this application to work properly.
This website explains the theory in mush better detail than I can for those that are interested and mentions that a capacitors dV/dt rating (the rate of change in Voltage with respect to Time) is very important (which it is) as mentioned by others here.
https://www.themagnetoguys.co.uk/condensers-theory
Thought I would offer a bit of information on the capacitor that I recommended as a replacement on the Model 88 unit that “The Boat House” has. The capacitor part number I suggested as a replacement for the Mercotronic unit does have a spec sheet that is shown on the Mouser website and the spec sheet shows the capacitor part number has a dV/dt rating of 10 volts / uS. While this does not come close to the spec of 600 volts/ uS that is mentioned on the website link I provided it is still 3 times faster than what I calculated would be required given the waveform illustration of the ignition coil I posted. The dV/dt rating will usually increase as the voltage rating of a capacitor increases – if you check the spec sheet a capacitor with the same microfarad rating but with a voltage rating of 1500 volts has a 25v/uS spec. The capacitor I chose as a recommend replacement is, like most things in life, a compromise that provides a quality component that should do the job at a reasonable price.
As I stated previously purchasing from a reliable electronics supplier such as Mouser, Digikey or Newark you are getting a product that meets or exceeds a manufacturers specifications and do have some recourse if it does not or fails because of a manufacturing flaw. Purchasing a product like a capacitor from ebay, Amazon or Alliexpress because it appears to be a better price can result in the buyer receiving a product that may or may not be a quality component that can do the job properly in an ignition circuit. If something appears to be a bargain there is usually a reason for it and it may be that the component does not meet the specification that it was supposed to and the manufacturer dumped it, the component is not a quality one that will stand up over time or it is old stock that is past its “best by” date. Not saying this is the case but these are questions that would be going through my mind if someone was offering a component for sale at a price less than what a reputable company is. Something else to keep in mind is that companies such as Mouser, Digikey and Newark usually offer a volume discount price on a component -the capacitor number offers a price break at a quantity of 10 and 100. Please note I am not affiliated with the three companies I have mentioned but have found they all offer decent products and good service over the years.
One other thing that should also be mentioned is that the Mercotronic tester will be only in use during a few minutes testing a coil and like most pieces of test equipment should be calibrated each year or two to make sure it is performing a test accurately and properly. This very rarely gets done because of the fact the tester is probably not used all that often and finding a company that could calibrate might prove challenging and quite costly in today’s world. Compare this to an engine that may be in use for several hours at a time and subjected to extremes in temperature of heat and cold and the importance of selecting a good quality and properly spec’d capacitor in an ignition system becomes very important. This becomes even more crucial in an engine that is used in aviation or a marine application as the first step if you have to walk in either application is a very long one.