Effect of failing/failed condensor?

[AustinWood]

Retarded ignition has a similar effect.

[Henry Harris]

I have a spare A7 condenser wired up with two fly leads with crocodile clips. One to the body and the other to the terminal. All well insulated with tape. I carry it as a spare and it can be clipped on if required without removing the exsisting condenser. It is also good for testing out suspect condensers.

John Mason.

If the original condenser is failing with old age it's likely to be short circuiting internally.
Fitting another condenser externally without disconnecting the original isn't going to do anything to help.

[Graham O]

John is right, a failed capacitor generally has a spurious discharge path (which might only show under operation, not with a multimeter's test voltage). That discharge path will stop a good capacitor from helping.

The capacitor does two things:
it eliminates sparking at the breaker points 
it creates a form of resonance with the coil primary winding inductance to increase the spark level.

This resonance starts when the breaker points open and the coil primary winding's magnetic field collapses due to a lack of current.  The collapsing magnetic field creates a high primary voltage that discharges through the capacitor.  When the capacitor charges up to this new voltage (higher than the initial 6v from the battery), it then discharges back through the coil's primary winding to create a reverse voltage in the primary and hence the secondary.  So the capacitor and coil primary inductance work together to increase the initial 6V to a higher level to increase the spark.

You can check this by playing with a good system and removing the capacitor.  There will be a spark, but it is much better with the capacitor.