Sorry - my use of English wasn't clear enough! I meant that without the capacitor, you might find it difficult to see the spark. The difference with and without the capacitor should be totally obvious - in the former case, you will see (and hear) a strong bright blue spark; in the latter case, you might struggle to see the very weak spark. If you can't see a difference then it suggests the capacitor is not doing it's job.
This assumes that the coil is working correctly. If there is a problem with the primary winding (eg partially shorted), it might not generate the correct HT voltage across the spark plug. If you have a digital voltmeter, you can measure the resistance of both the primary and secondary windings. If ok, they should be about 2 Ohms and 10,1000 Ohms, respectively.
The capacitor used is electrolytic which will normally only work properly when connected the right way round. On this basis, if you have a capacitor designed for a -ve earth system, it might not work with a BSA +ve earth system.