Capacitor, keep or remove?

Danno said:

I guess they hung the blue can on there for decoration. Take one off a stock Norton, pull the battery and try to kickstart it. If you are correct, it should fire right up. The rotor is a permanent magnet, The stator does nothing until it gets a tickle from the electrical system, either from the battery or the blue cap.

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The stator puts out volts as long as the rotor is turning. You can disconnect the alternator wires, start the motor (with a battery) and measure AC voltage off the stator. A capacitor filters the ripple (as LAB posted) to keep the DC voltage up between the peaks of the waveform. Without the cap the average voltage during kicking would be lower.

Danno, you're not quite right. When moving a conductor in a magnetic field, you get an electric voltage difference between the ends of the conductor.
In an alternator, the rotating magnet field produces electricity in the copper stator windings. So even at the low speed when kickstarting enough power is generated to get a spark. As the alternator produces AC, it must be rectified before going to the ignition system. But in order to feed the ignition system properly, the voltage must be smoothed as L.A.B shows in the diagram above. Hence the MC2.

MexicoMike said:

OK, makes sense - I'm familiar with RFI and how it can effect electronics but I don't associate it with having an effect on vintage systems like the Commando. However, in thinking about it, I recall some folks have mentioned having trouble reading Norton voltages with some digital multimeters and responses have suggested it's an RFI issue that can effect certain E-ignition systems. I haven't personally seen that but it has certainly come up on this site. I have always ASSumed that such RFI was due to a non-resistance secondary ignition path, not anything to do with AC ripple voltage from the charging system but maybe so...

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Yes, I experienced this with my digital multimeter, which works fine for checking DC Voltage with the engine running on my old Triumph car, but not on the Norton. I'm wondering if my capacitor might be getting past it. I also noted reference above to it potentially supporting spikes in electrical demand. Ive also noticed a delay from switching my indicators on and them starting to blink. Related?

Or maybe, must stop looking for faults and just ride the damn thing!

With the 2MC disconnected, I noticed the charge warning lamp flickered more and took another 200-300 RPM before it went out.

Smoothing the ripple of a rectified output using a capacitor is only advantageous if the battery is not good, or not connected.
If the battery is in first class condition and kept well maintained, there is no advantage in having a capacitor as the battery's capability of smoothing that ripple is far better than a capacitor can do.

...in fact with some reg/recs it can confuse the circuitry, as they are monitoring the battery voltage to determine whether charge is required or not.


The capacitor does not do much to attenuate RFI or EMI - a ferrite choke is better suited for this (or resistor caps/plugs on the high tension side of the coil), so don't run a capacitor thinking you are solving that issue.

This is why Tri-Spark sell a choke for when Tri-Spark classic twin ignitions are used with Podtronics reg/recs as mentioned already.