Magneto Questions

My '54 Velocette MSS came to me with a modern BTH magneto fitted. This is a fully electronic type, no points, and has an externally mounted coil with HT lead attached. The coil is about the size of a modern flasher relay. Have heard some describe it as a CDI unit.
Can someone describe to me how this works? Does the magneto still put out a high voltage to the coil unit, which then puts out an ever higher voltage? The leads connecting the mag to the coil are just thin 18 or 22(?) AWG wires.

Checking for spark with plug against head, I can only see a very faint spark...nothing like I see on the Commando with standard coil ignition. According to the FAQ on the BTH webpage, this is normal as the HT output is much higher tension:
"Question 13: Is there something wrong with my new BTH magneto as the spark seems very thin?
Answer: This is normal as the spark from our magnetos is very high tension that produces good performance. It is sometimes difficult to see the spark at the plug gap, especially in bright light."
Can someone elaborate on this? Why would a higher tension output make for a weaker looking spark?

Also, the FAQ mentions its OK to run a 5k Ohm cap but then later on states not to run resistor plugs as they may damage the CDI. Seems to be in conflict with previous point, no?

My knowledge of a BTH magneto is very thin, but AFAIK, it is not a true magneto. While it does produce its own low voltage energy, and thus needs no battery dependency to function, it is in effect, an electronic ignition, electronically saturating the coils and electronically collapsing the coil field. Practically, it is little or.no different in function to any of the EI's. It does not increase spark energy with increased engine rpm.

I would agree that a higher spark energy should produce a more visible spark. Anything else makes no sense to me.

I have heard the admonition that resistor caps are OK, but not resistor plugs. This is not peculiar to BTH, but I do not remember the reason why.

If the bike runs well, I would not obsess over it, unless like me, you are, at heart, a magneto man.

Slick

texasSlick said:

My knowledge of a BTH magneto is very thin, but AFAIK, it is not a true magneto. While it does produce its own low voltage energy, and thus needs no battery dependency to function, it is in effect, an electronic ignition, electronically saturating the coils and electronically collapsing the coil field. Practically, it is little or.no different in function to any of the EI's. It does not increase spark energy with increased engine rpm.

I would agree that a higher spark energy should produce a more visible spark. Anything else makes no sense to me.

I have heard the admonition that resistor caps are OK, but not resistor plugs. This is not peculiar to BTH, but I do not remember the reason why.

If the bike runs well, I would not obsess over it, unless like me, you are, at heart, a magneto man.

Slick

Click to expand...

Bike does start and run ok, so this is more about trying to understand more about the system.

Was thinking if the coil output truly is much higher tension than standard coil ignition, maybe this gives a different light frequency/colour that is less visible to our eyes, like more into the UV end of the spectrum or something. Or perhaps spark duration is much shorter so we cannot detect it as well.
Another thought, maybe its simply that when kicking over during spark test we do not make enough voltage from the mag/coil to give a good visible spark.

If the resistance (or impedance) reading across the low tension terminals of the coil is less than 1 ohm, then it is probably a CDI type system.

That means it does not saturate the coil and then collapse the field like points ignition or analogue Boyer. The CDI unit in the magneto sends a pulse maybe about 500 volts to the coil and the coil transforms it to a brief high voltage HT pulse.

Yes, the spark is sort of small and harder to see than in most other old bike ignition systems. Some say that’s a bad thing, but if your bike starts and runs, it’s fine.

Did some reading on the Wikipedia page for CDI systems. A high voltage capacitor is charged either by AC direct off an alternator or by a DC source. When charging is interrupted, capacitor discharges typically 400-500 volts to a low impedance coil which then transforms it to HT megavoltage ranges. Seems they are typically twice the power output of induction type coil systems, 50 MJ vs 25 MJ. Very short charge and discharge periods, some 10x to 20x faster than induction coils, which can benefit higher rpm performance.