Magneto Drive Rotation !

[James Anderson]

I am faced with a slightly odd problem that I hope someone can help me with. I sent a completely 'dead' Blic Magneto to be rebuilt. On the casing, it has an arrow engraved on it indicating a clockwise drive. However, when it was rebuilt, I was told that it was orientated to an anti-clockwise drive ! In that direction of rotation, it has a brilliant spark. I wondered if anyone knew if there was a way to alter its internal configuration to suit clockwise rotation (? a different cam) or some clever gearbox device that I could fit externally to reverse the direction of its drive?

[stuartu]

James, there will be someone who knows much more about this than I do but I do not think you need an external gearbox. Here is what that notoriously unreliable source, Wickedpedia, says about inductance type mags like the BLIC.

"One or more times per revolution, just as the magnet pole moves away from the coil and the magnetic flux begins to decrease, a cam opens the contact breaker and interrupts the current. This causes the electromagnetic field in the primary coil to collapse rapidly. As the field collapses rapidly there is a large voltage induced (as described by Faraday's Law) across the primary coil."

 It seems to me that the critical thing is the relation between the contact breaker cam and the rotating magnet. In my innocence I think you can alter this by retiming the internal gears in the mag.

If I am wrong I am sure someone will rush to correct me and if I am right someone may be kind enough to be specific about how. I think some magnetos have timing marks. It seems odd to me that your mag. person couldn't fix this.

Regards,
Stuart

[Steve Jones]

Like Stuart, I'm no expert on such things but what I do know is that some years ago, by re-timing a Lucas magneto that was anti-clock (from a Meadows 4ED engine), we were able to make it work for some weeks on a SV Anzani engine that rotates its magneto clockwise. Like Stuart, I'm surprised that your magneto person couldn't sort this for you.

Steve

[Phil Kingdom]

Just throwing this in, but the clockwise drive could be referring the drive of the main gear, therefore the armature and rotor arm being driven from this gear would rotate anit-clockwise. Someone please correct me if I am wrong but is it not the polarity of the capacitor that dictates the direction of the discharge, the older paper type capacitors are connected + and - whilst modern replacement capacitors will work in both directions.

[James Anderson]

Thanks for your usual quick replies. I thought that the direction of rotation would not matter too much but I've just taken the one off the Chummy it was destined for and the direction of the Rotor Arm is the same but it 'sparks' quite well if the Mag. drive is rotated clockwise but not at all if spun in the opposite direction. opposite to the one that's just been rebuilt. Points to some sort of internal shenanigans that's beyond me! The chap who rebuilt is very experienced and seems to think it is the cam that's at fault but I'm not sure why. By re-timing, Stuart and Stev, do you mean taking the main gear off and altering its position to smaller one ? I would've thought that no matter what its position, it should spark in a clockwise direction.

[Adrian Payne]

Hello James
The spark is generated when the poles just separate and the points open at the same time. The exact timing is a little more complicated than this but that is why rotating in the other direction does not work.
This can be re-timed by re-positioning the cam with the rotor and rotor arm in the correct positions.
However, the points are only supposed to run in one direction. In the correct direction the points will effectively open themselves whereas in the wrong direction they will try to dig into the cam and result in excessive wear. The points housing has to be of the correct rotation.
Some magnetos with an extended pole (for better spark in advanced and retarded position) cannot be reversed.
I can photo my BLIC when I get home if required.
Adrian.

[David Witton]

Once again this highlights the amazing depth of knowledge of the contributors to this great forum.
Long may it continue

[stuartu]

By re-timing, Stuart and Stev, do you mean taking the main gear off and altering its position to smaller one ?

Yes James, that is what I meant and I suspect Steve meant the same. However I think Adrian has just pointed out the snags!

Regards,
Stuart

[James Anderson]

Thanks, Adrian I've looked at the points of both magnetos and the two sets look quite different and like you say this is probably because they are orientated to a different rotating direction. If I find another Blic, if I change this bit over and re-time (?how!) then I might get closer to a solution. I'll have a rummage about.

[Adrian Payne]

Hello James
The relation between the large and small gear wheels only controls the position of the rotor arm when the mag fires, important but not the same as the mag internal timing. There are many articles on the web regarding internal timing.
Some mags use a keyway to fix the internal timing.

   

   

Adrian