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Hi all,

I have been rebuilding the Lucas MO1L magdyno from my 1955 Royal Enfield 350 G2 Bullet over the last few weeks and I think I now have a pretty good understanding of how everything works.

There are still some grey areas and I was hoping someone could maybe help to clarify things for me regarding the magneto points cam and it's advance/retard mechanism?

The magneto section from my bike is anti-clockwise rotation with a right hand spring (which I believe refers to the advance/retard cable entry) and a left hand HT pick up. This is as viewed from the drive end (timing side of the bike).

If you refer to the first of the attached images you can see the cam in position in the housing in (what I believe to be) the fully retarded position. Looking at the cam from this end view the rotation of the contact breaker carrier is clockwise and you can see that there is a rectangular notch at the bottom of the cam that allows the cam to rotate by approximately 32 degrees. The edges of the notch stop it rotating any further in either direction by hitting the stop screw in the magneto drive end housing. You can see the ramps that the contact breaker tappet ride over at about 7 o'clock and 11 o'clock. This means the points open at about 11 o'clock, as the the tappet is pushed up onto the raised section of the cam. The points stay open for about 270 degrees until the tappet goes down the second ramp at about 7 o'clock and then they stay closed for about 90 degrees. Rotating the cam with the advance/retard mechanism in an anti-clockwise direction will advance the ignition, as the ramp which opens the points will, in effect, be getting hit earlier in the cycle? I believe I am correct in this thinking and if I am then the ignition will be fully advanced when the advance/retard cable spring is fully compressed? Turning the handlebar control lever to un-compress the spring will make the spring push the advance/retard plunger down and turn the cam anti-clockwise to the fully advanced position?

This fully advanced position is as seen in the second image.

So, my first question is am I right in how I believe the advance/retard mechanism works? How would you normally go about using it in normal operation?
When I used to ride the bike regularly, as a youngster, I cant even remember having an advance/retard lever on the handlebars?

Secondly the notch that the advance/retard mechanism engages with is not alone. There are three of them. I am guessing the one at about 4 o'clock will be for housing with the advance/retard cable entry at the other side? But what is the third notch for, which is in about the same position as the top cam plate ramp?

Thirdly, what is the mark stamped close to this mystery notch? Is it the letter 'L' or is it an arrow?

All of these things I would like to know in an effort to make things clearer in my head. I'm never truly happy until I know that I have got things right and my understanding is clear of grey areas. My wife says I am autistic, my friends call it anal. lol

Cheers,

Kev Ev

Yes. If the points rotate backwards (as compared to the direction the tyres turn) then rotating the cam forwards advances the timing.



There’s a spring in the plunger mechanism at the mag end of the cable. The spring extends to push your cam to fully advanced, when turning the cam forward is advancing.

Normal riding is done at full advance. You may have to retard slightly to prevent it kicking back when starting and maybe when idling or travelling at walking speed.

Retarding with the lever makes the spark weaker, which of course can make starting difficult (or even unlikely).

Hi triton thrasher,

I was a little confused at first about your reference to the tyres but I think I know how you meant it now.

I'm just looking directly at the cam and the points rotation and basing my thoughts on this.

Can you explain to me how retarding the ignition makes the spark weaker? I don't get that bit?

My understanding is that their is an electrical current generated in the magneto armature primary windings, as it rotates inside the magnetic housing. This builds up a flux in the core and when the points open and break the primary winding circuit it creates the back emf, which induces the HT current into the secondary windings to make the spark. This points opening happens once every revolution and I cannot see what difference it would make to the strength of the spark when this opening takes place within the 360 degrees of movement? The strength of the spark comes from the back emf created by the imploding magnetic flux that has been generated in the primary winding. This surely is the only factor regarding the strength of the spark.

I appreciate that the spark, at the spark plug inside the engine, may be under a lesser, or greater load from the amount of compression depending on the position of the points opening (ignition timing). But the actual spark would still be the same and wouldn't be stronger, or weaker because of timing?

Cheers,

Kev Ev

Hi again triton thrasher.

I've just realised that the strength of the spark will of course be affected by the position of the points opening because of the relative position of the rotating armature winding and the fixed poles of the magnet. The magnetic flux will only be at its strongest at one point and therefore the points opening needs to be done at the right time to create the strongest back emf.

Thank you for making me think about this.

Kev Ev

I think that you will find that the "L" denotes left hand rotation, as your magneto is.
The cam stop is eccentric to adjust for optimum output but would be difficult without the proper test equipment.

Of course you can have a MO1 magneto built for taut cable advancing or slack cable advancing by your selection of the points housing. The Gold Star started using the taut cable advance and in 1953 went to the slack cable version. I suppose with a broken cable it was easier to race on or ride home with the engine at full advance then at full retard.

Gordo