Convert K2FC to auto advance for racing

I understand the magneto is the same. The Auto advance is done within the drive sprocket if it is fitted? But the manual advance mechanism has to be removed from the cam ring. Confirmation needed please.

You can use an auto-advance drive gear on a manual mag.

heyup Bob and Al - When the mags were built at Lucas ( or British Thomson-Houston who built the BTH mags ).  The position of the cam ring peg, at the bottom of the points housing, was rotated so that when the foot of the points assembly opened the contacts it coincided with the point where the magnetic field induced in the primary windings of the armature coil was at a maximum value ie the point of maximum flux density.  On the 650SS with the automatic advance kit this is positioned at fully advanced (springs fully extended) so that maximum induced voltage in the secondary windings (plug) occurs where the fully advanced position is before top-dead-centre.  So when the engine is stopped the springs bring the points position away from this position to a point several degrees earlier on the increasing curve.  Obviously this is no longer the position at which the maximum flux density occurs.  Many people blame this fact as the reason that the 650SS can be difficult to start especially if fitted with h/c pistons.  When a manual mag is fitted the fixed sprocket is ‘timed’  with the spring-loaded cam ring at the slack cable position but this is always at the point of maximum flux density.  As the cable is tightened against the spring (ie retarded), the cam is pulled clockwise retarding the ignition so as the flux density decreases from the maximum point following the decreasing curve,  it collapses from a high value when the points open and is sufficient to give the necessary voltage to start most engines.  The manual K2F mags are generally a stronger body-type with a larger points-end ball bearing, better bolt-on pickups, screw-on end cap etc.  Often an auto cam ring is used instead of the slotted manual one but the ignition is now fixed in the fully advanced position.  If a manual retard is required the cam ring must not be a loose fit in the housing.   Of course when the engine is used for racing purposes the manual mag is preferred because the fixed sprocket is a third of the weight of the advance/retard unit. Also it does not suffer from the wear and broken or detached springs so often the curse of a racing 650SS or Atlas motor.  Regards, Howard
    

Howard said: "On the 650SS with the automatic advance kit this is positioned at fully advanced (springs fully extended) so that maximum induced voltage in the secondary windings (plug) occurs where the fully advanced position is before top-dead-centre.  So when the engine is stopped the springs bring the points position away from this position to a point several degrees earlier on the increasing curve.  Obviously this is no longer the position at which the maximum flux density occurs."

No- the action of the auto-advance magneto drive pinion has no effect on the relationship between position of points opening and flux density.

Michael describes the theoretical advantage of auto advance.
The first problem is finding an auto advance unit working the correct way round.  Most are (I understand) the opposite direction because Triumph were opposite to Norton.  There must be hundreds of Norton ones from all those electronic conversions...

I'm not saying Lucas or BT-h auto-advance pinions aren't ramshackle devices.

I have changed an auto advance mag to manual. You do need the complete manual cam ring and housing to do it and not forgetting to cable and lever.
For racing, the moment the engine starts you shove the a/r lever to full advance and leave it there.
And yes, retarding the ignition does move the points opening away from the position of max magnetic flux.
I have 3 road bikes with manual mags. The only time you retard the ignition is for starting and when idling at traffic lights. 

The drawback of manual advance is the spark not being at the best field collapse point when retarded.  But the benefit on my Inter is that it's very sensitive to ignition point when kick starting.  The drawback is that if you make a mistake with the lever position, you can break your Achilees tendon...

David why worry about direction of advance? The Triumph unit is gear driven, the Norton twin unit is chain driven.

The auto advance moves the sprocket relative to the shaft when the rpm rises.  If it moves the wrong way, it retards instead of advances.  I don't know anything about Triumph magnetos - but anyway it's important to use the correct one.

If it's manual (cable operated) you need to disconnect the cable (make sure it's locked on full advance to get the best spark) and replace the fixed magneto sprocket with an auto advance one.