Magneto timing

[AndrewM]

I've been recommissioning my 1946 16M for a couple of months now and am almost ready to register it for road use again. Last job on the list was re-doing the magneto timing, as I had never been able to get the engine to idle smoothly at low tickover and had a feeling it was set too advanced, as I could never get it to run smoothly at anything but fully retarded on the lever. It could also have a nasty kickback if you were half-hearted with the kickstarter. Carbie has been replaced with a brand new one due to petrol leaks due to thread damage, and I am happy that idle mixture is very close to correct, so felt the problem was more likely to be ignition than carburation.

Finally got around to checking the timing today. Cleaned and set the points, got out my 0.001" feeler gauge (no fag papers here unfortunately), made up my timing stick from a cut-down skewer from the kitchen, found TDC and marked 7/16" to get the furthest advanced piston position. Engaged 4th gear so that I could rotate the rear wheel to rotate the points easily, set the A/R lever to full advanced, checked the points and found them fully open at 7/16" before TDC. Rotated the rear wheel backwards to find the point at which the points just opened - about 1 1/4" before TDC!

I was a bit baffled so checked the points opening position with the A/R lever set fully retarded, was about 3/8" before TDC.

My conclusion is that either I have totally misunderstood the timing procedure and have been measuring the things wrongly, or someone before me set the timing with the A/R lever fully retarded instead of fully advanced.

Comments appreciated, as this is the first time I have done this so wanted to be 100% sure I was doing things correctly before pulling and resetting the sprocket on the end of the camshaft.

[Groily]

I think you have understood it right and the last bloke got in a tangle!
Should just be opening - not fully open - at your 7/16ths BTDC if that's the chosen figure - at full advance of the magneto.
Retarded, they'll open around or just after TDC typically.
'Advanced' at the magneto is when the cam is moved to its full extent against the direction of rotation of the magneto. The magneto turns anti-clockwise from the drive end. The points turn clockwise looking at things from their end, so the cam moves anti-clockwise as it advances, from that end.
The timing was set at full retard, almost for sure. No wonder the idling was 'sensitive'!
Your 1 1/4" to 3/8" BTDC figures (7/8" overall movement) would translate into 7/16"BTDC to 7/16" ATDC if set correctly.

Don't need that much retard I don't think as that's a lot of degrees on mag and crankshaft and the spark gets progressively weaker the more retard is applied - but about half-retard give-or-take should do it I reckon, for starting and for steady idling.

[Les Howard]

I was a bit baffled so checked the points opening position with the A/R lever set fully retarded, was about 3/8" before TDC.

The normal way of setting the timing is with the ignition of FULLY ADVANCED not retarded. This is the state it will be at for 95% at least of the time when running so it's critical to get that setting correct. Normally get the piston at the correct distance from TDC at the compression cycle. Then have the centrifugal ADVANCE and RETARD unit locked in the fully advanced position OR as in your case, set the LEVER in the fully advanced position.... BUT note some levers work by pulling the cable in different ways IE: Some are fully clockwise and vis-versa. So before proceeding to check the points make sure the fully advanced position you THINK you have on the lever is REALLY pulling the points plate in the magneto AGAINST the rotation when the engine turns. If by moving the lever, the plate moves the same way as the plate turns, the lever is RETARDING the ignition. Now after being confident you are moving the lever to the fully advanced position, set the magneto points plate in the normal manner. I am assuming you know how to do that. Then recheck the points gap and do a final opening position check...PS: Most engines run faster but quite roughly when fully advanced at tick-over speeds.

[SPRIDDLER]

In short...........


If the A/R control cable enters the mag on the right of the mag body (when viewed from the points side) full advance is with the cable under tension - i.e. when pulled.
If it enters on the left - i.e on the frame downtube side, full advance is when the cable is relaxed.

[AndrewM]

I bit the bullet today, did the tappets which had been less than perfectly set, then did the magneto timing. Learnt a few lessons along the way, including that it is easier to take the link with the clip out of the timing chain, then use a puller on the bare sprocket, rather than mess around with the tire levers/screwdriver method recommended in the 1946 manual.

Les and Spriddler, thanks for the info, my mag is cable entry on the right, tight wire advance. I double checked that before I started just in case my thinking was arse about face.

Got the timing wrong once, think I failed to account for the backlash in the magneto drive chain so my first try was probably 10 degrees out, but nailed it on the second try. Starts easily now at about 40% on the A/R lever, and can drop down to a very low tickover once warm by retarding it a bit more. Outstanding result!

Thanks for all the advice gents, as Groily said, it looks like I was correct in my assumption that the last bloke had timed it with the lever at fully retarded.

[Groily]

Result!

[Andy G]

Great that's sorted.

Here's something that has often baffled me tho...
Given that we only ever use a small fraction of the magneto's available retard adjustment, is there a reason for having the rest?
I've never tried, but I imagine that when properly set up at full advance, using the full sweep of the retard lever would put timing waaay after tdc and result in a very weak spark?
...So why design a magneto with that option?

[SPRIDDLER]

Great result and so satisfying.

Learnt a few lessons along the way, including that it is easier to take the link with the clip out of the timing chain,

The mag drive chain (and the dynamo chain) *should* be endless, i.e. no joining link. More important for the dynamo chain which runs very close to the primary chain and a clash of chains could result in serious damage, not least to the chaincase.

I may be repeating what you've seen in the manual but anyway.........

then use a puller on the bare sprocket, rather than mess around with the tire levers/screwdriver method recommended in the 1946 manual.

Not sure which sprocket you loosened but I find setting the firing point more controllable if I loosen the (lower) cam axle sprocket rather than the one on the mag (the uppermost one). Preventing the mag armature from moving whilst you tighten its sprocket nut is quite tricky because on full advance the contact breaker opening point (the firing point) is at the mag armature's 'flip point' which tries to make the armature rotate away from where you want it as you tighten the sprocket nut. The cam axle has more resistance to unwanted movement as you tighten the nut than that of the mag armature. Before tightening the nut on the cam axle it helps to 'set' the sprocket in place on its axle if you put a socket larger than the nut against the sprocket and give it a sharp tap.

Got the timing wrong once, think I failed to account for the backlash in the magneto drive chain so my first try was probably 10 degrees out, but nailed it on the second try.

That's not unusual. However carefully I've set mine up I usually need two or three goes before it ends up spot on.

[Groily]

I ask myself that quite often!

An ATD will give typically 11-13° of movement on the mag drive, so 22-26° on the crank. That's what goes on the SR1s on our machines.
(The widest movement of any ATD made by Lucas was 18.5°, so 37° on crankshaft, and that was for MAC and MSS Velos for 1952-1954 only.)

A typical single cylinder face cam magneto like N1 or MO1 magdynamo provides 20°+. Up to 22° or 23° with a bit of wear. That's 40+° on the crank and quite unnecessary I would say. And not just me - it must be OTT if the factory was happy with little more than half of that on the very same engine when using the ATD system.

A typical manual advance twin cylinder Lucas K2F, as on our twins, offers a bit less than face-cam mags - but still way more than's needed.

I haven't owned many singles in my life. Three of the four I have had had coil ignition and alternators (late B31s and also a C15 when I was a callow youth). The other one was an Ariel VH with MO1 magdynamo, and I never used more than half-retard on that for any purpose.

My Mod 20 and 31 twins will start at full advance without breaking a leg - but I am a large person and twins are much easier anyway.

So . . . I also think it's overkill to have so much retard available on the majority of machines. It almost invites starting problems if folk use 'full retard' when the spark is at its most feeble, especially when hot.
And it is also a major contributory factor to the jammed A/R plunger and cable syndrome, which is a direct result of there being too much movement on camplates and camrings for the plunger's mushroom head to stay engaged at full stretch either way when parts start to wear.

[Les Howard]

I always set my bikes that have manual ignition with just a touch more advanced than the recommended setting. This allow me to tweek and adjust the timing for the best feel and performance from the engine. It doesn't need much, as usually the given one if usually very near the optimum, but if you set to the exact position you never really know if you HAVE set it spot on, plus the fact that different fuels, mixture strengths, temperatures, conditions etc. etc do effect the optimum timing. This position can be easily returned to when on a journey from memory but I like to fiddle with the lever. Please don't tell me I'm wrong as it works for me