ATD Springs

[b'knighted]

Hi Ken,

While this chart is educational it does not overcome my basic ignorance.
I know that I don't want any advance to occur until the engine is running but I have no idea at what revs I want the advance to increase or how quickly it needs to go to full advance. Please can you tell us which line on the graph is ideal for normal road use, for enthusiastic road use and, for other people, competition use.

 

[Ken Targett]

That's the engine-tuner's forte, Ian. But the ATDs I've seen that I imagine are much as they left the factory are probably on the orange line, or somewhere between the green and the orange. For racing machines, if an ATD were fitted, I expect that anything would do. I don't have any machines with manual advance, but when I do ride them, I tend to start them part-retarded, and then put them on full advance and leave them there. If struggling up a hill, I'll change gear, rather than play with the timing. It's much easier to forget to put the timing back on full advance at the top of the hill, than to forget to change gear again. But perhaps, I'm just lazy, or just forget that I'm riding a manual machine.
Ken.

 

[Martyn Goodwin]

The information in the graph is very interesting but I suspect it only tells half of the story. It is very clear what happens as the engine/atd revs increase BUT how does the ATD perform from the fully advanced position as the engine/atd revs fall? My recent experience was an erratic idle, generally at least twice the desired RPM when coming to a standstill - and at times the revs would remain high, but only sometimes would they drop to something I considered normal. The cause was traced to "stiction" within the ATD keeping it in a partially advanced state even when the throttle was closed. There was found to be just a few thou of interference between the ATD rear flange and the magneto body - so around 0.015" was removed from the rear flange of the ATD. At the same time the cam ring inside the magneto was re-greased. Both these things have helped reduce but not eliminate the problem. So I have ordered a set of stronger ATD springs from the spares co to see it that will get the ATD back under control as the revs FALL. More to follow once the stronger springs arrive and are installed. Martyn

 

[Ken Targett]

Interesting points, Martyn. It's only recently that I've built a drive adapter for my rig so that I can drive a mag through its ATD, so I haven't got masses of test data to go on. But the Lucas ATD that I was testing last weekend didn't show any signs of hysteresis, i.e. the amount of advance/retard wasn't dependent on whether the speed was increasing or decreasing. By contrast, with a BTH ATD that I tested recently, there certainly was hysteresis of the type you'd associate with friction in the ATD.

I had exactly the same problem you've been talking about with my Venom a few years ago. To be honest, I don't (now) know what it was I did to it to fix it. But thinking about the possible causes you've mentioned of interference between the rear of the ATD and the oil seal, and friction with the cam ring, I'm not at all sure they can be the real cause. Both, it seems to me, would tend to retard the ignition rather than keep it advanced.

All a mystery.

Ken.

 

[Martyn Goodwin]

Ken, I agree that the "stiction" I refer to would tend to slow down the rate of auto advance as the revs increase; The point is ,as the revs decrease it is ONLY the force from the ATD springs that can cause the ADT to retard. If the springs are not strong enough to overcome the "stiction" along with the reducing centrifugal force of the bob weights as the revs fall, then the ignition cannot be "correctly" retarded for idle speed. With the help of others I identified drag on the rear ATD bush plus drag in the magneto cam ring as possible causes of the "stiction". What other sources could there be? M

 

[Ken Targett]

The point is ,as the revs decrease it is ONLY the force from the ATD springs that can cause the ADT to retard.

Think I've got to disagree with you there, Martyn. The bob-weights do the advancing. The springs and the load of the magneto do the retarding.The more torque there is applied to the inner sleeve of the ATD to resist its rotation, the more it is going to try to retard the ATD. Possible causes for the effect that we agree does actually happen? ... ummm. 1. If the axial play between the inner and outer sleeves of the ATD is borderline, could the compression of the inner sleeve when the centre-nut is done up cause the sleeves to nip up? 2. If the radial clearance between the inner and outer sleeves of the ATD is borderline, could the radial expansion of the inner sleeve when the centre-nut is done up cause the sleeves to nip up? I remember that when I had this problem with my Venom, it only occurred when the thing was hot.

Ken.

 

[Martyn Goodwin]

Think I've got to disagree with you there, Martyn. The bob-weights do the advancing. The springs and the load of the magneto do the retarding.The more torque there is applied to the inner sleeve of the ATD to resist its rotation, the more it is going to try to retard the ATD. Possible causes for the effect that we agree does actually happen? ... ummm. 1. If the axial play between the inner and outer sleeves of the ATD is borderline, could the compression of the inner sleeve when the centre-nut is done up cause the sleeves to nip up? 2. If the radial clearance between the inner and outer sleeves of the ATD is borderline, could the radial expansion of the inner sleeve when the centre-nut is done up cause the sleeves to nip up? I remember that when I had this problem with my Venom, it only occurred when the thing was hot.

Ken.

OK – if it is the lack of radial clearance – given that theATD is riveted together – any suggestion as how to increase the clearancewithout destruction of the ATD??

M

 

[Ken Targett]

Martyn, although the part that you think was rubbing on the oil seal very often appears to be just one piece of metal, it is in fact a ring which is an interference fit on the end of the inner sleeve of the ATD. With the Vincent ATD, the pinion is much larger than the ATD, so that you can support the pinion around the ATD and press the inner sleeve out of the ring. Sometimes needs a lot of force the first time. You don't need to unrivet anything. There are some photos of an exploded ATD if you click on the two photo links at the bottom of this page: http://brightsparkmagnetos.com/library/LucasATDspecifications.htm. Once you've got it apart, you can clean up the engaging surfaces of the inner and outer sleeves, and when you press it back together again, you can decide how much axial play to let it have.

Ken.

 

[Martyn Goodwin]

On my ATD there is very little – unmeasurable – radial clearance. I ordered stronger ATD springs from theSpares Company, part number 416060 originally intended for a AMC bike – and fittedthem yesterday. They seem to have solvedthe problem for with a 130 mile run today I did not have one instance of aracing idle speed.

Fingers crossed that this is the end of the matter - was a lot easier than separating the ATDsections then repressing them.

Martyn

 

[Big Sid]

As for the benefit of critically selected return springs strength and the effect on general running smoothness and power on a street motor my guess is it's little . If all is free and the mechanism nicely returns to full retard for a satisfactory idle and kickback free starting then peace will reign . There's little more to be had by trying to match the springs to the motors needs beyond that .
Where the compression is higher than std and a tendency to pink at low revs is noticed then a little firmer spring might suppress this by retarding the advance curve . I had to do this on our Vincati to obtain the sweetness I desired from this highly tuned motor .
Took quite a bit of matching up the springs and the advance weights to acheve it but eventually I did and all was lovely . This was however with a Series D Dist . Unit and not a magneto s ATD . Sid .