That BTH advance unit I modified on that Velocette I had here a few years back was going to full advance way too soon........came very close to damaging his engine.........Probably getting to full advance by 2000 rpm or less........Not good.
E: Engine Unsteady Idle
- Thread starter Martyn Goodwin
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- amal concentric atd idle speed
Interesting, as I suspected fully retarded to fully advanced within 500 rpm difference, hardly a controlled advance curve, but don't forget this was early days of auto advance and the only other option was a manual adjust lever on the handlebars and this basic ATD did its function on thousands of bikes across multiple manufacturers. You cannot compare it against electronic controlled advance which came along 30 plus years later.
Simon, I´d say Lucas was stuck in prewar thinking way too long and didn´t accept the idea of placing their ATD at the contact breaker cam - like was done in car distributors or Bosch , Magneto Marelli magnetos etc . . Advancing the complete rotor is a lot more troublesome than just the little cb cam as was done many years before WW 2 on car distributors.
Even so, I´d think the Lucas device should be modifyable to reasonable smooth operation by keeping it well lubricated in places and experimenting a bit with springs - two different types, weak and strong possibly.
A very different approach when having the ATD in the magneto drive like Lucas was worked out in the Scintilla/Ronco/Vertex magnetos which I got for the Rapides - not an easy mod here. They went for a variety of weights in different shapes , no springs in there at all. By selecting different weights you can do variations of ATD curves. There must be files for these somewhere but I did this by experimenting on a variable speed motor.
Vic
Even so, I´d think the Lucas device should be modifyable to reasonable smooth operation by keeping it well lubricated in places and experimenting a bit with springs - two different types, weak and strong possibly.
A very different approach when having the ATD in the magneto drive like Lucas was worked out in the Scintilla/Ronco/Vertex magnetos which I got for the Rapides - not an easy mod here. They went for a variety of weights in different shapes , no springs in there at all. By selecting different weights you can do variations of ATD curves. There must be files for these somewhere but I did this by experimenting on a variable speed motor.
Vic
Vic
I woudn't say that Lucas was stuck in pre war thinking, it was probably more that Lucas didn't want to spend any money to make a better product. I remember reading once it was BSA that told Lucas they wanted to pay no more than X and so Lucas made them to that cost and the rest of the industry had to suffer the consequence as BSA was their largest customer.
When you look at the size of the weights in a car mechanical distributor generally they are a lot smaller and weigh less and so easier to control by springs etc. I suspect that is what is wrong with the Lucas magneto ATD and Martyn may be onto something reducing their size.
I woudn't say that Lucas was stuck in pre war thinking, it was probably more that Lucas didn't want to spend any money to make a better product. I remember reading once it was BSA that told Lucas they wanted to pay no more than X and so Lucas made them to that cost and the rest of the industry had to suffer the consequence as BSA was their largest customer.
When you look at the size of the weights in a car mechanical distributor generally they are a lot smaller and weigh less and so easier to control by springs etc. I suspect that is what is wrong with the Lucas magneto ATD and Martyn may be onto something reducing their size.
That story of BSA setting the market is spot on magnetos were all made down to a cost and once the alternator was available -(and dont forget the Triumph TRW had one in 1949), the writing was on the wall for the magneto using components from the car world was a better marketing trajectory redesign was never viable, they only continued to be produced while the smaller manufacturers continued with old engine designs till magnetos became uneconomic and forced a change (Panther never did- Velo only just)
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Hi Simon,
I actually gave some thought to building a test rig composed of a variable speed motor with tachometer (electric) with a shaft that I could attached an ATD to then using a stroboscope I would be able to see exactly what happened and when as I played about with the weight weights and spring tensions.
Back of the envelope math quickly scrapped that idea for financial reasons - I reckon the test rig would cost a motza to build. Well beyond my pension capability.
I can tell you I am VERY happy with my weight reduced bob weights
Martyn
I think the idea behind the magneto turning the whole rotor is to get a strong spark at any Point of Timing.If you turning the Points and the rotor stays you get a weaker magnetic flux and a weaker spark.Even with the not ideal sparking on a V Twin it is better to turn the whole rotor.And of Course with low revs ,when starting the engine a strong spark is essential.Erik
I have yet to understand the thinking behind manual ADV/RET in the sense that advance is normally at the point of maximum flux and retarded is away from that, just when you need the best spark for starting.
On my 60 degree JAP engine there is in effect no front cylinder spark at kickstart speed, I have to have it almost on full advance to start, once running it can be fully retarded ok.
When I first got the bike the mag was duff so I built a magdyno with a HD adv/ret and twos sets of points inside (I made a bearing cartridge to go inside, machined some of the magnet away and glued a lead strip in so the magnets didn't affect the bob weights) the thing would start with a gentle push on the kickstart and ticked over at about 200rpm.
I would have kept this system but for liking the electrical independence of a magneto.
On my 60 degree JAP engine there is in effect no front cylinder spark at kickstart speed, I have to have it almost on full advance to start, once running it can be fully retarded ok.
When I first got the bike the mag was duff so I built a magdyno with a HD adv/ret and twos sets of points inside (I made a bearing cartridge to go inside, machined some of the magnet away and glued a lead strip in so the magnets didn't affect the bob weights) the thing would start with a gentle push on the kickstart and ticked over at about 200rpm.
I would have kept this system but for liking the electrical independence of a magneto.
Well a magneto does have a "Range" of travel per the spark output.......On the test bench you can see quite easily that moving the cam ring (advancing and retarding the spark) in the full advance position the spark will be strong and if you advance it a little more, the spark drops off entirely, just like an on/off switch. This is because the "Break point" at which the end of the armature segment has passed the magnetic pole at the top and bottom of the mag housing, at this point the magnetic field collapses. then immediately starts to build again but with the polarity reversed (a magneto is an AC device) as the armature turns and the next build up of electrical energy starts again........As you retard the spark the magneto will still give an output due to the stored energy available within the armature core, until the contacts open and this electromagnetism collapses thus roughly the 200 volts built up in the primary winding x the secondary windings at a ratio of about 1:60 gives a secondary output of about 12000 volts.........This range as you retard the magneto varies from one to another........As you know a lot of prewar mags had a range of about 20 degrees (40 at the crank) A couple of reasons the mags loose some energy from when they were new, is loss of magnetism, but another very important one, is the diameter of the armature is very critical........An armature that has been machined down by say 10 to 20 thou on its OD will definitely have a weaker spark especially at slower turning speeds........So this coupled to a loss in the magnetism will definitely make for hard starting especially on a big engine that is hard enough to kick over........this machining down of the armature core is very common and irreversible.