Auto-Advance Magneto question

[flatdeck]

Hi, My 1949 A7 Star Twin has an auto-advance magneto, whether it was original or not I don't know. Last night's game was to get the timing right before trying to balance the carbs. So took off the crankcase cover, took the plugs out and turned the crank over ... I found that holding the auto-advance in the advance position was just a case of pushing the weights apart. When letting go I expected the springs to pull the weights back in quite strongly ... what actually happened was that I let go and the weights stayed where they were. Seems to me that it never gets retarded, which would explain why it is such a PITA to start. The good news is the points seem to be just about right ... So the questions are: should the auto-advance snap back to the retarded position when you let go of the weights? and is the likely cause going to be very tired springs? Have to say the springs don't even look man enough for the job ...  Is there anything else that in there that causes the return to retard?
Cheers,
Dave T

[Brian]

Hi Flatdeck, the weights should return when you let go of them. There is something wrong with yours. They shouldnt "snap'' back but should return easily. As you say your ignition wont be retarded at start up and will probably be at full advance at idle, both situations you want to avoid. These things are quite prone to wear so may well need much more than a couple of new springs. C@D autos in England have the springs, I just bought two sets myself. If you need a new or reconditioned unit the only ones I know about are at SRM in England. There is probably other sources to get one from and maybe someone else on the forum can suggest who to try. To set the timing I have a small piece of wood cut in a wedge that I use to hold the unit at full advance. Hope this is of some help, good luck.

[flatdeck]

Hi Brian,
Thanks for that. Is it only the springs though that actively try to return the weights? I read somewhere that if the full advance stops are worn away then it could go too far and stick. I don't think mine is as bad as that because it does return smoothly when done manually. If I replace the springs and that does not work I'll be starting to take it apart from that end to investigate, clean and lube it. There is also alot of debate about the pros and cons of replacing it all with something electronic.
Cheers,
Dave

[Brian]

Hi Dave, its just the springs that return the weights. Its the small pins that the weights and the small pieces that the springs connect to that usually wear. I believe there are some forms of electronic set ups available but I am a big fan of magneto's. The first thing I do when I buy a bike is get the magneto rewound, I've been riding these things most of my life and have not had one fail yet. I have had a few generators stop working though and if you have electronic ignition you need a charged battery. I think of it like this, if your engine is a bit worn it may use a bit of oil and rattle a bit, if your carby is worn it may run a bit rough but will still run, if your magneto is crook your bike will be hard to start or may not start at all, as soon as it gets hot it may well stop and not start. So I consider the magneto the most important part. If your advance unit is beyond repair for some reason you could replace it or maybe look for a manual advance set up.   Cheers, Brian.

[fido]

I would also recommend getting a manual advance magneto and plain pinion without the auto advance. Other sporty models like the A7SS had manual advance so it would make sense on the Star Twin. If you take the springs off the weights should flop about easily, if there is resistance to movement you will probably find the pivots have rusted.

[flatdeck]

Had both ends apart last night to understand how it all fits together. Cleaned it all up. One tooth of the fibre gear missing. Cleaned and oiled the cam ring. Put it all back together. Tested that the advance unit returned on the spring ... and it did ! Turned the engine on the crank and tested it a few times. Seems to be working. Balancing the carbs next ... Thanks guys.

[groily]

Hi Flatdeck,

If you've got a tooth missing then you need to take some action pdq obviously! Fibre pinions without the ATD in the middle are available, I think, from SRM to replace the original without having to buy the whole shooting match. If you think you've fixed the ATD with new springs and/or a bit of attention to the weights and the pivot pins, then that's great and well done - wear on the pins is a pain as it can make the weights move in a cock-eyed plane compared to the face of the pinion itself, and they can get stuck or sticky. Getting an ATD out of its fibre pinion strait-jacket ain't that easy though - needs a press on the collar and it can take several tons in some cases to do it. Big hammer not recommended and vice on bench will only occasionally do it.

You can always go for a manual system if you like - you'll need a cam ring with a notch for the cable end, and a mag end-housing that'll take the cable and adjuster gubbins (all usually available from several places) BUT -

- it's worth bearing in mind that an ATD system working properly is genuinely technically better than the manual system because with the ATD the spark is always delivered at the optimum point (of max flux) of the mag armature (a few degrees after 0 and 180 degrees of its slip ring's rotation) whereas if you go for manual you're asking the mag (other than at full advance) to generate sparks at sub-optimal moments. All very boring, but true.). That's why the bright (joke) chaps at Lucas dispensed with manual - and the point is made perfectly in relation to V twins, where the armature is asked to deliver sparks well outside the optimum range for a device that works best in equal intervals of 180 degrees. I don't know why some of the more 'sporting' parallel twins of the day retained manual advance-retard. It was not necessary and not a great plan in theory even if in practice it was OK. Maybe die-hards preferred to think they were in control of something that almost all of them would have left at full advance as soon as they had got the engine running. Control freakery and 'gimme more levers' sort of thing. Given that with a bent twig down the plug 'ole most of them wouldn't have got the basic ignition within 5 degrees of maker's, it's all nonsense.

My money would be on replacing the pinion and ATD in one unit, even though it's not a cheapie decision, cos that way you get peace of mind and a zillion hassle-free miles, easy starting, decent idling etc etc. I wouldn't go electronic myself, cos that's non-original and more importantly needs batteries and dynamos wot work. Mags are generally far more reliable than dynamos (and regulators, especially old mechanical ones). And if they'll start the thing at all, they'll give progressively better sparks as the revs rise, owing to the way they work.

Good luck!

Groily

[flatdeck]

Groily... I am unhappy with the lost tooth but...  This AAU has not been apart since 1976 that I know of. When we checked the timing Monday night for the first time it was pretty much spot but I was concerned that the AAU was not returning to the retarded position (potentially causing the hard starting). Since the gear does not appear to be slipping I think I'll leave it as is for the moment. At least it is easily accessible. I am keeping a note of all the little "issues" I am finding with a view to dealing with them as parts/repair opportunities come to hand.  Cheers, Dave

[fido]

I prefer the manual advance as it gives you some form of adjustment for the ignition timing. The magnetos I've used all have some damage to the taper (yes, even the one I recently fitted which I had reconditioned) so they tend to go out of adjustment when I tighten the bolt. I don't have infinite patience for this job so it usually gets left once it's "near enough". With manual advance I try to err on the side of getting it slightly too advanced then find the optimum advance using the lever position i.e. get it pinking then retard it slightly. The published timing figures were only ever a guide anyway.

[groily]

Very fair point Fido - I know just what you mean and I have often relied on the same benefit of the manual lever, especially if I've had to do repairs in strange places. I've got one manual ignition non-BSA twin and the A10 with ATD.

A mate of mine who's much smarter than me has successfully modified a twin cylinder mag to give variable positioning of the cam ring - which is a very effective way of getting the same fine-tuning capacity the manual lever offers. The reason he did it was cos he wasn't satisfied that after a rewind the points were opening at exactly (like within a degree or 2 of) the right spots in relation to the position of the slip ring on the armature and this was the easiest way of ensuring that. The practical side-benefit is to deliver a neat fine-tuning capability within limits (those being the optimal spark position of the armature and slip ring, blah blah).

What he did was extract -carefully! - the little peg in the mag end housing that locates the cam ring (there's a tiny plug knocked into a small hole in what is in fact a small (BA I think) screw rather than the simple pin it looks like). But it does all come out. Then he drilled and tapped the housing for a bigger 5/16ths BSF or Cycle set screw with an eccentric machined on the end bit which goes into the notch and a slim locknut on the outside. He has got 10 or 12 (crankshaft) degrees of movement available, and it gets him over the point you make about not getting things spot on cos of lousy tapers etc. It also looks very neat. The original peg does have a small eccentric on it I think, which presumably the man at Lucas would have set in place to get the points to open at the position of maximum flux etc etc bore bore, but with the bigger one you get meaningful and useful adjustment.

But Flatdeck, coming back to your dental problem! If one tooth has gone, there's been stress on the thing (specially if the tooth broke off due to the pinion slipping on the taper, which can happen) and I truly think it might not last tho' I truly hope it does. . . the adjacent teeth are likely to be weakened so even though the loss of one ain't enough to cause the timing to jump . . .

Least I'd do is carry a plain steel gear (w/out ATD) in the tool box, cos even if (agreed) it's easy to get at, there's no easy way home if it strips on the road. I've had it happen twice in my life with fibre pinions when i was a poverty-stricken student a zillion years ago. It's always raining, dark and miserable when they go. The battery's dying, the lights are flickering. And the pub's shut. I hate pushing and the sainted memsahib hereabouts hates coming out to rescue me! Good luck!

Groily

[flatdeck]

Very interested in looking at the cam ring option for giving some fine-tuning. I am getting a lot of help from an ex-Rolls Royce engineer (my mate who's much smarter than me!) and I'm sure if he read your last post Groily he'd say "that'd work!". We have been shaking our heads wondering why it was designed so we have to take the crank cover off to adjust it. We are very close to getting this baby running well though. I see the price of a new fibre drive gear is about 8 quid but then I understand fitting it can a bit of a PITA ... It's all fun.

[LJ.]

Blimey! "an ex-Rolls Royce engineer" bit of an over kill for our BSAs isn't it? 

[Brian]

While we are all on this subject has anyone used or currently running a auto unit with a alloy gear. I am in the process of building up a auto unit and I have a new alloy gear that came with a bike I bought. I have good fibre gears to use  but was thinking about putting the alloy one in. I am not quite sure why BSA used the fibre gears, the singles [B33 etc] all use steel gears with no problems. SRM sell their ones with fibre gears so maybe there is a good reason to stay with fibre.

[flatdeck]

LJ, I would be screwing things up badly if I was left alone :-) The ex-Rolls Royce engineer won't let me use the wrong tools on anything ... having said that he has a hammer named "Excalibur" that he says he only uses for "gentle persuasion" . He does have a hydraulic vice that does several tons so I could look at replacing the fibre drive gear (see Groily).

[groily]

Not sure why fibre gears came in, but they arrived when the switch was made to ATD from manual I think. Can't remember seeing a steel pinion wirh ATD in, but maybe they exist too? One theory is that the fibre pinion offers a failsafe in that if the mag seizes up the teeth on the fibre pinion strip rather than more expensive bits of the timing gear train busting. Sounds unlikely - BUT I have another very smart mate who did lose a fibre pinion just like that - the insulation in the mag melted on a very hot day here in France after a 150 mile local club run, turned to araldite inside as the bike cooled down (uniting the armature and the rest of it in a lump of goo), and when he tried to start the thing - whack - one busted pinion, which also slipped about 90 degrees on its taper. It unseized the goo too. The resultant back fire - yes there was still some HT activity in the thing - broke the kickstart quadrant and various other bits - a classic bad hair day!

An alloy pinion would probably be great if you were to live with fixed timing or go to a manual system. Fibre pinions are quite cheap - but you'd need that hydraulic vice on the collar of the ATD I suspect! Given the price of ATDs from SRM or wherever, it's probably the best and most cost-effective route to go though.

Groily