Any advance curves available for Auto advance units?

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Any advance curves available for Auto advance units?

Postby Jdraper » Sat Jul 13, 2019 9:57 am

Just a thought - does anyone have ignition advance curves for our machines? After 60 odd years most springs in auto advance units must be pretty knackered through heat cycling and general wear and this will have an effect on starting and general running.

Auto advance unit curves can be tested, but without a template to work to who knows what they should be doing, apart from advancing the spark, somewhat...

Also, I have no way of knowing if the unit is for a 350 or a 500 - and what difference does it make?!

Having just spend a fortune on getting the magneto rebuilt (and reducing the possibility of a heart attack trying to start a recalcitrant bike), I would at least like to know that the expensive sparks are being used effectively.. Getting a car's distributor rebuilt and and having its advance curve accurately set up is half the price of a magneto rebuild. Having the advance curve set up correctly on the bike would be the icing on the cake.

Jeremy

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Re: Any advance curves available for Auto advance units?

Postby SPRIDDLER » Sat Jul 13, 2019 10:16 am

Here you go.......chapter and verse from the website of our very own 'Groily' (Brightspark Magnetos).

http://www.brightsparkmagnetos.com/faqs ... %20ATD.htm


Advance curves (640x415).jpg


(I don't do icing ;) )
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Re: Any advance curves available for Auto advance units?

Postby Groily » Sat Jul 13, 2019 2:47 pm

Blimey! You did well to find that Neville - I'd have struggled myself to put my finger on the graph if someone asked me!

But the key point here is that the performance of an ATD - at least in the controlled tests we did some years ago now - is surprisingly little affected by some of the things we worry about! The fact that there are mechanical and electrical loads, including friction galore, within the magneto itself as well as in the ATD unit, somewhat negates what we'd think of as the very important issue of the state of the springs and their preload. However, still better to have good ones than weak ones, or none at all!

Some ATDs have more than the 12° of advance shown in the graph. Up to 18° is the common range we see (36° on the crankshaft that is), but I'm sorry I'm not dead sure what is recommended for what engines / marques. The Lucas parts lists might help, but I go goggle-eyed poring over them far too often to go look and see if there is any way of telling! Some judicious filing of the limit stops on the ATD can always increase the movement a bit if someone really wants a bit more retard.

Personally, I don't care much about the actual ° of movement in regard to my own bikes with ATDs (Beesa and Notrun twins) because the important setting is the full advance one, which is of course what we use for setting the basic timing.
What I like about ATDs is that the magneto is always working at its best, with the internal timing at its optimum; manual mags are always compromised when set anywhere other than at full advance, in terms of spark strength. This is easy to demonstrate on a dynamic test, where for example a perfect K2F might make all sparks, using Lucas-spec test gaps of 5.5mm, from say 120 rpm fully advanced - but need 150rpm or more fully retarded. That's mag speed, so when translated into kickstart / crank speeds, you need to be kicking things quite hard to get a good spark at full retard on a manual mag. While with an ATD, no difference.

If you compare an AMC single with manual N1 mag to one with an SR1 and ATD:
The manual mag will have, typically, at least 20° of AR movement on the camplate, which is 40° on the crank. (I see 45°, even 48/50° not infrequently on camplate mags/magdynamos - far more than anyone could need.) Many owners hardly touch the thing once running, using retard for starting, and maybe for ticking over. Otherwise, most of the time they're on full advance I'd guess. There must be those who try to match revs and advance, with half a thought for load as well, but not so many I dare to suggest!
The ATD could maybe have 36° max (crank), but more likely a good few degrees less. And whatever it does, it does it all on its very own . . . which is a plus of sorts, if it's working somewhere close to how it should
So - Same engine. Same full advance setting in terms of piston before tdc. Maybe 10°+ difference in firing point at full retard (if ever used on the manual one, that is). No big deal methinks, and I've never heard anyone tell they won't use an SR1 + ATD 'cos there isn't enough AR movement'.

If you compare performance on twins (not ours, as very few have ATDs, although it IS possible to fit them, just, with some work on the mag pinion front), the story is a bit different: a typical manual K2F has less movement on the camring than an N1 has on its plate, so the effective firing point on full retard using a 15° or 18° ATD might be similar to manual. That said, on my AMC twins, I hardly ever use anything more than a tad of retard on the manual levers, to smooth out the tickover really, and about half the lever movement for starting.

In terms of the actual curve . . . Well . . . the graphs Sprid refers show roughly what happens, or what we saw anyway. I reckon most engines with ATDs in average condition are running at full advance by 2500 rpm. There is no more available, so that's it!! Whether this is GOOD or BAD is a much discussed question and I really don't know. Someone asked me the other day to try to get one to hit full advance at 2800rpm . . . nearest I could get was around 2500 with any parts I could dig up. AFAIK pattern springs don't come with little stickers saying 'use this one for delayed advance, and that one for a shorter curve!'
What I personally think might have been interesting in this regard would have been a system as per car distributors, using inlet manifold depression via a diaphragm to get some instant retard (overriding a manual system temporarily, or working in tandem with an ATD by the use of a movable camplate or ring), for big handsful of throttle / heavy engine load at comparatively low revs.
But I imagine the good people at BTH and Lucas knew far more about it than us, and they didn't - for any magneto application I have seen. Too complicated, or just over the top, or maybe there isn't enough depression available in the typical manifold of a m/cycle without having to use a mega-size diaphragm, dunno!. . . It was something the late 'KenF' Targett and I were going to start playing with some years back, in light of the sensitivity of some alloy-head hi-compression engines, Beesas especially, to serious pinking under load . . . But one thing and another and we never did.

One final thing, on Jeremy's last point on Costs . . . a significant part is the HT winding for mag repairs. If you were to strip out the costs of messing with the armature, the rest of the job wouldn't look so bad in terms of ££! Unfortunately, coils don't come off the shelf for these things, and the supply of some of the other essential parts is low volume and can be patchy. Ensuring quality isn't always easy either!

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Re: Any advance curves available for Auto advance units?

Postby SPRIDDLER » Sat Jul 13, 2019 5:05 pm

Thanks Groily, that's most useful.


Over the past 40 years I've spent many a long couple of seconds worrying about this as I've only ever had manual A/R mags but if it helps here's some data from a test I carried out on my slightly warm engine in a pub car park this lunchtime in an ambient temp of 25.3 degrees at a cost of only a couple of pints .
It's only a guide as other's procedures may vary a bit, e.g. non-smokers.

N1 magneto graph Final (640x485).jpg
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Last edited by SPRIDDLER on Sat Jul 13, 2019 6:05 pm, edited 5 times in total.
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Re: Any advance curves available for Auto advance units?

Postby clive » Sat Jul 13, 2019 5:05 pm

Icing? I reckon Jeremy has got the blooming cake as well with this tome from Groily.
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Re: Any advance curves available for Auto advance units?

Postby Jdraper » Sat Jul 13, 2019 6:16 pm

Fan-flippin-fantastic response, however candles not quite lit yet. Any difference between 350s and 500s? ATD (spring) part numbers are different, but no idea why... yet.

I am just breaking in my rebuild after a slumber of decades sitting in my back garden while the kids were born, grew up & left home. Now I have the time to consider the icing.. and have some experience of classic car stuff.

Gradually doing more miles & building up the safe distance from home, where I can call home for a lift back to get the spanners to fix whatever has dropped off/failed/gone out of tolerance.

Jeremy
PS the magneto was 'repaired' by a company in Sheffield in the early 80s and this nearly broke my connection with the bike as it remained an unreliable starter, hence sitting in the garden for 30 years gently rusting away. Dave Lindsley fixed this problem with the application of a large wad of cash..

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Re: Any advance curves available for Auto advance units?

Postby Groily » Sat Jul 13, 2019 8:30 pm

This list gives the broad ATD variations, including showing those Lucas part nos. for springs Jeremy. You probably saw it or an equivalent list already:
http://www.brightsparkmagnetos.com/libr ... ations.htm
Whether, however, the various different springs are available, and whether the effect of fitting one pair over another is more than pretty modest, I'm not sure. Both 350 & 500cc engines use the 11-13° AR unit, and given the variables, I doubt if, being quite honest, it'd make a lot of difference in most cases. Stronger springs would lengthen the curve towards full advance on rising rpm and encourage rapid retardation on declining rpm, and vice versa - but by how much, in reality? Hard to say!

Be interesting to have two 'as new' ATDs, one with each set of springs, verified correct, to mount on a test mule mag, to see what the real differences might be when measured up and down the rev range. On the basis that manufacturers only ever do things that involve them in hassle for good reasons, it would be nice to see exactly . . . . The data must be out there somewhere I s'pose, but if it is, haven't seen it myself.

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Re: Any advance curves available for Auto advance units?

Postby Jdraper » Mon Jul 15, 2019 9:45 pm

So if I understand this correctly, ignition timing is set with full advance, which gives 37degrees BTDC (from the manual). The ATD will only give a max retardation of the spark of around 12 degrees, so it will never run less than 25 degrees advance. Seems a very high level of advance to kick start the engine. With tired springs, that will be even higher, with the engine going to full advance way too soon. Maybe that's where the tales of broken ankles comes from... Too much advance also causes overheating, something else I see talked about on the site.

- what am I missing?

I found an advance curve for BSA singles as attached which suggests a much softer ignition timing at lower revs.
PAM2-advance-single 2.gif


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Re: Any advance curves available for Auto advance units?

Postby Groily » Tue Jul 16, 2019 10:08 am

Not quite. 12° on the mag is 24° on the crank cos the mag goes at half engine speed. So far less advance at tickover than you are thinking, as long as the ATD retards correctly.

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Re: Any advance curves available for Auto advance units?

Postby Pharisee » Tue Jul 16, 2019 10:34 am

Not particularly relevant but I thought I'd chuck it into the pot anyway.
This is the advance curve for the electronic ignition sytem I fitted to my '71 Bonneville. It's the yellow line that you should be looking at. Full advance is 38°

triumph curves.jpg
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