The BSA A7-A10 Forum
Bikes, Pictures, Stories & more => Service Literature, Scans, Links, Documents => Topic started by: groily on 05.09. 2021 05:41
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Lucas Twin-cyl magnetos: Equalising points gap & firing interval
A common probem with the K2F and KVF Lucas magneto is a seemingly inexplicable visible difference in the points gap on the two lobes of the cam. Equally common is an error in the firing interval between cylinders, which may or may not be related to an unequal points gap (and which we can't see).
A typical problem would be a multi-thou difference in gap, and a several degrees' difference in firing point as the points open, between the cylinders.
Bearing in mind that 1° error on a half-speed magneto = 2° error on the crankshaft, there isn't much leeway for error if smooth running and correct timing across the rev range are to be maintained.
While the problems occur on magnetos with fixed camrings and auto-advance units, they are more common on magnetos with manual advance and retard, where there are more points of wear owing to the requirement for the camring to move in its housing.
The symptoms of unequal points gap are obvious, those of uneven firing are less so and only really identifiable and measurable using a strobe on both cylinders, or a dynamic magneto test rig with the facility to display and measure the spark interval.
On a parallel twin, the interval should be 180°:180°, and on a V twin the interval will reflect the degrees of the V angle – ie on a 50° V, the interval would be 155°:205°. An error of 1 degree on the mag is probably as good as many will get, and it is said that at the factory they were often no better, but errors of up to 5° and more at the magneto are often seen. That is 10° on the crankshaft and makes a mockery of accurate ignition timing as well as, in extreme cases, risking holing pistons, overheating, ruining any chance of a smooth tickover, etc.
The following are some of the things that indicate possible problems:
1. Sideways slop in the fit of the camring in the housing. (Denoting wear in housing, outer side of camring or both.)
2. A visible difference in the thickness of the lobes of the camring, or serious rust pitting, grooves or gouges on the camring near the ramps which open the points. A vernier gauge or micrometer will tell you if there is a major difference, although it won't help with the profile of the ramps on the lobes, which is all-important for the firing interval.
3. Play in the bearing at the contact breaker end, up and down and /or end float. Ideally, you want close to Zero float at room temperature.
4. A worn heel on the opening point; or wobble on the pivot post it's mounted on.
5. An ill-registered camring housing which can be be moved about a bit if you loosen the screws that secure it to the main part of the magneto.
6. The invisible but common problem of a defective bearing insulator cup behind the outer race of the contact breaker end bearing.
Replacement of camrings and housings is expensive, and may not in fact cure these problems if the cause lies elsewhere, so it is worth going through some hoops to try to identify the actual reason for the problems.
Remedies and Workarounds:
Sideways slop in the camring can be taken up by using self-adhesive stainless steel shim tape on the outside of the camring. It can be had in thicknesses down to a couple of thou, and it is durable enough to survive the limited (but repeated) arc of movement of the manual advance for many miles. It is an excellent long-term remedy on fixed camrings. Shimming is usually needed at about 3 or 9 o'clock looking at the camring end-on, to ease the thing across the housing one way or the other to make the points start to open earlier than they would without it on the lobe that is 'late'. If the upper lobe is the one that the points are 'early' on, then 3 to 5 o'clock-ish may help; if the lower lobe is the problem, then 9 to 11 o'clock. It's a bodge, but it's a good one and it can save a small fortune on a new camring or new housing, or both.
A felt lubricating strip, if there isn't one behind the camring in the groove designd to hold one, will reduce fretting in this area.
Shim tape can also compensate for a worn lobe.
Using any wiggle in the fit of housing to magneto body can play a part here too. A small amount of unintended 'adjustment' is very often present because the magneto's main elements have been mixed and matched over the years, and the housing isn't the one originally fitted to the magneto body.
Trouble at the cb end bearing is perhaps the most common problem, and it is often caused by disintegration of the insulating cup behind the bearing outer race. If the race isn't concentric with the housing, or is loose in its seating, the points gaps will be uneven and the firing interval upset. A new insulator cup can seem like a miracle cure.
Rectifying this requires the end housing off, the bearing race out and the new insulator cup fitting - with the added pain that the armature end float may well need resetting. Hopefully by using a large brass shim between the housing and the magneto body to compensate for the use of a new and thicker insulating cup. If the bearing itself is worn, then it's more trouble as replacement needs the armature out and possible reshimming behind the bearing inner race at the slipring end, which would only normally be undertaken in the context of a complete overhaul as it requires special tools.
Wear on the points heel, or the pivot post, can have some weird effects, which may only affect one side even though you'd think it would do the same to both. (Note that slight eccentricity in the rotation of the cb unit on the armature will affect both sides the same, so although it can look alarming it won't be the cause of the imbalance absent other problems.)
Replacement of the points pivot post is a fiddly job, requiring nadgery skills - but a new set of points can often help, even if the post is oval in part. However, the life of a point with a round hole of correct size running on a worn post can not be indefinite . . .
Order of Play
Look at the opening point as a first move - and if in any doubt try another one.
If the camring is sloppy – ie can be pushed from side to side more than a thou or two - then shim tape as a medium term fix.
If one cam lobe is worn down, then it's shim tape or replacement; but start with shimming because replacement costs money and may not, on its own, be a cure. Availability may also not be straightforward.
The biggest difference will probably be made by reseating the cb end bearing with a new insulator cup. Often, it fixes things all on its own, but it is the hardest of the jobs to do, apart from making a new points pivot post, so probably best not to start there.
Next . . .
Assuming that one way or another the points gaps can be got back to reasonable - within say, 2 thou of 12 on both ramps, the outstanding question is 'What is the actual firing interval' when the mag is running? You can ONLY check that on the machine by using a strobe across the rev range on both sides, and it is very possible that the interval will vary at different speeds. Sometimes, an error at kickstart speed irons itself out at speed - which isn't so bad . . . but the other way round is fraught with risk if overheating, pinking or other classic ignition timing problems are to be avoided. Modest errors can be averaged out by setting the timing with a degree disc to a mid-point, but serious errors require further work.
As with all these things, do only one thing at a time, then check for the effect. In almost all cases, the mag can be got back to correct, or close to correct. It would be rare not to be able to get the error down to a couple of thou on the points gap, and a couple of degrees (crankshaft) between cylinders. But it can be a labour of love unfortunately, and takes time.
Worst cases
If none of the above offers a solution, then there is only a small number of remaining options.
First, it is possible to grind back the opening ramp of the cam lobe which is opening the points 'early'. This is a fiddly business, and because material can't be put back after removal, a risky one requiring constant trial and error. A dremel-type tool rigidly mounted, and very steady hands, are the minimum requirement for a clean job. Good results can be had, and the effect on the dwell (points closed period) will be pretty minimal.
But if owing to wear and the ravages of time a good result and a good finish can't be obtained, then it's replacement of the camring and / or the housing, which is expensive unless good 2nd-hand bits can be sourced.
Footnote
In the context of this discussion, it's important to remember that the firing interval is more important than a perfect points gap (but it does have to be 'sensible'). A magneto that fires at 180:180° with points gaps of 10 and 14 thou on the respective lobes is better than perfect gaps and an error of several degrees.
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There must be many bikes running with unequal firing intervals. Engines condemned as being rough and unbalanced simply all down to that camring and its pals.
Makes a modern upgrade to a slightly different ignition system all the more attractive.
Thanks Mr G. Got me thinking......
Swarfy.
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My mag-timing setup > https://www.a7a10.net/forum/index.php?topic=1375.msg9915#msg9915
Uneven timing work > https://www.a7a10.net/forum/index.php?topic=1375.msg9862#msg9862
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(sorry fro somewhat offtopic, but related)
For those who have not seen these photos (which I am still quite satisfied with :O) closeups of k2f sparks in action > https://www.a7a10.net/forum/index.php?topic=1412
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Well done Groily - Very thorough coverage.