Newbie Questions- timing

Information relating to the Matchless G9 or AJS Model 20 500cc twin
Groily
Member
Posts: 2151
Joined: Thu Jul 04, 2002 1:00 am
Location: NORMANDIE FRANCE

Re: Newbie Questions- timing

Post by Groily »

Wow. That speaks to an unusually serious imbalance . . . More than a 12 thou difference, if you had set the rh cyl with a 12 thou gap as per book.

You can set things on the other lobe, or open out the gap till you get say 10 thou on the tight side, but you'll probably have way over 20 thou on t'other, which is a) too big for half-decent operation of the contact breaker and b) will without doubt be reflected in a horrible firing interval between the two sparks.

Questions:
1. Is there a lot of sideways slop in the fit of the camring in the housing? (Denoting wear in housing, outer side of camring or both.) Is there a felt lubricating strip in the groove in the camring at 6 o'clock-ish?
2. Is there a visible difference in the thickness of the lobes of the camring, or serious rust pitting, grooves or gouges on the camring near the ramp for opening the points on the tight gap? 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. Is there 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. Is the heel on the opening point in reasonably smooth unmarked condition, and does the opening point float or wobble on the pivot post it's mounted on?
5. Is the housing for the camring able to be moved about a bit if you loosen the screws that secure it to the main part of the magneto? Quite often there is some wiggle and holding it this way or that while tightening up the screws can make a significant difference.

Remedies / 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. It is usually needed at about 3 or 9 o'clock, 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. If the upper lobe is the one that the points aren't opening on, then 3 to 5 o'clock-ish may help. Yes', 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 (which would set you back £150+ for brand new). A felt strip, if there isn't 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, of course.

Trouble at the bearing is the most common thing with these mags probably, 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. Rectifying this requires the end housing off, the bearing race out and a new insulator cup fitting - with the added pain that the armature end float may well need resetting. Hopefully by using a 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, Ouch, so let's not go there for now.

Wear on the points heel, or the pivot, 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.)
Replacement of the points pivot post is a fiddly job, requiring nadgery skills - but a new set of points can often help quite a lot, even if the post is a bit oval in part.

As an order of play, I'd adjust the points to see just what the extent of the problem is, as you suggest.
Then, I'd look at the opening point as a first move and if in any doubt try another one.
If the camring is sloppy - by which I mean it 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 shimming or replacement; but start with shimming because replacement costs money and may not, on its own, be a cure.
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.

Let's assume, then, that one way or another the points gaps can be got back to reasonable - by which I mean not worse than 10 thou one side, 14 the other.
The big Q then is 'what is the actual firing interval' when the mag is running? You can't check that other than by using a strobe across the rev range, 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 as an error at high speed can do things we don't want to think about if it serious.

As with all these things, it's best to do only one thing at a time, then suck it and see. 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. In the very worst cases, it can take a grinding session using a dremel-type weapon rigidly held, to take a bit off the ramp of the cam lobe that is opening early . . but again, let's not go there in too much of a hurry. Anything removed can't be put back, so a Bad Plan if the reason is something other!
User avatar
clive
Member
Posts: 5657
Joined: Mon Jan 01, 1990 12:00 am
Location: LONDON UK

Re: Newbie Questions- timing

Post by clive »

Groily I think you should copy and paste that and send it to the technical tips. Very full technical info.
clive
if it ain't broke don't fix
Sams Dad
Posts: 21
Joined: Mon Apr 19, 2021 6:49 pm
Location: Lancs UK

Re: Newbie Questions- timing

Post by Sams Dad »

Thank you Sir, for such a thorough explanation and troubleshooting plan. I am done for today, but hopefully I will get on to it after work one night this week. I will let you know how I get on. The points gap was huge before - I didn't measure it as I was aiming for 12 thou and reset to that but 20 thou or more i would guess at and yet it did start, but the carb was missing the float needle and choke so I could not easily keep it running. I think the a/r cable has pulled the timing out when i reconnected the repaired cable as it would not fire at all after all was fitted. It is all a step learning curve, but I am so grateful to those who have assisted and helped me.
Groily
Member
Posts: 2151
Joined: Thu Jul 04, 2002 1:00 am
Location: NORMANDIE FRANCE

Re: Newbie Questions- timing

Post by Groily »

clive wrote: Sun May 23, 2021 10:37 pm Groily I think you should copy and paste that and send it to the technical tips. Very full technical info.
OK Clive, thanks. I have done a slight reworking - now in Tech Articles at jampot.com. (Complete with a careless minor error I can't edit because I can't get back into the text, but I will if I ever find out how!)
Sams Dad
Posts: 21
Joined: Mon Apr 19, 2021 6:49 pm
Location: Lancs UK

Re: Newbie Questions- timing

Post by Sams Dad »

Wow this is proving worse than I expected. I am set at 10 thou on the right cylinder and after getting my left and right side of my brain in sync I timed up the right cylinder until i was happy with it with a fag paper. Then checked the left. About 20 thou gap and timing about an inch early on my timing stick. Can i go down to say 5 thou then try again ? The crazy thing is it did fire up before I started trying to set it up correctly and before i set everything up on the carb with the choke and float needle. Am i going to need to pull the cam ring out? There does not appear to be any side to side slop either..I guess next step is pull it out and get it measured up to at least see if that is the issue.
Groily
Member
Posts: 2151
Joined: Thu Jul 04, 2002 1:00 am
Location: NORMANDIE FRANCE

Re: Newbie Questions- timing

Post by Groily »

I think it was inevitabe from what you initially reported that there'd be a massive difference in gap on the points.
I doubt the piston position varies by an inch though, as that's a third of the entire stroke of the engine pretty much. Assuming the pistons are the same type on each side!

I'd start with the points - look at the heel on the one that opens for funny wear patterns, check for wobble on the pivot. Or just try another one, even a complete contact breaker assembly, if you can get your hands on one.

Pulling the camring off is easy enough - they fall out near enough on manual mags, coming off their locating peg at 6 o'clock and the plunger for the advance and retard at 9-ish. You could then measure the thickness of the lobes at various places to see if they're miles different - but that won't tell you if the ramps that open the points are 180° apart. If you have access to a rotary table, that can be a good way of working out the firing interval (static) or you might be able to play with a protractor and a ruler on a bit of paper to get a rough idea, if you mark the camring before removing it to show exactly where the points start to open on each side.
For comparison, a new camring has lobes 150 thou thick, with the thinner parts at 100 thou.

Otherwise, there's no choice but to work through the possibilities. The bearing race insulator cup has to be a prime suspect I think.

The chances are, being honest, that the thing needs a proper going over from end to end to get it into decent working condition. I fear the old adage about silk purses and sows' ears comes to mind!

There's no real purpose in changing the points gap and trying again, you'll get the same variation, and in any event 5 thou is much too small.
Sams Dad
Posts: 21
Joined: Mon Apr 19, 2021 6:49 pm
Location: Lancs UK

Re: Newbie Questions- timing

Post by Sams Dad »

I took the cam ring out a and removed breaker backplate. The ring appears ok lobes 153 thou whilst non cam areas are 115 thou. Not a lot of pitting although it looks cleaned up more on one lobe than the other. I loosened casing and closed the disparity down to about 8 thou. No side to side play in the spindle so I was going to order points but none in stock! The points do look ok except a little pitted on one side. Where will i fine the bearing race insulator cup ? I have the magneto out for ease of access.
Groily
Member
Posts: 2151
Joined: Thu Jul 04, 2002 1:00 am
Location: NORMANDIE FRANCE

Re: Newbie Questions- timing

Post by Groily »

You can get insulator cups from various places, such as Dave Lindsley, Priory Magnetos etc, or you can order online from us here at
http://www.brightsparkmagnetos.com/shop/shop.htm. You'll need an insulator for an E15 bearing and I'd order in each of the 2 thicknesses as you don't know what you're going to need until you have the bearing out. With any luck the bearing race will come out quite easily with a bit of heat and some careful playing with little levers, but worst case it might take 'a tool'.
Camring sounds quite good which is good news due to their cost, hope the clean-up someone did didn't move the on ramp on that side and thereby cause or exaggerate the error. The one I measured was a new replica, not a Lucas one, so your measurements are probably perfectly OK too. If the ramp is 'off', you can take a fraction off the other side's opening ramp to get the 180° interval back - with great care! - after you know the bearing race is concentric and the housing is in its 'best' position given there is some wiggle. Another wheeze is to rotate the camring half a turn to see if the gaps are more equal and the firing interval better that way round - but if it does have a good effect, you then have to grind new notches for the stop peg and for the AR cable. Symmetry only goes so far!
It's boring that points are so hard to get at the moment but a bit of pitting will clean up 90% of the time. If the heel is in a decent state and the thing doesn't flop around on its pivot you're probably OK there. And it's good there's no real side-to-side between ring and housing - or slop on the bearing.
The gods may be with you!
Sams Dad
Posts: 21
Joined: Mon Apr 19, 2021 6:49 pm
Location: Lancs UK

Re: Newbie Questions- timing

Post by Sams Dad »

Hi Groily, I think i had a major break through. The points housing is held on by only one bolt in to the mag main body.The points cover was screwed on to that housing, but it looks from the manual diagram that it should be held on by a much longer stud with a nut on top.I think this is the problem as the one retaining bolt is drawing the cover over to one side, whilst the two studs are missing. Slacken the bolt, hey presto similar points gaps ! Do you sell these studs ? Are they the same thread as the singular bolt? The female thread on this is bigger on the body on mine. That may be an issue
User avatar
clive
Member
Posts: 5657
Joined: Mon Jan 01, 1990 12:00 am
Location: LONDON UK

Re: Newbie Questions- timing

Post by clive »

Stephen could you private message Groily please. We don't allow adverts in the general forum and if Groily replies I would have to sanction him :lol:
regards Clive
aka the Adverts Police
clive
if it ain't broke don't fix
Locked