Ignition timing query

Information relating to the Matchless G9 or AJS Model 20 500cc twin
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Ignition timing query

Postby rob4586 » Fri Mar 17, 2017 9:50 pm

Good evening all,

Could somebody explain to me how the magneto on my Matchless G9 works?

Currently my bike is firing on the off side cylinder, but isn't firing in the near side cylinder. I was doing some work on the ignition cam as the wine bottle kept jamming, so I think I've disturbed the points putting the timing out on one cylinder.

Does the G9 only have one set of points for both cylinders using a wasted spark system? However, if this was the case, then it can't be ignition timing that is out as every rotation is causing a spark on both cylinders, so if one cylinder is firing perfectly then the other cylinder should fire?

I appreciate the help.

Regards,
Rob

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Re: Ignition timing query

Postby dave16mct » Fri Mar 17, 2017 10:33 pm

Hi Rob,
not wasted spark. The points work on the cam which has 2 bumps, so open twice. The armature in the mag has a slip ring which has a brass contact. As it rotates it contacts the pickup carbon brush and travels along the HT lead to the plugcap. It rotates to the other pickup etc. So start at the non firing plug, remove it put the cap back on and see if it sparks when lied on the head when turned over on the kickstart. If not try another plug, then the cap, then the HT lead, then the pickup. Carefully remove the pickup the carbon brush should project and you should be able to lightly press it in with your finger. You may need a new brush if it's worn down.
Have fun, Dave

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Re: Ignition timing query

Postby robcurrie » Sat Mar 18, 2017 12:46 am

It is critical that the points gap at both cam positions are spot on, otherwise the ignition timing could differ between the two cylinders.

Rob C

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Re: Ignition timing query

Postby rob4586 » Sat Mar 18, 2017 9:43 pm

Dave & Rob,

Thank you for your replies - it has certainly aided my understanding. The slip rings explain how one set of points can fire two cylinders independently if I'm understanding it correctly.

However, would it be fair to say that if one cylinder is firing correctly then it can't be the points that need adjustment as they're exactly 180 degrees out, so if I adjusted the point it would alter the timing for the cylinder that is firing perfectly?

Regards,
Rob

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Re: Ignition timing query

Postby aobp11 » Wed Mar 22, 2017 9:01 am

Hi Rob,

Exactly 180? Ask Groily (Brightsparkmagnetos) for other stories!
Did you check the points gap at *both* lobes of the cam ring? In the worst case the points just open at one lobe but just not at the other lobe. But when both gaps are about right then both cylinders should work, regardless of a few degrees difference in ignition timing. Also check that the metal spring and its tiny retaining screw do not touch the lobes. There can be some radial play in both the cam ring and the magneto shaft, so you have to apply some pressure in the worst direction while doing such checks.

Albert

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Re: Ignition timing query

Postby Groily » Wed Mar 22, 2017 9:46 am

Ha! As Albert very rightly hints, there be a few stories to tell on this one! All a bit anoraky though . . .

The firing interval on K2Fs is often a real pain to get right, even if the points gaps do seem even (which is a good start). But often, it's better to tolerate uneven gaps if you can get a perfect firing interval between sparks (as long as you're not talking more than say three thou or so difference with neither bigger than about 15/16 thou to avoid them hammering away too much.
Measuring this isn't so easy though. Using a rotary table and 'buzzer' one can get a good idea of the static situation to within a degree or so - but the problem is that at speed things don't always stay the same. And it's at speed that it matters more.

The most common causes of error are probably wear between housing and camring or in the fit of the cb end housing to the mag body (especially if bits have been mixed and matched over the years), followed by eccentricity in the seating of the bearing at the cb end. Reseating the outer race with a new insulator very often 'cures' a significant error (and messes up the armature shimming and makes other work 'n all!). Wear patterns on the heel of the opening cb point can also do funny things. It's not usually the camring, although it can sometimes be if one is very worn, rusty, pitted or someone has attacked it with a dremel in search of 180:180 sparks.
I've seen several mags with not far off 10° of error between cylinders at the magneto - which is 20° on the crank and potentially very harmful/fatal. Most are within 1° to 4° at the mag when I first look at them, and some are - alleluia! - perfect.

Something that can be quite startling, too, is the difference to the timing setting (and sometimes the firing interval as well) that follows simple points replacement. The unworn heel on a new set will almost always open the points earlier than the old set, with consequently several degrees of advance in the basic setting. Boring, but it is actually worth checking the setting against tdc or in degrees after swapping a set of points. And wear on the cb pivot post for the moving point can cause some or all of the above too.
I've seen it said that when they were all new, errors of up to 2° at the mag were common, and I believe it!

BTH twins don't do this nearly as badly, as they seem to me to have been made with better tolerances in many respects. But with horizontally-emerging HT pick-ups (and a thicker flange at the drive end) they don't fit easily on all machines, sadly - or I'd use them in preference on my own.

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