Magneto timing

Information relating to the Matchless G3 or AJS Model 16 350cc Heavyweight
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clive
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Re: Magneto timing

Post by clive »

I was going to write something similar, certainly I stick to 1/2 inch before TDC which was later recommended. If the engine does not feel right I ease back on advance. Having said that I use 1/2 inch before TDC on the bikes with an SR1 auto advance too and have no option to fiddle!
clive
if it ain't broke don't fix
SPRIDDLER
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Re: Magneto timing

Post by SPRIDDLER »

Les Howard wrote: Wed Jan 25, 2023 2:46 pm I always set my bikes that have manual ignition with just a touch more advanced than the recommended setting. This allow me to tweek and adjust the timing for the best feel and performance from the engine.
(I've just been mulling this over on my trip around the supermarket).

I do the same. Occasionally when trying to identify the cause of a problem I have set the timing (say 1/2" BTDC) with the h/bar lever at a marked mid range point which in effect is my chosen 'full advance' and the strongest spark position. Thus, if the full range of movement of the mag face cam plate is around 20 deg (as Groily mentioned above) I have !0 deg of advance and 10deg of retard to play/twiddle with on a test run.

There is a fail-safe element to setting up the ignition timing with the h/bar lever at the extent of its range for general riding since a). It's easy to determine just by feel with a gloved hand that it's (still) set exactly at the full advance position (a bit like the habit of checking the zip on your flies), and b). There's no risk of running over-advanced and holing a piston when you might not be able to hear the engine pinking above the engine noise and when wearing a helmet. On my G3LS the full advance position is with a tight/pulled control wire so I tend to check that the lever hasn't crept back retarding the timing. It isn't necessary when full advance is with a slack control cable.
The extent of movement of the h/bar lever between the full advance and full retard positions is relative to the diameter of the 'winding drum' of the h/bar lever. If the diameter of the winding drum on the h/bar lever was larger there would less movement required to move the mag face cam through its 20 deg range. Thus a small movement of the h/bar lever would result in a proportionately larger alteration to the ignition timing.
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AndrewM
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Re: Magneto timing

Post by AndrewM »

SPRIDDLER wrote: Wed Jan 25, 2023 2:00 pm The mag drive chain (and the dynamo chain) *should* be endless, i.e. no joining link. More important for the dynamo chain which runs very close to the primary chain and a clash of chains could result in serious damage, not least to the chaincase.
The one I have has a joining link so I'm stuck with that for the time being. Will order a new one and install it once I get everything else on the bike sorted.
Not sure which sprocket you loosened but I find setting the firing point more controllable if I loosen the (lower) cam axle sprocket rather than the one on the mag (the uppermost one). Preventing the mag armature from moving whilst you tighten its sprocket nut is quite tricky because on full advance the contact breaker opening point (the firing point) is at the mag armature's 'flip point' which tries to make the armature rotate away from where you want it as you tighten the sprocket nut. The cam axle has more resistance to unwanted movement as you tighten the nut than that of the mag armature. Before tightening the nut on the cam axle it helps to 'set' the sprocket in place on its axle if you put a socket larger than the nut against the sprocket and give it a sharp tap.
Had read this advice from you on previous searches of the forum and did exactly what you recommended. I immobilised the contact breaker by clamping my original LTK-12 adjustable spanner around it and resting its handle on a convenient part of the cylinder base, was able to clamp it at exactly the right angle.
SPRIDDLER
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Re: Magneto timing

Post by SPRIDDLER »

AndrewM wrote: Wed Jan 25, 2023 10:02 pm
The one I have has a joining link so I'm stuck with that for the time being. Will order a new one and install it once I get everything else on the bike sorted.
O.K. but don't let your XXXX tinny get warm worrying about it. The chain is lightly loaded driving a mag so the most force it will have to take is when the engine kicks back on starting as obviously the mag rotation reverses direction and this is also the time when a sprocket may slip on its shaft as the sprocket isn't keyed to the shaft like the dynamo sprocket is.
Had read this advice from you on previous searches of the forum and did exactly what you recommended. I immobilised the contact breaker by clamping my original LTK-12 adjustable spanner around it and resting its handle on a convenient part of the cylinder base, was able to clamp it at exactly the right angle.
Excellent.
:oops: But I don't claim originality for the suggestion as much fettling lore is absorbed over the years from other owners, admittedly trying the tips out on my own bikes first.
I envy you down there in mid summer. :cry:
'There is a tide in the affairs of men
Which taken at the flood............'
AndrewM
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Re: Magneto timing

Post by AndrewM »

SPRIDDLER wrote: Wed Jan 25, 2023 11:07 pm I envy you down there in mid summer. :cry:
You wouldn't have envied me yesterday working in my metal-roofed garage on a stinking hot day doing tappets and timing, took me hours, and it was like working in a sauna. I'll pick a cooler day next time.
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