Ignition timing - 650 CSR

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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JEAN-NOEL
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Ignition timing - 650 CSR

Post by JEAN-NOEL »

Hi,
My A/R cable was too long to run properly from the A/R lever to the mag. I got one new, wich is perfect. But I noticed a problem : when the cable is released in position full advance, the plunger that actuates the cam goes down too far, and the cam become loose from full advance to full retard, and the lever some times remains solid. It is difficult to have a good setting, i.e. vith the cam pushed by the spring against the little screw wich limits the cam rotation. Fully released, the cable does not actuate the cam, slightly tight, there is a play between the plunger and the cam slot. Have I missed something ? Your advices welcome !
Cheers.
J.N.
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Duncan
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Re: Ignition timing - 650 CSR

Post by Duncan »

Hi J.N.

I have had a similar problem with the cam ring moving and locking in position (usually near TDC), I have tried different wine glass shaped plungers, running the outside of the cam ring dry as it jumped out more frequently when lubricated, swapping the mag end caps, putting gaskets and "o" rings under the cap to hold the cam ring in amongst other desperate measures.

On inspection the cam ring I had in the magneto the cutaway for the wine glass shaped plunger cut into the running surface for the points by about a quarter of its width, I have swapped this for another where the cut away does not extend so far and also put my spare magneto on the bike, in about 500 miles it has not dislodged since.

As a long shot it could be possible that if the bearings are going in the mag the slight oscillation may work the cam ring out, just a theory at the current time .
Groily
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Re: Ignition timing - 650 CSR

Post by Groily »

This will happen from time to time Jean-Noel, usually owing to wear on the plunger 'mushroom' or the edge of the notch in the camring. It usually shows up when you replace bits, just as you have!

The position of the little eccentric pin that limits the movement can sometimes cause it, if the pin is at the extreme end of its adjustment (in your case to the 'right' looking at the magneto from the points end). The reason there is adjustment there is to fine-tune the 'internal timing' of the magneto to get the points to open at just the right moment (which is a very few degrees after the 'flip' point of the armature). It might be possible to rotate the pin a little to the 'left' as it were, which will retard the internal timing a tiny bit and stop the camring going so far 'down' in the housing. This might have a very small effect on the points opening compared to 'perfect', but might be a price worth paying if the camring and plunger stay engaged. ('Perfect' is a movable feast anyway, as the wear that occurs on the heel of the opening point causes the magneto very gradually to retard itself; setting the pin is always best done with new points fitted, which will open earlier than a used set.) It is worth remembering that the points gap itself is also part of the game here - a larger gap gives earlier opening, so a tiny bit of 'retard' can be overcome by increasing the gap by, say, a demi-dixième or 2 thou.

Sometimes the problem of disengagement can be avoided by careful adjustment of the cable itself to ensure that at full advance the plunger does not go past the point at which it will slip out of its slot. If you can get a result where the camring will go to full advance after adjusting the position of the pin very slightly, then you will probably be OK.

Quite often, a replacement plunger will cure it. It is also possible to recover a worn plunger with a bit of braze or weld and a lathe or just a file - but it is a fiddly business and probably not worth it for the cost of the thing.

More extreme would be to replace the camring (expensive!), or to take it out, carefully cut a new large notch in the opposite side, then make a second new notch for the plunger and refit it turned 180°; but this has to be done extremely carefully or the timing may go badly wrong. If the points open too early at full advance there will be no sparks - and you only get one shot at this generally speaking!

It is also worth remembering that playing in this area can have an effect on the interval between the two sparks if there is any 'slop' in the fit of the camring to the the housing on a manual magneto. It is very common to find there is an error here after trying to resolve these sorts of problems. A degree or two is sort-of OK, but if things end up with a firing interval of, say, 178:182° or worse, then it's not so good.

If you get sick of playing with it, I'd be happy to set it up and test the interval across the speed range if you wanted.

A+
Bill
JEAN-NOEL
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Re: Ignition timing - 650 CSR

Post by JEAN-NOEL »

Hi,
Thank you Duncan and Bill.
The mag was fully serviced with an excellent report...
Obviously, the culprit is the plunger. The "mushroom" is too small ( Dia 9.20 mm). What is the diameter of a new one ?
Cheers.
J.N.
Groily
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Re: Ignition timing - 650 CSR

Post by Groily »

Usually about 9mm + or - J.N. from measuring some used ones (haven't got a new one to hand)- so yours is probably close to spot on. The notch position is the most likely cause to be honest . . .
Best, Bill
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robcurrie
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Re: Ignition timing - 650 CSR

Post by robcurrie »

J.N. What cap does your magneto have and is it securely in place? It holds the camring in position.

Rob C
JEAN-NOEL
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Re: Ignition timing - 650 CSR

Post by JEAN-NOEL »

Thank you Rob,
My mag is a K2FR, with a threaded cap, in good order. Even with the camring fully in place, the mushroom of the plunger escapes the notch in certain positions...
I have a headache !
J.N.
JEAN-NOEL
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Re: Ignition timing - 650 CSR

Post by JEAN-NOEL »

Hello,
problem solved !
A drop of braze around the "mushroom" to enlarge it, two strokes of file, and job done.
No problem now, the A/R lever does its job and doesn't fail.
Thank you for your help.
King regards.
J.N.
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