Timed the bike, twice now,using your standard average procedures. Used both a fag paper and a multimeter to get an accurate(?) points opening point and a marker down 'the little hole' for TDC. Yes I know taking the head off would be less likely to impart an inaccurate piston position.

The problem is that when it runs, which it does - very readily, and you operate the lever it picks up lovely until it reaches a particular point (maybe half the lever travel) and then the engine dies. Back the lever off and you are back in business.

It suggests the timing is out and it is 'going too far' but when checked everything appears to be happening at the right point gap, point of opening, btdc etc. Anyone else had this ?

Duncan

email (option): cruiserchooser@hotmail.co.uk

You've definately set the timing with the ad/retard lever in the fully advanced position ?
& looked in the mag to see that the backplate is turned fully in the anticlockwise direction before setting the points to open at the correct amount before the top of the stroke.

Is there any chance that the movement is causing something to ground out within the unit? Seems odd to immediately die out at a specific point only to return by backing off.

Yes CFred, I've checked all the obvious mistakes! and it has me puzzled. Interesting point Scott, I had not thought of that one but damned if would know what to look for there or how to check for it, particularly if it was something really internal - but it would have to be something around the moving cam plate area for it to be associated with the lever movement? Going back out for an hour or so to give it the once over again!

email (option): cruiserchooser@hotmail.co.uk

Yes, I did risk stating the obvious :-D
But a friend of mine bought a G3L & it ran badly last week I tested the timing & it had been timed retarded .. so that's why I mentioned it.
Make sure your not using the Choke lever (joke)

I dont mind the obvious being stated, it sometimes help trigger a thought process that isnt always obvious. 'Fraid not on this occasion though triple and quadruple checked and it should be right - but it still cuts out if I try to wind the lever the full travel. Up to that point it goes like a good 'un and it is easy to recover if you tweak it too far. It just has me baffled as to what is going on. No matter, I had a good blat on it today!

email (option): cruiserchooser@hotmail.co.uk

Something I did think of, at some point in the 50's the camplates changed in thickness, that means its possible to have the housing for a "thick" plate, but a "thin" plate in there. So if you havnt checked, check the cam plate can't move in & out (even if the clip is present) as I read some info that states camplates reduced in thickness from 5/32" to 3/32" in the late 1950's & Lucas supplied only the thin ones later on & included a little spacer/packer to make up the slack in early housings. As parts get swapped arround its certainly the next thing I would carefully check on yours.
Occasionally people have "thin" housings & can't get the clip in the groove due to a thick type camplate, this is obvious on assembly, but reverse the situation & its possible to miss it.

We live and learn! I'd not heard of that one before, it is something I will check. Did find out, following a bit more testing, that the misfire only happens when the bike is under load i.e. moving. stationary I can get full lever travel and the bike hardly falters. So now I am thinking of either the plug giving up the ghost under load or maybe the mag might not be delivering full juice. More to look in to, isnt life fun?

email (option): cruiserchooser@hotmail.co.uk

Ah, well in that case I would try substituting your HT lead as I have had vehicles that fail under load be caused by HT lead / connections.
Its worth doing before condeming the mag.

Hi Duncan..I assume you are describing retarding the ignition using the Adv/Rtd lever. If that is the case you won't be able to retard it fully with the engine running at normal riding speeds without detremental effects..The range of movement provided by the lever means the engine will be firing well after top dead center when the lever is set in the fully retarded position. (completely the wrong place for riding speeds).
Retarding the timing, either partialy or fully is only neccessary for when the engine is running at low revs, such as for starting, making slow running adjustments and if you are riding very slowly under load..
From the moment the spark occurs the combustion process happens at a constant speed. Setting the spark at 7/16" BTDC for riding speeds allows the piston to be in the correct position just after TDC as the 'explosion' reaches its maximum potential..in other words the correct position for it to be driven down the stroke.
At much lower engine speeds, such as tickover speeds, the piston is travelling more slowly so takes longer to reach the optimum position but combustion is still happening at the same speed....In that case the 'retard' facility is used to make the spark occur later, thus giving the slower moving piston more time to reach the correct position.
Generally once you have started the bike you would put the lever in the fully advanced position as soon as you pull away and leave it there...The only real exception to this when riding was if, for example, you were travelling VERY slowly in heavy traffic.(when engine speeds would be low).
Is it possible that you are expecting the bike to do something it isn't designed for in this regard?..Ian

email (option): ian@wright52.plus.com

CF I take your point about the HT lead, and I suppose I will add the plug cap and the mag connector to that as well! The lead happens to be brand new from a roll that I have used without problem on another vehicle, so in fairness I will trust that - but the point is fairly made and someone else checking the threads might gain help from the suggestion. I am looking in to the magneto setup as the model number on mine is not as per the book, so unless it is an 'approved' substitute or has been creconfigued to be the correct M20 setup then this may be something to do with the problem?
Ian. Yes the ad/rtrd lever is what my whole world is revolving around at the moment :-) I have set/checked the timing as per the book and I am confident it is correct with the points just opening at the 7/16 btdc as prescribed, with the lever at 'full throw' in the fully advanced position. And the points opening to the correct amount also. The bike starts fine with the lever unmoved in the retarded position. On the stand the lever can be moved to fully advanced and the engine goes with it with only, now and again, the vaguest hint of a misfire as the lever gets to full throw. On the road though it is different. Under load I only get about half advanced (difficult to say really in moving traffic conditions) before the bike gives a bit of missing to start with but then if you move the lever to full advance it cuts out completely. Back it off quick enough and it cuts back in. Performance is fine. I initially thought I had the opening point wrong and that the spark was moving beyond the point where it worked, but not now. I feel I have eliminated that by the repeated cross checking. Which is why, currently, I am thinking of some other component giving up under pressure. But always glad to hear input from others.

email (option): cruiserchooser@hotmail.co.uk

In case it hasn't already been stated. These mags don't like to run through a plug suppressor cap. Ron

email (option): ronpier@talk21.com

Scott's suggestion sounds like a good thing to check for. You could check that it isn't shorting easily enough with a meter or a bulb and a battery. Move the advance/retard lever and see if the bulb lights up when advanced.

email (option): horror@blueyonder.co.uk

Although it's not a system that I like, have you tried experimenting by setting the timing at TDC on a slack wire (ie lever fully retarded ?)

You'll probably find it a little over-advanced on a fully tight wire but only to the extent that it pinks a bit.

Some photos might be helpful, just to see if all your parts look correct. Another daft idea but your mag hasn't been built up with a slack wire advance, has it ?

When you move the lever from retarded to advanced with the bike stationary does the engine speed up or slow down?..It sounds like you might be retarding it,
not advancing it..If the mag was not off an WDM20 you may have a 'slack wire' fully advanced set up, not a 'tight wire, fully advanced..which means it will be operating in reverse for a WDM20..An even worse possibility is that you have a Norton mag fitted that runs in the opposite direction to an M20 mag..that is completely unsuitable.
So, does the adv/rtd cable enter the mag body to the left or right of the contact breaker assembly?.. and when you have the lever set in the 'fully advanced' position do the points open at approx. 11 o'clock as you look at the breaker assembly?...Ian

email (option): ian@wright52.plus.com

Thanks for all the suggestions fellas, I will check out all your various bits of advice.
Ian, moving the lever from retarded (slack wire) to advanced (tight wire) speeds up the engine. The cable enters the body to the right of the contact breakers - the side furthest from the barrel. regarding the 'o'clock bit' I will have to check in daylight tomorrow to remind me of their exact orientation.
Cheers again.
Duncan

email (option): cruiserchooser@hotmail.co.uk