Lucas K2F Magneto

[Diana3899]

My friend has recently had a K2F magneto rebuilt by Paul Lynford. When on the bike with the spark plug attached to the HT lead and held against the cylinder there is no spark, so compression is not an issue. However when held in his vice and spun using his electric drill there is a big blue spark. The mag has an associated advance and retard device attached. He is trying to get the bike running as he has a potential buyer. Unfortunately he has advanced prostate cancer and has only been given a short time to live. So I would appreciate any ideas as to why the mag will not spark when on the bike as I would like to help him get it going so that the bike can be sold.

[Groily]

We can assume I think that the mag goes the same way on the bike as it is being spun on the bench?!

This sort of head-scratching problem can be to do with the earth of the magneto - and can be very bewildering.

You should see continuity if you place a meter between the spindle of the armature and the magneto body, or from the spindle to any good earth elsewhere, such as the cylinder head. (A good earth from magneto body to frame / engine should be a given, in light of how the magneto is attached.)

I'd suggest removing the earth brush from under the big screw next to the mounting flange at the drive end and checking the brush is making good contact with the brass bit of armature you can see down the hole. Rationally, for it to work on the bench, it OUGHT to be doing its job (as the armature is insulated from the body by insulators behind the bearings) but . . . .

If the auto-timing device has a fibre pinion as per many BSAs and Triumphs, then the earth brush absolutely has to be working properly as there's no secondary path from magneto spindle to earth via a metal drive train.

Other things to check include the engine 'stop' arrangement if there is one, and a general check of the rest of the contact breaker area just to be sure nothing is going funny when a sideways load is placed on the drive end.
If the contact breaker is the later steel type, be sure that the spring on the opening point can't kiss the camring under any circumstances, as that can kill sparks dead (but would do it on the bench too, of course.)

I know from experience how annoying this can be - but it IS, almost always, 'something to do with the earth'.

[Yves.Astein]

As Groily has pointed out, it sounds like an earth problem. Instead of measuring continuity, you could just touch the Sparkplug to the Magneto housing, instead of somewhere else on the Bike, while kicking the kickstarter. If you see a spark then, then your magneto is missing the contact to the rest of the bike.

You could also check your HT Cable to the Sparkplug. It could be damaged inside and when it’s bent differently when testing the magneto on the bench, the internal lead might make contact.

All the best to your friend

[bitza]

If you have Carbon fibre leads and they sometimes have a internal break, the spark then needs to jump across two gaps,
As been suggested check the H T leads if they are any age, cheers.

[Diana3899]

Thanks chaps for the advice, I am going down to see him next week and I will check out the earthing and HT leads as suggested.

[Groily]

If you have Carbon fibre leads . .

. . . then the thing to do is to put them aside for a modern vehicle.
Always best to use traditional copper-cored cable and (unless local laws demand otherwise) non-resistor plug caps and plugs - because if there's one thing a magneto appreciates it's a clear run from HT pick-up to plug!

In this case, we have sparks in plain air on the bench, but not when the magneto is fitted to the machine.
If there were an HT lead problem it would likely be that the insulation is damaged or melted, maybe where the leads pass through the inlet manifold if routed that way for tidiness, or that the HT pick-ups are so leaky that their proximity to the rear of the cylinder and the gearbox / engine plate / whatever is promoting a short. But it would be fairly unusual to have that happen to both sides at once.
All worth checking though, certainly.