Magneto coil conversion

[Mick BaileyParticipant @mickbailey28509]

My 84 year old friend has a Velocette Venom with starting problems and I called round to check things over and discovered that the K1F magneto has a very weak, intermittent spark. Even with the most vigorous kicking with the plug removed it produces a thin, barely visible whisker of a spark that will not jump more than 1/16".

The magneto is physically in good condition otherwise. Even if the unit was in good condition there's another complication in that my friend is barely able to kick the bike over, made worse by the low kickstart gearing. I'm thinking that a coil conversion would help him out, giving a fat spark regardless of rotation speed.

I can think of two possibilities – the Thoespark unit which replaces the magneto points, which is an attractive option. The second is if the points can be used (with an external condenser) to operate a coil – has anyone done this, or is there a reference? I recall seeing an article a while back but can't remember the specifics.

[Mick BaileyParticipant @mickbailey28509]

[Chris Evans 6Participant @chrisevans6]

I can speak from experience about the "Thorspark" conversion. I fitted one to my 1929 built (1930 model) BSA 350 Sloper. 9 years or more and still first kick starting. You do have to set up timing marks and do final timing with a strobe. The conversion is reversable and economical to do.

[not done it yetParticipant @notdoneityet]

I’m not heard of the Thorspark conversion (but I will check it out).

May not help you aged friend, but one common reason for a weak spark from the magneto is that the permanent magnet is no longer as strongly magnetic as it was. They can be remagnetised to overcome this failing. The timing of the generated pulse may also be mis-timed slightly. I’ve only fixed much larger magnetos than motorcycle sized ones.

On some vehicles where the magneto coil has failed, the magneto points have be used (but in the tried and tested Kettering ignition system arrangement).

One question:  Do these old lucas magnetos have an impulse coupling, for low speed tripping of the magneto, to improve the spark at cranking speeds?  If so, that may not be latching and releasing.  Most of the mags that I have, are fitted with them.

Edited By not done it yet on 07/07/2022 09:47:23

[mike robinson 2Participant @mikerobinson2]

My Velocette Venon Clubman (1960) also had the same problem even after a magneto rewind. Going to electronic ignition is fine but it does put a strain on the dynamo and usually requires that upgrading plus a larger battery. In the end I replaced the electronics with a new BTH Mag. It looks pretty much like a K1F but has a series of (maybe 8) radial high power magnets and coils that then feed a separate coil. The spark is very strong. It has built in electronic advance and retard and a setting pin to lock the rotor in postion when tightening the mag retianing nut. More in keeping and no reliance on the battery. In getting this link I just checked the price and its a shocker, its £660inc VAT https://www.bt-h.biz/BT-H-Magnetos/

[mike robinson 2Participant @mikerobinson2]

PS: I fitted mine about 6 years ago when it was about £300

[HopperParticipant @hopper]

Sussex Motorcycles have the kit listed at 150 Quid these days. I might have to look into one for my Norton Atlas. The maggie insists on firing 5 degrees out between the cylinders even with a different cam in it Most annoying. Trouble is then I will have to fit a battery and make sure the reg/rec is working properly instead of the present direct lighting with the old original Lucas rectum-frier and ancient Zener diode..

Edited By Hopper on 07/07/2022 09:57:05

[john fletcher 1Participant @johnfletcher1]

As not done it yet says above, the magnet might need re magnetising a quick and easy job for the repair shop, apparently now expensive, count think why.as it is a quick flip job providing you have the re magnetising transformer.. I had a Norton ES2 many years ago and did a home brew conversion on it and have done the same on several two strokes. I used the same points, condenser and a 6 volt Morris Minor car coil on the ES2 which I had in the shed. I'm sure 6 volt coils are still available other wise to go to 12 means changing the battery and the light bulbs. I would suggest you check out the condenser first, for a rough test get your friendly electrician with his 500 DC Megger to charge it up, leave it for a few minutes and then see if you get a good cracking sound when you short the lead to the case. I assume the points are clean not pitted and have correct gap. I hope my experience is of help to you. John

[duncan webster 1Participant @duncanwebster1]

I had a Venom many years ago, it started OK from cold but from hot I had to bump it. I think at 84 years old with a bike as problematic as a Venom I'd be looking at an electric start option. Must be available in UK starter

[Mick BaileyParticipant @mickbailey28509]

The setup is quite basic and I'm not aware of impulse coupling with the K1F, and my MAC 350 didn't have this. I have a couple of insulation testers and will check out the condenser out of interest in finding out what's actually wrong – a failed condenser is a common failure with this type of magneto. On first inspection the points, pickup and lead are all in good shape.

I saw a Venom the other week with a really frail chap struggling to get on the bike. I thought "he'll never get that started". A button push and it instantly sprung into life – I never heard it turn over a second time. The price is the killer (£2040), though this would be perfect.

My friend's Venom already has 12v electrics, a decent battery and an alternator conversion. This would make fitting either a Kettering setup or Thorspark simpler in that nothing else would need to be changed or uprated.

[Chris Evans 6Participant @chrisevans6]

Iv'e seen a Velo crankcase ruined when an electric start conversion was fitted. Apparently the bike kicked back when attempting to start and the conversion did not incorporate a sprag clutch. Like a lot of things great when all is working well.

[HopperParticipant @hopper]

Old magnetos can be a can of worms. They not only lose magnetism over the years, but the insulation on the windings breaks down too. And the slip ring insulation can crack, ditto the pickups. Condensers fade away. If not fitting a modern electronic system it can be worth sending the old maggie away to one of the specialist repair experts who advertise in the classic bike mags. Everyone I know who has done this is pleased with the result. When in good nick, they actually work very well.

[Mick BaileyParticipant @mickbailey28509]

The bikes fitted with magnetos I've owned have been fine. I'm not sure though that even with a good magneto that starting a Venom is going to be easy – they're difficult at the best of times. I'm a good 20+ years younger than my friend and even when the bike ran well to was a real pig to start. Part of the problem is age and mobility, height (he's not so tall), the position of the kickstart, and the gearing ratio of the kickstarter. He could spend a fair sum on getting the magneto reconditioned only to find that he still had problems. Comparatively, fitting (say) a Thorspark would eliminate the need to achieve a decent rotational speed in order to start the bike effectively.

My thought on using the mag's existing points with a separate coil won't work – the K1F has a ring cam and the points rotate, making this mechanically complex to re-engineer.

[garyParticipant @gary44937]

i have owned a mss , viper, venom, and a thruxton all of them were good starters hot or cold get the magneto fixed and the timing and carburettor set up properly and it will start easily. it is also a myth that velo clutches are troublesome. gary

[duncan webster 1Participant @duncanwebster1]

Posted by Chris Evans 6 on 07/07/2022 18:07:55:

Iv'e seen a Velo crankcase ruined when an electric start conversion was fitted. Apparently the bike kicked back when attempting to start and the conversion did not incorporate a sprag clutch. Like a lot of things great when all is working well.

Can't see what a sprag clutch would achieve. If it backfires it is trying to back wind the starter, sprag would still drive. If backfire is a problem then a shear pin or other overload protection is needed.

[JAParticipant @ja]

There is only one sensible answer to the poor performance of old magnetos. Get it re-built by someone who knows what he is doing. Otherwise you are just messing around and going no where. I know from years of experience. Using electronic ignition is all very well until the dynamo fails (which is less reliable than the magneto). Joe Lucas built these things cheaply without a thought above longevity. The rider's hand book for my 350 suggests that the dynamo and magneto should be checked at a Lucas depot between every 10,000 and 15,000 miles.

OK, there are complete modern electrical systems for old bike but they are far more expensive than getting the magneto rebuilt

If you want recomendation for persons who do rebuilds send me a PM.

One other comment: The person who rebuilt the magneto should tell you "if it fails within a month or so bring it straight back to me". He is not after more business. The HT side of the magneto coil has thousands of turns of very thin copper wire that can be damaged while winding. He has no way of checking the continuity over time so you are the inspector. If he is any good the repair will be free. Obviously Joe Lucas would have just taken another magneto off the shelf.

JA

John Fletcher's tip works and is useful for getting you home. How you do it with a K series magneto I am not sure but it is an easy job with an SR1.

Edited By JA on 08/07/2022 15:38:34

[Mick BaileyParticipant @mickbailey28509]

The bike doesn't have a generator – it has a modern 12v alternator fitted, SS rectifier/regulator and sealed battery Comments about easy starting with a good magneto are all accepted, but my feeling is that given the bike already has modern electrics for the rest of the system that a triggered coil would produce a far better spark at VERY low RPM than a magneto in good order.

[Chris Evans 6Participant @chrisevans6]

Posted by Mick Bailey on 08/07/2022 19:59:23:

The bike doesn't have a generator – it has a modern 12v alternator fitted, SS rectifier/regulator and sealed battery Comments about easy starting with a good magneto are all accepted, but my feeling is that given the bike already has modern electrics for the rest of the system that a triggered coil would produce a far better spark at VERY low RPM than a magneto in good order.

I think you have it sussed re the very low rpm. My single cylinder BSA Sloper fires up with a gentle prod on the kickstart.

[HopperParticipant @hopper]

Posted by Mick Bailey on 08/07/2022 19:59:23:

The bike doesn't have a generator – it has a modern 12v alternator fitted, SS rectifier/regulator and sealed battery Comments about easy starting with a good magneto are all accepted, but my feeling is that given the bike already has modern electrics for the rest of the system that a triggered coil would produce a far better spark at VERY low RPM than a magneto in good order.

If you already have all that, then yes the final step of modern electronic ignition would make sense. Get good old Joe Lucas right out of the picture! The electronic ignition with a properly charged battery should give good spark at very slow kicking speed.

[JAParticipant @ja]

Posted by Hopper on 09/07/2022 09:33:43:

If you already have all that, then yes the final step of modern electronic ignition would make sense. Get good old Joe Lucas right out of the picture! The electronic ignition with a properly charged battery should give good spark at very slow kicking speed.

There was never anything good about Lucas, whatever they sold – bike, car, aero engine.

It is obvious that everyone advises electronic ignition, OK. The bike is a 500cc Velocette. These were never the easiest to start and I have always felt, as an observer, that the difficulties were due to the kickstart gear ratio and a badly positioned spark plug. My 350, not a Velocette, fitted with a good Lucas SR1 magneto and new AMAL carburettor, will start first or second kick, hot or cold, as long as the engine is not flooded.

My other comment, and I am in my mid 70s, is that bike manufacturers would have never thought that anyone over 70 would be riding one of their products.

JA

Edited By JA on 09/07/2022 11:43:32

[HopperParticipant @hopper]

Agreed. My Norton Atlas with maggie starts first or second kick. But it's easy to kick over and being a twin you get two bites at the pie per kick.

I think Lucas, like Amal, could make good quality gear, eg their racing maggies and carbs, but the bike factories demanded they supply maggies at so many Quid each and carbs at so many Quid each and so Lucas and Amal were forced to build down to the price set by the factories.

[Mick BaileyParticipant @mickbailey28509]

Here's an update on the Thorspark; Fitting everything was straightforward and my friend had already located the coil, so it was just a case of replacing the points and ring cam with the pickup unit and wiring everything up. A little consideration needed to be given to routing the wires, as well as providing a much better earth from the battery to frame. We roughly timed it and got a fat spark right away and now the bike starts first kick. Strobing the bike was not entirely successful; I'd marked the engine-speed alternator with TDC and 36 degrees full advance, but could not get a static view of the marks – they were shifting all over the place. I did notice though that the engine pulley serial number strobed up fine, with a stable image. I assume this must be due to slack in the belt drive. We were running out of time so will leave strobing until another day. The bike was running so well with the initial timing that we left it to see how it performs on the road.

The earth brush and main HT pickup were removed from the mag, so the armature is now just a drive shaft that rotates the magnet assembly.

[garyParticipant @gary44937]

we.ll done and good luck. gary

[Chris Evans 6Participant @chrisevans6]

Well done, after years of service my Thorspark equipped BSA still starts first kick.

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