Wont Start When Hot (Magneto?)

[Joker_Bones]

1955 Matchless G9 - K2F magneto, manual adv/ret

New spark plugs, HT lead and pick-up connections cleaned, suppressor caps that were fitted removed, points cleaned and re-set, ignition timing re-set. Carb stripped and cleaned, new fuel.

While I was working on it, it would start from cold usually 3rd or 4th kick. As it is new to me, I have not really figured out the best technique but I get the impression that its more likely to start with the ignition in the fully advanced position rather than retarded any amount.

First time I rode it, after about 5 or 6 miles, was ticking over at a junction and it just stopped. It hadn't misfired or spluttered, it just cut out as if it had been switched off.

Tried to re-start it but without success. Checked for a spark, couldn't see or hear one. Checked again after about twenty minutes, spark was back, appeared weak and intermittent but the bike started. Rode home but the bike was running slightly hesitantly and gave the impression it would stall on tickover unless it was kept on the throttle. Pulled up outside and let it idle, after about ten seconds it just cut out, and would not re-start. Came back after an hour or so, started and ran on idle OK.

Since then I have taken it out a couple times, rode 4 or 5 miles round town (keeping pretty close to home ). It ran fine and did not cut out, but each time I could not re-start it immediately after I got back. After an hour or so.... started OK.

I was thinking 'there is a problem' but also wondering if I was not getting the technique right when trying to start it hot.

So, read some posts, articles and manuals and went to the Brightsparks website.... Most suggested likely cause (as Alf said) a failing old condenser. Thought.... If I can fit an Easycap condenser that would be a good place to start.

Took off the magneto and took out the armature. From what I could see it looked like an old Lucas condenser, not potted in resin, and the LT connections to it were accessible, LT resistances were the right numbers, looked promising.

Then I checked the resistance of the HT circuit from the slip ring to armature earth, looking for the 4-7k ohms suggested on Brightsparks.... Open circuit! Doh!

How can I have an open HT circuit, the bike was running? Remembered one article and went back and re-read it.... It said you can test open on the HT circuit but the HT current can jump a small break in the circuit when cold but not necessarily when hot. (wondered why when cold but not when hot? increased resistance?)

My conclusion.... I suppose not much point in replacing the condenser alone if I have an open HT circuit.

Options.... An armature rewind together with new condenser or electronic ignition, Thorspark perhaps?

Do you agree? or am I barking up the wrong tree? Any other suggestions?

Ta

Russell

[SPRIDDLER]

I get the impression that its more likely to start with the ignition in the fully advanced position rather than retarded any amount.

A mag will produce its strongest spark at a full advance setting as the points are opening at the mag's point of maximum flux. Therefore a failing mag (dodgy condenser, break in HT windings) when ignition timing is retarded just won't produce enough oomph (techy term) to get the plug sparkling.
You have classic symptoms of failing condenser, coupled with the likeliehood of a break in the HT windings as you've already worked out.
Don't abandon the mag for an electronic ignition set-up (IMHO dynamos are nowhere near as reliable as magnetos) but have the mag sorted and it'll see you out......

[robcurrie]

I fitted a Thorspark in December after discovering that my mag's slipring was at the end of its life. I didn't have the time or inclination to remove the mag, send it in for rebuild etc. I had to sacrify the earth brush in the bakelite cap, but could always return the mag to working condition if I had masochistic tendancies. I did a 300 mile run with other classic bikes and on the return trip I was averaging 80 mph to keep up with some modern classic Japs.

My CSR has an alternator, but the Thorspark will probably be able to run all day on a fully charged battery so a dodgy dynamo shouldn't be a problem for daytime riding. Ignition was easily set with a strobe timing light and will not need to be constantly checked with fag paper and compromised between the two cylinders etc. The Thorspark runs on one coil so both cylinders timing is exactly the same.

Rob C

[Janet]

I did a 300 mile run with other classic bikes and on the return trip I was averaging 80 mph to keep up with some modern classic Japs.

I'm impressed. What kind of road were you on to average 80mph? Over here, unless you live at the edge of a motorway, that would be nigh on impossible, even allowing for ignoring all speed limits, because of having to get out of town and then down roads A roads that go through villages and have road junctions requiring slowing down or even stopping, plus the added obstructions of other traffic. All these mean that maximum speed has to be very high to give an average like that. Many owners would be delighted with it as a top speed.

[Groily]

Hi Russell, It is just possible that the HT open line is caused by a poor connection of the spike on the end of the coil into the slipring, so you shouldn't condemn the coil until you can test it with the slipring off the mag. Which is a pain, owing to the need to pull the inner race of the bearing that's in the way. However, you'd have to pull it to get the armature apart if you were going to dig out the old condenser, which will almost certainly be leaky. Your symptoms are classic for a problem with coil continuity and/or condenser as says Spriddler.
Not going to argue about Thorsparks etc because there's no doubt they're really excellent from what Rob and lots of others say. But all the same, with a dynamo I'd prefer a mag, every time. I do have Boyer Bransden systems on a couple of alternator bikes (which could run magnetos), and that's fine: dead spark/spot on timing and a programmed advance and retard curve. All very nice. But a mag in decent shape will last for ages, as Spriddler also says, and needn't cost a fortune to fix. That's what I'd do, but I'm biased. (Glad you found the info on our website at least partly helpful!)
Bill

[bunners]

Russell (Joker Bones) I see you live in Dorset, anywhere near the Isle of Purbeck? or better still Langton Matravers? there is a guy there you does a magneto service, Moathouse Magneto's, not the cheapest but will check do a free report, estimate and repair to your requirements. he also does an armature exchange.

His name is Dave Fisher. I have his address if you want but here is the phone number for starters.
01929421255

[SPRIDDLER]

Ignition was easily set with a strobe timing light and will not need to be constantly checked with fag paper

Aren't we all different in our attitude to our old bikes!
I derive much satisfaction from getting all elements of 'old-fashioned' engineering working well and the bike set up right and to be reliable in its original form. There's something mysterious, alchemy almost, in getting a spark out of an assembly of whirring metal components; a magical process akin to achieving perpetual motion.
Strobe lights? Nah! Fiddling about with fag paper and c/b points and adjusting sprockets on shafts is an opportunity to bond with the ol' girl.
My new Honda has fuel injection, electronic ignition, disc brakes, leccy start, rev counter, 'neutral' indicator light, fuel guage..........nothing to fiddle with, and it's boring; just draughty transport.
But then I'm retired.

[robcurrie]

What kind of road were you on to average 80mph

We were a group of 100 classic bikes about 50% older than 50 ( bikes and riders). Our outbound trip was a leisurely all day ride taking in breakfast, lunch and afternoon 'tea', visiting bike museums and a racetrack along the way. A few of us couldn't stay over that night and took the more direct road home. The guys on the Jap classics went to Johannesburg and I had to go 50 miles further to Pretoria. The country is mostly wide open spaces (farmland) and the distance between intersecting roads is sometimes 30 or 40 miles. We had one stop in a small town and joined a highway from there with the next stop less than a mile from my house. Most of the time my speed was around 85mph touching 90mph occasionally.
In a different thread you talked about recovery services, here you have to provide your own, and we had a couple of trailers following the group. The return trip was sans backup.
Here is a link to the overnight venue: http://www.lakeavenueinn.co.za/

Rob C

[Joker_Bones]

TY for your replies, gives me confidence I on the right track.

Spriddler.... I hear what you are saying.... It does seem magical , keeps things 'original' and is independent of the dynamo / battery.

Bill.... I hoped the lack of continuity in the HT circuit would be at the connection to the slip ring (maybe could have done something about it). I didn't pull the bearing / slip ring.... I shaved the insulation away with a Stanley blade from the back of the HT spike (where the HT spike enters the slip ring) until the back of the spike was just exposed. Tested for continuity from there to the slip ring, which was OK, from there back to armature earth which was 'open'. So I figured the discontinuity was in the winding. Which was disappointing, If the HT winding was OK, I was hoping to fit an Easycap condenser which doesn't (if you can get to the LT connections) involve dismantling the armature, you can bypass the old condenser and leave it in place. For the £13 one costs I am still tempted to fit one, just to see if it is the HT winding or a failing condenser that is the main cause of the problem, but ultimately I assume the windings will have to be replaced.

Bunners.... I am in Dorchester, thanks for the info, I like the idea of an armature exchange and Worth Matavers is about 25m away so within easy travelling distance.

I saw this exchange armature service being offered on ebay for £110
http://www.ebay.co.uk/itm/Lucas-K1F-K2F ... 33898317b9


Rob.... I have not dismissed the idea of electronic ignition.... I see on the Thorspark website and you say 'Ignition is timed with a strobelight' How is that achieved? There are no external timing marks that I am aware of, is it done on the unit fitted in the magneto points housing or do you have to run the engine with say the chaincase cover off?

I fished out my old gunson stobe light to check it was still working.... It wasn't, it gives a few weak flashes and then stops.... failing capacitor?

Russell

[robcurrie]

Russell, I removed the primary cover and made a pointer and marks (they were redrawn in white later for better visibility).



Here is the installation in the mag