74 SF2 - Ignition help needed

Paul Marx said:
If there's feed to coils and points it's obviously not ignition or kill switches.

Any idea of the history of the bike. Was it running when it was laid up or did somebody try to repair it?

Points gap is 0.35 to 0.45mm.

Did you or not get a spark from the bare HT lead to earth?

Paul
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Paul - thank you for chiming in. I have no history with the bike as it was flipped to me from an estate by a third party back in Oct 2020. I have some original entry info but that was 1974. No invoices (or one or two) of parts purchases and no service info or memorandum. I have a service manual that I got with the bike and, believe or not, the owner's manual. While the bike is mostly complete, it's easy to see that it sat for quite sometime. The engine turns over well and makes the right noises and the batt is new. I think the owner was attempting to make the bike into some sort of SFC since it has clubman bars, a cafe seat and 2-1 exhaust. The rest of it is complete and stock and it does not appear to have been screwed around with much. The brakes need a rebuild and I redid the Dellortos in prep for the start up. I should have checked for spark first!! I am a somewhat experienced vintage bike guy, but this thing has me going in circles!

Yes our points gaps are at the high end of that range; I did not do the HT lead test that you mentioned. Yes, re your feed comment, I agree.
 
Ernesto said:
Do the 12V at least arrive at the points?
If not I guess the coils are shot. At least the primary circuit is interrupted.

All the little insulating parts there? Washer, enough rectangular isolator plates?
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I have to double check that the points are insulated properly - have the diagrams and will do. Will double check 12V at the points but I think we checked that and was OK.
 
Thank you everyone for the comments.

I think my "to do" list is as follows:

Check to see that the point have all the insulating parts and are not grounded
Double check for power at the points
Check points base for having 12V - it should not
Get some new HT leads and proper caps
Get new condensers
Make sure coils (through body) and engine are grounded properly

Do I have it all and am I correct?
 
Ernesto said:
The points are the earth.

Ernesto
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Yes Ernesto, they're the earth for the coil primary 12V circuit. But I was talking about the secondary high voltage side of the coil. The lead is connected to the spark plug but to complete the spark circuit the coil has to be earthed to the engine, via the frame. A bad connection somewhere in that circuit will kill the spark.
 
That's exactly what happened to me heading home on my own from a Snowy Run on my 3c. I stooped at a petrol station for lunch and fuel and the bloody bike was dead when I tried leaving. The station owner gave me a push but NADA. So I pulled the luggage off and started looking. No spark at all, I started wiggling wires and still nothing, as a last resort I tightened the coil mount and suddenly had spark, packed up and it started and ran well the next 300ks to home. My best roadside fix, pure arse of course.
 
Vince said:
That's exactly what happened to me heading home on my own from a Snowy Run on my 3c. I stooped at a petrol station for lunch and fuel and the bloody bike was dead when I tried leaving. The station owner gave me a push but NADA. So I pulled the luggage off and started looking. No spark at all, I started wiggling wires and still nothing, as a last resort I tightened the coil mount and suddenly had spark, packed up and it started and ran well the next 300ks to home. My best roadside fix, pure arse of course.
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Vince - thank you, will be all over this during the upcoming weekend
 
The Dynas I'm fitting have a plastic body (as do most modern coils on motos) and I can't see any way they'd be earthing, Cam. My original front Bosch SF coil was held with a clamp to the painted frame and had a rubber pad between it ands frame as well. No way the coil body was earthing.

+ for power to, - for igniter/points from, HT to spark plug, which earths to motor. I don't get what you're saying?
 
I told John Wilson about losing spark and getting it back by tightening the coil mount and he didn't get it either. I was happy to get home though.
 
Coils earth via the points and plugs.
Maybe if you squeeze them hard enough they yell of maybe moving them a bit cures a loose connection of which there are two. 1+ feed and 1 earth to points.

Paul
 
piranha-bro2 said:
The Dynas I'm fitting have a plastic body (as do most modern coils on motos) and I can't see any way they'd be earthing, Cam. My original front Bosch SF coil was held with a clamp to the painted frame and had a rubber pad between it ands frame as well. No way the coil body was earthing.

+ for power to, - for igniter/points from, HT to spark plug, which earths to motor. I don't get what you're saying?
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Depends on the internal wiring of the coil. Some coils need to be earthed to the chassis, some don't.

Some coils (like your Dynas) have the secondary HV earth connected to the negative terminal of the primary circuit (that goes to the points). But the coil doesn't earth the HV spark current through the points. Well, not directly anyway, as a moment's thought will tell you: The coil actually fires as the points are opened, so there's no earth path via the points just when you need it. However, being a transient pulse (as distinct from DC) the current needed to make the spark can flow into the condenser (capacitor) that's wired in parallel with the points, charging it up. Spark plugs typically only draw a very small current so the capacitor doesn't need to be a monster thing to hold a gazillion Coulombs. The capacitor will discharge when the points close again, so the capacitor provides a kind of delayed earth via the points.

These types of coils that earth via the negative primary terminal won't work with a dud condenser because there's nowhere for the spark current to flow.

A coil that has its secondary winding earthed via the frame may still spark without a condenser because the HV earth path is always connected. But if the condenser is stuffed, you'll get arcing at the points as they open, so it's not a clean current break. The magnetic field in the coil won't collapse as quickly as it should, making a weak and erratic spark.
 
Dellortoman said:
Depends on the internal wiring of the coil. Some coils need to be earthed to the chassis, some don't.

Some coils (like your Dynas) have the secondary HV earth connected to the negative terminal of the primary circuit (that goes to the points). But the coil doesn't earth the HV spark current through the points. Well, not directly anyway, as a moment's thought will tell you: The coil actually fires as the points are opened, so there's no earth path via the points just when you need it. However, being a transient pulse (as distinct from DC) the current needed to make the spark can flow into the condenser (capacitor) that's wired in parallel with the points, charging it up. Spark plugs typically only draw a very small current so the capacitor doesn't need to be a monster thing to hold a gazillion Coulombs. The capacitor will discharge when the points close again, so the capacitor provides a kind of delayed earth via the points.

These types of coils that earth via the negative primary terminal won't work with a dud condenser because there's nowhere for the spark current to flow.

A coil that has its secondary winding earthed via the frame may still spark without a condenser because the HV earth path is always connected. But if the condenser is stuffed, you'll get arcing at the points as they open, so it's not a clean current break. The magnetic field in the coil won't collapse as quickly as it should, making a weak and erratic spark.
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Doesn't the capacitor (condenser) discharge when the points open? Charges up while the points are closed (dwell) so that the points don't burn?
Interesting stuff this, despite points being old history for most people and I doubt they are even in the tech school text book anymore, it is good to learn more stuff.
 
I have been doing this stuff a long time and I still don't completely understand it. I think this bike is going be my final teacher in this area.

Thanks everybody for the commentary, article links and info. It has been a giant help. Will be getting back on this thing this weekend and will advise.
 
Tippie said:
Doesn't the capacitor (condenser) discharge when the points open? Charges up while the points are closed (dwell) so that the points don't burn?
Interesting stuff this, despite points being old history for most people and I doubt they are even in the tech school text book anymore, it is good to learn more stuff.
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I can't claim to be an ignition expert either. I'm just going by electrical theory I learned while studying engineering. A capacitor doesn't actually pass current as such. It just charges up and discharges, but as the voltage increases on one plate, it will induce a voltage in the other plate. In a steady state with DC voltage applied to one side, nothing will happen on the other side. It's only when the voltage is changing that something happens to the other side of a capacitor. When AC is applied to one side of a capacitor, the fluctuating magnetic field will induce AC on the other side. You can think of a capacitor as a DC insulator and an AC conductor. I dunno whether that helps when trying to understand ignition systems though.

As I understand it (and ignition experts might prove me wrong) when the points are closed, the condenser will charge up to 12V in the time it takes for the coil to saturate (dwell time), then no further current will flow into it. When the points open, the voltage pulse from the coil is much higher than the 12V that the condenser is already holding onto. So it'll charge up a whole lot more to around 40,000V as it accepts the pulse of coil current. Note that a transient pulse from a coil isn't steady state DC, it's milliamps for milliseconds, so the condenser will happily accept it. When the points close again, the capacitor will discharge that high voltage to earth, and it'll start to charge up to 12V again via the coil's primary circuit.

Sorry, I'm using the terms capacitor and condenser interchangeably for the same thing. Buggered if I know why they have two names. But they are usually called condensers in ignition systems.

Something that Ernesto mentioned to me in a PM is quite relevant to coil earthing. A sealed coil that has no provision to earth the arse end of the primary circuit (by that I mean the other side of the HV circuit to which the spark plug is connected) shouldn't be earthed via the common "earth" pin that goes to the points. It'll simply earth out the points and kill the ignition process. You could wire a kill switch that way if you wanted to.

Anyway, the short version of this long story is to identify what type of coil you're using and only earth the secondary side if it needs to be earthed. As a general rule, coils that have the metal core sticking out of each end are the type that need to be bolted to the frame for their secondary earth connection. Sealed plastic coils that just have the two primary circuit connections and a plug lead don't need to be earthed via their mountings.
 
OK, I am starting to get it. But the monkey in my wrench is that both cylinders are dead. If I had a bad condenser, would it not be unlikely that both would be bad at the same time? OK maybe, but down on the probability scale methinks. Wolfgang our parts guy tells me that the Bosch condensers last forever. Nevertheless, yrs ago, we had supposedly bad condensers on an old Triumph that was doing what I have. Both cyls dead. We replaced them with new to no avail. We were on a trip so we flew in new parts and, as a last resort, installed the new new condensers and the bike ran. So, Ok, the first new parts were bad but it would not make a spark w/o good condensers.

So, what is the common link between the two cylinders? The points and their common mounting plate I would think. If the points are grounded to the mount then the game is over and I feel that this is the likely scenario. I think @Vince mentioned this and I agree with him that there are only so many parts in this system, but there are a lot of little parts - insulators - that need to be perfect for the points to work.
 
To put it short, the tasks of the condensers are
1.) to reduce the voltage at the points and so reduce wear
2.) to increase voltage at the plugs

The only symptons I heard of shot condensers are that the bike won't run well,
backfiring etc. and you can see big sparks at the points.

Another 2 cents
Ernesto
 
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