My on going investigation is showing that I have a voltage drop of around 4.5V at the coil, and 2A, even with a freshly charged battery. Google AI reckons this might prevent starting. Certainly, it fires the first time I kick it, but, if I don't catch it, then it is all over on the second kick, and nothing from then on.
The resistance of the whole loom at the battery, is 3 Ohm and the coil is 2 Ohm, so the numbers stack up. Maybe I need to whittle down that extra Ohm.
And, yes, I did clean the points.
I'm half tempted to do a 12V conversion, even if I don't do a full electronic ignition conversion. These stray fractions of an Ohm would be less of an issue.
i feel we may need to clarify "voltage drop"
your battery (fully charged and connected to the bike - nothing turned on) should show about 6.5volts when the red and black croc clips of your meter on the 20volt scale are connected straight across the battery.
ok - turn the ignition key ON and watch the ammeter kick to the - side , showing you have a discharge - ensure the points are closed will confirm this.
Check what the battery voltage has dropped to at the meter - it will drop as the primary of the HT coil is consuming power - maybe 1 volt ish .
but it is the time that this voltage stays stable that will show if your battery is good or bad health.
if it stays stable for a few minutes (bear in mind the HT coil will start to get warm , so don't leave too long) then you are probably fine
you can try to turn the headlamp on as well and this will make the ammeter kick further into the - side , the battery will drop further and if it stays stable for a little while , then you have a happy battery.
are you saying that your battery drops from 6.5v to 4.5 volts and stabilises there with stuff turned on ???
I spent years as an AA patrol sorting out dead cars with ignition and charging faults and my best friend was my £2.50 HT neon tester that went in between the HT cap and the top of the plug - showing me (without hesitation if the car's issue was loss of spark or something else.
trouble was , I got the car running and waved them off into the sunset leaving my Neon tester on their car - must be hundreds out there!!
or are you saying that the voltage drops to 2 volts showing a volt drop of 4.5v ?? if this was the case, then the battery is not healthy at all (but was it fully charged before you did the test??
you must never do charging checks or battery check if it has not been on charge for a good while
You may have something shorting to earth. Disconnect everything except the coil and wire it direct then see what happens. It should run fine without charging for a good 30 mins.
Its also possible the points are shorting under the little plastic ferrule but this usually causes no spark.
Where were you based as a Patrolman?
Central south west London, Surrey and West country
I was an AA patrol for a year in the early 70s (based in Birmingham) but left when they brought Relay in as I didn't want to be a glorified lorry driver. The crunch was when I was called 10 minuted before the end of my backshift when they wanted me to take a broken down car to Leicester, which I politely refused.
Barnsey states that the voltage at the coil drops to 4.5V. He didn't say that the battery voltage drops to 4.5V. It's possible to have a high resistance somewhere between the battery and the coil causing the volt drop, like the ignition switch. He should check the resistance of the loom from the coil back to the battery.
I was based in Basingstoke
And North Hampshire. 26 years with the company last 3 years on the Technical Help Desk before taking early retirement.
If he's measuring voltage at the coil, he'll get a low reading if the points are closed......
with the points OPEN, the voltage at the coil should be virtually the same as that voltage across the battery.
the best/only scale to use to discover "volts drop" is the 20V voltage scale as this is when the circuit under test is actually working
end to end with the 2 clips - 1 on battery + and the other to the points wire or (if points are held open) the actual point face.
any voltage drop will show on the meter - any more than 0.5volts is excessive , preferably 0 volts
I powered the bike up tonight, from a bench top PSU, hense no voltage sag at the battery. The PSU is giving 6.4V and 2.01 A. The voltage drop across the coil, with the points closed is currently 3.7V. I would expect it to be more like 5V. With the points open I have the full 6.4V on the coil.
I am losing almost nothing in the points (I did clean them), but, weirdly, I am losing about 0.6V where the distributor mates to the engine block, so, maybe I need to take the distributor off and give the mating surfaces a good clean. However, this would only get the coil drop up to around 4.2V, so it isn't the whole picture.
there are always 2 sides to a voltage drop - the live side and the earth side
from the look of your reading with points open and points closed , you're pretty much ok - 6.4 going down to 3.7 v points closed - i would prefer the 3.4 to be about 4.9 v ish really.
the 0.6v concerns me - try this test--
meter on 20v scale
any colour clip on the earth terminal of the battery ( are you POS earth if so red clip for ease)
and the black clip on the cylinder head fin or cylinder head nut - in other words on the engine - not the frame
good and tight
everything off no ignition or anything
now turn your ignition ON with points OPEN - what reading do you get on the meter ?
now points CLOSED , what reading do you get on the meter ? (I am assuming 0.6v)??
let us know
Okay, I've finished for tonight, so I'll take a look tomorrow after work.
The 0.6V would indicate a resistance on the earth side of the contacts. With the points closed I would expect the voltage across the coil to be the same as the battery volts assuming no resistance in either the supply side from the battery or the earthing of the distributor. Obviously with the load from the coil the battery volts will have dropped a bit but I'd expect it to be at least 5V depending on the state of the battery.
The other 0.7V - 0.8V drop can only be due to a resistance in the supply side. As I said previously, check the resistance between the coil and the battery - with the battery disconnected. Also with the battery connected and the ignition on check the voltage measured between the battery terminal and the coil with the points closed and with them open. This will give you an idea of any losses in the supply side.
I took some measurements after work tonight. I have the full 6.5V on the engine block, but I am losing 1.5V between the engine and the coil positive. Most of this appears to be between the engine and the distributor.
If I measure between the engine and the coil positive I am seeing the 5V which I want. I guess that this also implies that I am losing 1.5V on the negative side.
It seems to me that there are three possible solutions.
1/ Clean and reseat the dizzy, and go around the wiring remaking all connections. This might become a full loom replacement, and switch replacement, as on my other bike.
2/ DO a 12V conversion, as a 12 V coil has a higher internal resistance, and all these other traces of resistance will have less effect.
3/ Go the whole hog, and do a full electronic conversion with 12V to boot. This should knock it into a cocked hat as there will be no current through the dizzy, all current going via the magic black box.
Since advice is in that direction, I will persist with 1 for the moment, but I am strongly tempted to skip 2 and go straight to 3.
Simple test
Clip a wire to the engine head nut or rocker box.
Then then other end of this wire hold onto or clip into the distributor body with a Croc clip
Does this fix/reduce your volts loss ?
Yes, no surprises. I added a wire from a rocker cover nut, to the spring clip which holds the distributor cover on. 3.5V at the coil became 4V, and 2A became 2.25A. SO, not quite a smoking gun, but definitely a useful improvement.
At the weekend, I'll take the Distributor off and give it a clean, maybe I'll add some copper grease when I re-seat it.
To remove the distributor, is it a simple case of undoing the adjuster screw and pulling it out?
Put a mark across the distributor/crankcase joint so you put it back in the same position.
Just install a dedicated earth wire direct from the battery positive terminal direct to the rocker cover nut.
No need to remove the distributor.
It is held in with its clamp(albeit hidden in the timing cover) and this clamp screw is screwed into the engine so the earth path is there.
So did it start with your earth lead installed ?
Quote from: Barnsey on 22 Sep, 2026, 17:09 1/ Clean and reseat the dizzy, and go around the wiring remaking all connections. This might become a full loom replacement, and switch replacement, as on my other bike.
Separate every bullet connection between the battery and the coils and clean the surfaces and refit one at a time. Also check the resistance across the ignition switch when it is switched on. I used to have a Ducati 907ie and every time I used the indicators the rest of the lights would go bright/dim/bright/dim in time with the indicators. It was dropping 1V across the switch but this dropped further with the extra 3.5A load of the indicators affecting the lights.
The voltage drop across the ignition switch was also affecting the ignition and injection circuits. I fixed it properly by replacing the switch with a 30A relay and modifying the wiring so that all the ignition switch did was turn the relay on and off. A bit more drastic than what you need I think.
Quote from: Barnsey on 22 Sep, 2026, 17:09 I took some measurements after work tonight. I have the full 6.5V on the engine block, but I am losing 1.5V between the engine and the coil positive. Most of this appears to be between the engine and the distributor.
If I measure between the engine and the coil positive I am seeing the 5V which I want. I guess that this also implies that I am losing 1.5V on the negative side.
It seems to me that there are three possible solutions.
1/ Clean and reseat the dizzy, and go around the wiring remaking all connections. This might become a full loom replacement, and switch replacement, as on my other bike.
2/ DO a 12V conversion, as a 12 V coil has a higher internal resistance, and all these other traces of resistance will have less effect.
3/ Go the whole hog, and do a full electronic conversion with 12V to boot. This should knock it into a cocked hat as there will be no current through the dizzy, all current going via the magic black box.
Since advice is in that direction, I will persist with 1 for the moment, but I am strongly tempted to skip 2 and go straight to 3.
if you are going to check connections in the circuit The batt pos terminal is first, then both ammeter terminals, cable then goes to terminal 12A on the ignition switch, probably brown-white and comes out of terminal 13 probably white,to the pos side of coil. If you monitor at these points with ign on and contacts closed you willbe able see if the voltage drops along the route to the coil and hopefully pin it down to a dirty connector or high resistance in the ignition sw
Not tried starting it yet. Time was short. Resistance from the battery to the engine was negligible, the resistance being between the engine and the distributor, so I think a ground from battery positive to engine might not help in this case, but no harm in trying. I'll go around and check the connections before I put the tank back on and try firing it up. It's a pity that there is nothing on the distributor that I can permanently connect a ground wire to.
It's worth bearing in mind that C15's have been running happily for close on 70 years without the "dizzy" needing a dedicated earth.
That said, it's always worth having an earth wire running to somewhere on the engine, moreso if the frame has been powder-coated or painted recently.
Quote from: Barnsey on 22 Sep, 2026, 20:16 Not tried starting it yet. Time was short. Resistance from the battery to the engine was negligible, the resistance being between the engine and the distributor, so I think a ground from battery positive to engine might not help in this case, but no harm in trying. I'll go around and check the connections before I put the tank back on and try firing it up. It's a pity that there is nothing on the distributor that I can permanently connect a ground wire to.
Hope you dont mind suggestions, just trying to get a good picture of your problem, looking at your last post but one, you get 6.5v presumably between the batt Neg and the engine casing which is expected and you get 5v between the coil POS and the engine casing. If the points are closed at this point you should be seeing very little voltage at the coil +ve as should be grounded through the points/distributor/engine/frame. Have I understood you correctly in that the distributor is not pulling the coil voltage down to near ground in which case there is resistance between the coil +ve and the engine casing? If thats the case, my first thought would be loose or rusted connection where the points spring terminal is fixed to the distributor connection screw or a corroded terminal on the wire where it connects to the distributor.
With the meter on volts, connected between coil +ve and the engine casing, with points closed (showing the 5v), try putting a piece of wire between the distributor terminal and the engine case, if voltage drops the problem is in the distributor as you suspect. then put the wire between the distributor body and the engine, if volt drops then that proves your diagnosis that the distributor is not making good contact with the engine. If the problem is between distrib body and engine you could do a short term fix and attach a lead to the cap fixing screw and run it back to the earth point under the seat.
If I have misunderstood and you see 5v on the coil +ve with the points open that suggests a different problem because with points open there should be no current draw on the coil so you would see the full 6.5 volts on on both terminals. if this is the case it suggests earth leakage through the dizzy. If you take the connection wire off the dizzy and voltage goes up to 6.5 then maybe failing capacitor leaking to earth or conductive gunk bridging the points.
Thanks Shoal, Yes that ties up with what I have been doing, and I definitely have a drop across the distributor. Unfortunately this one has the cap held on with a clip. not a screw, so nothing to attach a wire to.
I tried slacking off the adjuster screw and rotating the distributor, to see if it would improve the connection, but no dice. It might be a case of having to remove it.
I will be off work tomorrow, so expect an update soon.
An update. I ran a grounding wire from the supply terminal on the frame, to the distributor and clamped it on with a jubilee clip.
Actually I was quite proud of this, it was a smart bodge as bodges go. I cleaned the body of the distributor first with emery, and soldered a piece of copper, from a central heating pipe onto the ground wire, which is clamped between the jubille clip and the body of the distributor.
Anyhow, I have got the current from 2A up to 2.5A. In theory, 6V into the 2 Ohm coil would be 3A, so I suspect that 2.5A might be as good as I can hope for.
I am busy with family stuff today, so I'll put the bike aback together tomorrow, inc refitting the tank, and give it a kick over.
An update for this week.
Fitting a ground wire directly onto the distributor unit, made all the difference.
The bike started second kick. Now that I have adjusted the carburettor it is starting FIRST kick!
I have also fitted the new 6V regulator.
Thanks to everyone for your help and support!
Good result.
I always fit a dedicated earth wire to the crankcase (although have never found it necessary to do so with the distributor itself). Expecting electrons to flow through painted / corroded surfaces and bolts is a bit optimistic in my view. I also run earth wires to rear light assembly and headlight for the same reason.
Given that the dizzy is metallic and has a large clean surface area, and the engine unit the same, the resistance should be so low as to be irrelevant.
if you have a measurable drop in resistance with this dedicated earth wire it points to a poor engine/frame electrical path rather than a poor dizzy-engine unit.
Simultaneous posting.
you are looking for 7.2 ish volts across the battery when running , to prove the RR is working fine.
but you must do this with a charged battery