Twin sparking a single cylinder. Plug gaps and caps?

[stinkwheel]

I'm going to twin plug my big enfield bullet to get better combustion and frankly, because the decompressor valve is causing issues so if I'm going to blank it off, I may as well fit a plug in there (the hole for it is threaded identially to the sparkplug hole). It's an 87mm piston so the flame front has a fair distance to travel.

I have a twin coil with the same primary resistance as the original and it's on points ignition.

My understanding is it's effectively the same spark on both plugs, it goes up one and down the other.

So my questions for the panel:

Does this mean I should use supressor caps with half the resistance since I have two in series? (I suspect yes or use one supressed and one unsupressed)

Do I need to reduce the plug gap? (I suspect no, it's either got enough ass to jump the gap or it hasn't)
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Easy-X]

I have the entire parts book for the Thunderbird Twinspark, fancy a read? I'd have to message you a link though as it's 20MB.

Anyhoo, the plug caps are different:

500905/C "Spark Plug Cap M14" x1
580397/A "Spark Plug Cap M10" x1

However, I suspect it's only the angles that differ. Curiously though, it uses two ignition coils with different part numbers:

581027/B "Primary Ignition Coil" x1
147096/A "Secondary Ignition Coil" x1
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Royal Enfield Continental GT 535, Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[stinkwheel]

[jeffyjeff]

[stinkwheel]

[WD Forte]

Initial thoughts / Top of my head shit as I haven't looked into this in detail

If you're using points then two coil primaries will double the LT current
across the points possibly causing early burn out even with a good capacitor/condenser

A lot of 12v scooter coils are physically small and many run for tens of thousand of miles
with no faults so I'd be looking at those for compactness
Resistor plugs or plug caps? I'd experiment with this but probably start with a resistor at first

Power:
If I couldn't fit a chopper (opto) or Hall effect thingy instead of points to the distributor, I reckon I'd try
and use the points as a signal source to drive a power transistor or MOSFET coil driver with direct power.
the low current needed for this would cause far less, if any, arcing

IIRC typical 12V conventional or electronic ignition coils have around 2.5/3 Ohm primaries and say 10K secondaries
CDI coils 0.8ohm primaries 8K secondaries ( typically)

I'd also be looking at driving them in parallel not series
ATM I cant see any benefit in driving them in series
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bikers smell of wee

[stinkwheel]

Back to my standard grade physics. If I run two standard coils in paralell, I'll be doubling the current but halving the resistance right?

Sounds like lower resistance CDI ones would be a bad idea then.

So would using higher resistance coils be a good plan? Standard ones have a 5-ish ohm primary winding.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[WD Forte]

Yep don't use CDI types as they usually run at a much higher primary voltage if they're AC CDI types
2 coils in parallel would use twice the current of a single one
so 2 strong sparks but at double the cost in power.

2 in series would use half the current of a single one due to the increased resistance
what we dont know is the effect on the strength of the electromagnetic field buld up between sparks at half the rated current and its effects on the quality and consistency of sparks when running.

The Enfield is not a high revving motor so it may work for all I know.
A single cylinde motor with no wasted spark only sparks about 8 times/second at 1000 rpm and about 50 times/second at 6000 rpm
At 6000rpm thats an interval of around 20 milliseconds for the EMF to build, so slow in electronic terms which may make it quite feasible.

I does occur to me the lower current of a series system will/should have a smaller current across the points any arcing that happens should be reduced
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bikers smell of wee

[WD Forte]

I wondering about the effect of having two primary coils in series
and the two fields will interact?
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bikers smell of wee

[WD Forte]

I just asked grok


Yes, you can technically connect two 12V ignition coils in series on a single-cylinder points ignition motorcycle engine (assuming this is for a dual-plug setup per cylinder, as that's a common reason for dual coils), but it's not recommended and comes with several negative effects.

### How It Works
- In a series connection, the primaries (low-voltage side) of the two coils are wired sequentially: 12V power to the first coil, output from the first to the input of the second, and then from the second to the points (which ground the circuit).
- Each secondary (high-voltage side) would connect to a spark plug.
- The points still interrupt the current to collapse the magnetic field and induce the spark, but the setup splits the 12V supply across both coils.

### Negative Effects
- **Voltage Division and Weaker Sparks**: Each 12V coil effectively operates at around 6V (half the system voltage), which reduces the magnetic field strength in each coil. This leads to lower secondary voltage output (potentially 20-30% less than a single coil setup), resulting in weaker sparks. Symptoms include misfires, especially at higher RPMs, poor combustion, and reduced engine power.
- **Harder Starting and Idling Issues**: The diminished spark energy makes cold starts more difficult and can cause rough idling or stalling, particularly in varying conditions like wet weather or high altitudes.
- **Increased Resistance and Heat**: Typical 12V coils have 3-4 ohms of primary resistance each; in series, this doubles to 6-8 ohms, halving the current flow. This undercharges the coils, but it can also cause uneven heating, potentially shortening coil lifespan or leading to internal failures over time.
- **Spark Plug Wear or Failure**: The weaker output stresses the plugs, causing faster fouling, wider gaps needed for firing, or one plug failing intermittently (e.g., after a few miles of riding).
- **No Performance Gain**: If you're aiming for hotter sparks or better combustion (e.g., via dual plugs), this setup often underperforms compared to a single dual-output coil or two 6V coils in series (which better match the 12V system without voltage loss).
- **Other Risks**: Without modifications like upgraded condensers or points, the system may draw excessive current during dwell (charging) periods, risking points pitting or arcing. In extreme cases, it could strain the battery or alternator.

### Better Alternatives
- If you must use two coils, connect them in **parallel** instead (primaries wired side-by-side from 12V to points). This keeps full 12V across each but requires two condensers (higher capacity) to handle the doubled current and prevent arcing. Parallel is more reliable for dual plugs but may need points cam adjustments to avoid overcharging.
- For optimal results, use **two 6V coils (1.5-2 ohms each) in series**—this provides the equivalent resistance of a single 12V coil (3-4 ohms) and full voltage utilization for stronger sparks.
- Or switch to a single **dual-output (waste-spark) coil** designed for 12V points systems, which fires both plugs simultaneously without needing multiple coils.
- Always match coil resistance to your system (measure with a multimeter) and consider upgrading to electronic ignition if points are causing issues.

If your engine isn't set up for dual plugs, there's little benefit to adding a second coil—stick with a single high-quality 12V coil. Consult your bike's manual or a mechanic for wiring specifics, as mismatches can damage components.
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bikers smell of wee

[stinkwheel]

Well, an AI coming up with something useful! Amazing. I hadn't considered what I was doing with condensers.

I could hook one to each coil if they're in parallel.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Easy-X]

Thinking of the Old Fella's '70s Triumph, that's a wasted spark twin-cylinder and that uses two 6v ignition coils in series. You could follow their lead with regards to wiring.
____________________
Royal Enfield Continental GT 535, Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[jaffa90]

If all goes well remember the engine will run hotter, also how the hell are you going to kick start it without the decompressor valve?
I`ve worked on stihl stone cutters and cannot pull start them without the decompressor valve.

[stinkwheel]

It'll probably need the ignition retarding a couple of degrees with twin plugs. That will only help make it easier to start.

I've been running it with the decompressor blanked off anyway for the last 6 months. Long story short, it has an upgraded clutch which doesn't slip and I'm a big guy so I can ease it over compression by standing on it. You don't actually kick it until it's just past compression. It's a long-stroke crank so you only get one pop per kick anyway.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[WD Forte]

Having been guitly of overcomplicating stuff in the past, I must also
suggest we do not forget the K.I.S.S. approach.

A simple robust system trumps over a complex one with dubious
or unproven benefits in my experience, so is there a way to make your single coil/plug system better?

For the price I'd be giving this kind of thing a serious look and maybe even trying it

https://www.ebay.co.uk/itm/262890491220?
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bikers smell of wee

[stinkwheel]

At some point, the whole charging/ignition system is getting replaced. There is 10 degrees of backlash on the timing drive gears so it's a sum of unbalanced forces at the moment. Those CDIs are just making a spark, they still rely on the mechanical advance unit. Hitchcocks specifically recommend against the electronic conversions on their 612 motors. They killed two of their race engines with them and went back to points.

The long term goal is to fit a dynamo/small car alternator where the distributor is now for charging and then have a programmable didgital ignition on the crank end. This will either involve me finding a magdyno and refrubing it or somehow fitting a dyno/small car alternator into a lucas magneto housing or fabricating an adapter plate/housing to take one. That'll take several thick coats of looking at.

There are good "race" CDI systems which I can throw a more suitable advance curve on. Specifically I want it way retarded at kick-start revs for easy starting then to bring in the advance a good bit later from idle to bump the midrange. I've done this already to an extent with stronger springs on the auto-advance unit and it made a significant difference. Trouble with them is, you can't fit one of them AND a charging system in the primary case.

There's an off the shelf one from electrex but the charging system is weedy, it's got a fixed advance curve and it glitches a lot. If it advances on or just after startup, it can backfire which does bad things. I'm done bending pushrods and if it strips the oil pump drive, it'll lunch my engine. I strobed my electrex one and I don't know if it was just mine or if they're all like it but the timing was all over the shop at some rev speeds. Probably not an issue on a bike in a standard state of tune but this ones been heavily poked (612cc, long stroke crank, high lift cams, big valves, 9.5:1 compression etc etc.).

I'll want to twin plug it anyway. It makes a good fat spark as is (PAV coil, Bosche condenser), I'm convinced part of my issue is the offset plug is causing swirl issues. Worst case doing this, it makes things worse I disconnect the second coil and have a redundant ignition system sitting there.

I found some fairly neat and tidy, German made pattern ignition coils for suzuki triples which claim a 4.3 ohm primary winding.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Pete.]

I'd just fit a twin coil, two plugs and pull the gaps in tighter. That's what we did with turbo bikes to ensure a good spark when pushing some boost.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[WD Forte]

Your bike time and money so how you spend it is entirely up to you.

Been mulling this over and
if it were mine I'd fashion a pulser/pick up from the crank to avoid backlash issues and hack in a cheap, common,usually robust,
wasted spark electronic or DC CDI system.
Some timing adjustment to get it working optimally would be best imo
But with a well set up timing signal you could run it with single coil/plug and experiment with dual coil plugs later.
My preference would be a simple effective and robust system for what I believe
you have is basically a trials bike?.
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bikers smell of wee

[stinkwheel]

A programmable, crank mounted CDI is the long term aim but this one is a performance tuned road bike and there's not enough room in the primary drive case to fit both a CDI and a charging/lighting system*. I would like to set one up with a very retarded low rev (as in kick start speed) setting and hold a reasonable amount of retardation until further up the rev range than my lightened bob weights and stronger springs do currently for midrange grunt. Then let it go full-bore-roar at higher RPMs.

So to do this I'll need to work out a way of mounting a charging system on the distributor output. Current thinking is either a 1940's magdyno with just the dyno hooked up or to get a lucas magneto housing (which will bolt right on) and fit a small car/plant alternator inside it. It'll take a thick coat of looking-at.

I also have a trials bike which is in a relatively low state of tune and runs happily enough on the points ignition/single plug. In fairness, for the type of trials I intend to do, it also needs lights because the classic long distance trials often set off at 2 or 3am and do the first few sections in the dark.


*Unless you count the electrex world one, which I tried and found to be too erratic, it doesn't use a hall effect pickup, presumably the timing and advance curve comes from the high voltage coil waveform. After a lot of chasing faults I thought were fueling related, I strobed it and found far too much in the way of "wandering" timing. Also only 50W power output.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[WD Forte]

I can only make somewhat vague guesses as I dont know the RE in intimate detail
I think I get the backlash issue as the distributor I assume is driven off the cam shaft ?
I'd still be looking at a GY6 type DC-CDI setup and converting the generator to a 2/3 phase AC alternator similar to the mod I did to a guys 50cc Mobylette which had a piddling amount of power to utilise

The 'retard then advance' issue sounds like a programmable spark unit
would be best
A micro controller could do this but I'm also reminded of the early CX ignition system which used double wound pulsers.

One winding would trigger a spark at low speed/rpm which was somewhat retarded, the as the speed increased, the second winding
would reach the trigger voltage and precede/advance the spark.
In fact those cunning Honda guys had fitted a third set of pulsers in the back of the case to advance it even more !

It was clever for a 70's bike but I'd go for a micro nowadays.
Ignitech might do an off the shelf one you can tinker with settings yourself
the ones they did for the old CX worked well
https://www.ignitech.cz/en/
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bikers smell of wee

[stinkwheel]

Yeah, it's an odd setup. The ignition timing is taken from a distributor which is very indirectly driven off the crank. The original design had a gear driven magdyno. In the mid 1950's they moved the charging system to the end of the crank and used a magneto (the fittings for which are still there). Then they moved to a shaft driven distributor.

So this is the timing side. The bottom pinnion is the end of the crank. That drives the exhaust cam, which drives the inlet cam, which drives two idler gears, which drive the distributor shaft (formerly the magdyno/magneto).
https://lh3.googleusercontent.com/pw/AP1GczN19rSqxqmv4EhwdqYaoZ6R9kUm_tQ1RjaLppg9EF3eIyOLhPIjEK1B6NyE-gq0ph5WM4-1mY0SaNwWn_-NH0gYA92AsLNbHd5B4tuM8nv6F2HJs1pP8UTAT29VjL6UuJH1YKvpNSr4sgqyiC8K9qRv=w1252-h939-s-no

The distributor drives a points cam.
https://lh3.googleusercontent.com/pw/AP1GczNfLYoLv-Sco1Ouo0M-1_Nf4Ars2eJwVWWwXGwAban5w0UrQ95y7KNI1L1Jn3IA5mC2gp7W-GX7BDJ7fLEKu1SgoZHMn9tUB3opSBZVi6ppJ7nRtb57e-LnWtuvf9BjyGorpNHqRL1u_VE1lb-W4qa9=w1252-h939-s-no

Which has a mechanical auto-advance.
https://lh3.googleusercontent.com/pw/AP1GczN5ZOubX_Qeswzj12HC51dHn3lyT_9ixoJ85YysfPr0dnnqVrbuTy8vw5kbI8RAM3BCS2DSgb8srE0hlvga3odrC_X_7jNmB8XbBFS6mXRjY7hkjIHgL6g2euzEVeZ4bwxV-t9f6yzD7wy1mc__LeGg=w1252-h939-s-no

The charging system is a crank mounted lucas alternator (currently a single phase high output one)
https://lh3.googleusercontent.com/pw/AP1GczNeOZkLMDvRWDSJJhelNNRS8hz-QhjgjYTjz-WdbTcEUeJC41CtSOMYmm5fNnO4oTXJOIHVWl--fdQO3YFoEieq-Av9UCM_sHXoQim9tclHuI5QlH9wylbbaHECzy9SgDYvQ8slFTWfnqQV4aGfnN1W=w1252-h939-s-no

As you can imagine, fitting an electronic ignition to the end of that distributor shaft would be like putting lipstick on a pig.

For the moment I'm going for putting a twin plug setup onto the points.

I just found boyer actually do a twinspark setup to replace the points plate. It still uses the mechanical autoadvance and wibbly driveshaft to trigger it though which I don't feel gives me much of an advantage. They use two seperate coils, each with its own pickup and a double trigger so effectively two pickups mounted 180 degrees apart, each making a spark every 180 degrees. Seems complex.

So for the moment, I'll try the standard points firing two coils in paralell, see how it goes.

Longer term, I want to swap things around, so the distributor drive is turning a charging system of some sort. Be that an alternator or a dynamo. There are some very compact kitcar/kart/plant alternators out there so it would be all bracketry, the tricky bit would be getting a taper on the end for that drive sprocket but I know a guy with 15 lathes.

I want to move the ignition to where the alternator is now, on the end of the crank where it should be. That ignitech lot may be worth looking into. I had also seen a system by Rex Caunt. https://rexcauntracing.com/product/progammable-digital-ignition-system/ Effectively needs their system mounting on a backing plate and the crank taper. I would imagine they have done this before because pretty much every british motorcycle from the 60s uses the same lucas alternator. Certainly all the BSA unit singles. Even some of the early CCM ones I think.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[lingeringstin...]

I'm no leccy wizard but it seems to me that just bunging a dual output coil on and leaving everything else the same should work even if it is on a points ignition.

I'm using a twin lead snowmobile coil on a 360 degree twin so both plugs fire at every rotation and it's just a wasted spark alternatively, but then mine is a cdi ignition if that makes any difference, but it's a dead simple one without even an advance curve.

I would have thought the points/condenser part of the ignition system shouldn't be bothered how many outputs the one coil has on it.

[WD Forte]

Jeez !! 'backlash' seems an understatment !!

I'm struggling to think how even the best electronic system would cope with that.
OK You'd get a good spark... but when?

initial thoughts would be summat like this.
Yes its fixed, but a programmable system should be able to retard/delay it for starting, then gradually advance as rpm goes up



https://i.ibb.co/svnPPbv3/idea-1.jpg
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bikers smell of wee

[WD Forte]

incidentally I see the generator has an inboard rotor outboard stator
so rotational speed will be lower than modern bikes so less of a
field build up.
However with stator mods and higher/thinner windings you should be able to mod it a more modern/conventional 3 phase alternator with negative ground if you use led lights and bin yer poofy heated grips and seat warmer
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bikers smell of wee

[stinkwheel]

We run into constraints of having enough "real estate" to fit stuff in the primary drive cover. It was designed with just the primary drive sprocket where that alternator is. There isn't a lot of room between the cover and the primary chain, to the extent the wire sleeve on the stator touches the cover.

The electrex world ones do fit in there which have the external flywheel but it's by necessity very small and so quite weedy (pic below). Everything about it is very tight for clearance, you have to shim it all to get the sweet spot where it fouls neither the primary chain or the cover. I'm all about the GY6 ones because they just work but the rotor wouldn't fit in there, the EW rotor is just over half the size of a GY6 one.

https://lh3.googleusercontent.com/pw/AP1GczPCG3iadqzcbGdgbDYQZkxVqb2IdZCPR45ObM_AbA_k8oN65H18HMwe0bJRIVlazY90_ydSh5a-xCl4uu4_8fCT_6A-DHUBaSrVxtWxmq_NlihoRBxeXMYu9lKm-3H3-C7cXFt7VleWOSGyamX6DuG0=w1252-h939-s-no

Could probably fit a sensor and trigger on the outside of that somehow but even then, it has a pretty weedy power output. The lucas one is fine in that respect, it has a 180W output (in theory).

As I say, my current thinking is to see if I can fit a kubota alternator into a Lucas magneto housing and mount that on the original timing shaft location (it was designed to spin up a magdyno), then have a "proper" race ignition like the ignitech or Rex Caunt on the crank. It's a work in progress. I'm on the proof of concept stage with the twin plug thing just now. I will of course, keep things updated.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.