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)
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“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.
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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

[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.
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a.k.a 'Geri'

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