Dual ignition - double spark plug on mopeds?

Some East European moped enthusiasts advice to use/modify the hole of decompressor as a holder for second spark plug which will lead to a better combustion and cleaner exhaust gas.

There is always an arguement what is better - to connect both spark plugs parallelly or one after one.

How do you like this idea?

Re: Dual ignition - double spark plug on mopeds?

Jimmy Cincinnati /

I feel like the Japanese tried doing this in the 70's with little gain. You will see it on some older bikes but it disappeared after a few years. I would suggest plugging the decomp hole over putting a plug in it.

Re: Dual ignition - double spark plug on mopeds?

Dirty30 Dillon /

The second plug hole on Japanese bikes was not to double the spark, it was to allow you to tool-lessly swap to a fresh plug should yours foul. Old Suzuki TS's had this.

Having twice the spark gives no benefit that you will notice. Especially if they are sparking at the same time. All the second plug hole will do is decrease compression and require a non-symmetrical chamber design.

Re: Dual ignition - double spark plug on mopeds?

If my bike had a Cessna engine maybe....

IDouble the plugs is not always double the pleasure. At least for our lowly mopeds.

Re: Dual ignition - double spark plug on mopeds?

> Dirty30 Dillon Wrote:

> -------------------------------------------------------

> The second plug hole on Japanese bikes was not to double the spark, it

> was to allow you to tool-lessly swap to a fresh plug should yours foul.

> Old Suzuki TS's had this.

>

> Having twice the spark gives no benefit that you will notice. Especially

> if they are sparking at the same time. All the second plug hole will do

> is decrease compression and require a non-symmetrical chamber design.

I never understood how if a plug that has been sparking and running fouls out, that a plug just chilling there in the c.c. doing nothing is gonna work great. I know on bultaco, montessa and other manufacturers that made early enduro/trials bikes the decomp was typically used to generate engine braking effect on a 2t. I thought the second plug was so you could easily add or remove said feature.

Re: Dual ignition - double spark plug on mopeds?

Dual ignition is OEM on my 1957 Simplex :

It has been tested and proven that no real advantage is to be had with two plugs firing at the same time as opposed to a single plug .

I can only guess that there 'may' be a bit less carbon build up , but ???

Re: Dual ignition - double spark plug on mopeds?

All it does is making fouling plugs twice as expensive

Re: Dual ignition - double spark plug on mopeds?

Chöschi21 C.K. /

I know for a fact that it doesn't gain you anything, at least in the way we tried it, the only way i can think this might be helpful is if one Plug goes bad, but thats it, also, you would have to fire on both plugs, otherwise you will just foul the plug that isn't insanely fast.

Re: Dual ignition - double spark plug on mopeds?

Alright, so if the spark plugs will not fire simultanously, could there be any minor advantage though?

Re: Dual ignition - double spark plug on mopeds?

The advantage of a hole in yoir piston.

Re: Dual ignition - double spark plug on mopeds?

> Max Ginsburg Wrote:

> -------------------------------------------------------

> Alright, so if the spark plugs will not fire simultanously, could there

> be any minor advantage though?

The plugs in my Simplex fire simultaneously . Even if there were a difference , it would be something close to or less than a millisecond . Pretty much undetectable and without negative affect . (edited)

Re: Dual ignition - double spark plug on mopeds?

Chöschi21 C.K. /

Firing offset would probably not gain you anything, most Engines that use a Dual ignition use them either for better Emissions and a Cleaner Burn, or for more Power in the same way, this works because of the Big combustion Chambers, in wich you sometimes can't burn everything inside, that won't really work on our Engines witch most of the time have combustion chambers that are Hemispherical and really small.

Or they use them for reliability, for instance the Old Porsche 24h Racing engines had them and Planes use them so that if one System dies the Engine still works mostly fine. this would probably be the only reason to use them on a Moped, except for maybe coolness factor. (edited)

Re: Dual ignition - double spark plug on mopeds?

kevin Smellaflange /

actually the dual plug heads on a BULTACO ASTRO / PURSANG are stock. gavE A HALF HP MORE & CLEANER BURNING...but on a pencil thin piston it would need to be 3 to 4 heat ranges colder...the plugs that are closest to the exhaust port needs to be colder, but side by side, iv'e never seen before.

Re: Dual ignition - double spark plug on mopeds?

a well designed squish band negates the need for dual plugs

they used dual plugs on the milwaukee 8 engine because it was a large bore pent roof design, it had two dead zones and meeting emissions goals required a really homogenous burn so in order to get that large awkwardly shaped chamber to light off really well, certain operating conditions needed both plugs to fire. it was more for emissions than anything else.

if you have a really crappy designed head and there is nothing you can do about it- like a 'twingle' it can be a good option, but moped engines its much easier to just design a good squish and combustion chamber and use 1 plug

Re: Dual ignition - double spark plug on mopeds?

kevin Smellaflange /

absolutely...i had an old sears all-state mc that used 1 plug for both cylinders, it was a fuUuglly beast

Re: Dual ignition - double spark plug on mopeds?

All a spark plug does is ignite the fuel so it can be compressed. Having two spark plugs will do nothing because your still igniting the same amount of fuel and compressing it.

Its like using two matchs instead of one to light gas on fire

Yea if you have a horrible head design two spark plugs help the fuel burn more even. (edited)

Re: Dual ignition - double spark plug on mopeds?

My dad used to have a 1926 American la France fire engine which had switchable dual ignition, it did make a difference running on both, but that 600 cubic inch 6 cylinder had a mighty large bore. It also had a 3rd spark plug in each cylinder that was not connected to anything. Anywho....not needed on a moped.

Re: Dual ignition - double spark plug on mopeds?

> Rocco Taco Wrote:

> -------------------------------------------------------

> ignite the fuel so it can be compressed.

Please explain . ;)

Re: Dual ignition - double spark plug on mopeds?

🇮🇹💦 Of the Loin /

the two plugs in vintage bikes things are for "dual sports"

aka run one plug range while on the highway

one plug heat range while offroad

thats it

Re: Dual ignition - double spark plug on mopeds?

> Maitland Of the Loin Wrote:

> -------------------------------------------------------

> the two plugs in vintage bikes things are for "dual sports"

>

> aka run one plug range while on the highway

>

> one plug heat range while offroad

>

> thats it

Huh that had never occurred to me. Very interesting. I always assumed the foul out thing.

Re: Dual ignition - double spark plug on mopeds?

> Maitland Of the Loin Wrote:

> -------------------------------------------------------

> the two plugs in vintage bikes things are for "dual sports"

>

> aka run one plug range while on the highway

>

> one plug heat range while offroad

>

> thats it

That kinda makes sense , but , my '57 Simplex was never meant for off road . Not to mention that both plugs were the same from the factory .

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