Waste spark system clarification

Thread Starter

Autobike

Joined Feb 23, 2018
69
hello :) i have a small question about the waste spark system. i have this system in my Honda CBR 4 cylinder bike. one coil fires two cylinders at once. so according to the motorcycle manufacturer Honda and the spark plug manufacturer NGK, the secondary circuit is a sole complete circuit.



as we can see in the above picture the EMF generated in the secondary winding (due to sudden collapse of the magnetic flux) goes through spark plug "A". then jumps through its center electrode to ground electrode. after that it uses the cylinder head as a conductor and jump from ground electrode to center electrode of the spark plug "B". finally it goes to the other end of the winding and the circuit completes.

recently i had to pull out a plug cap while the engine was running. but what i noticed was the other plug of the same coil kept firing without any problem. confirmed it by measuring the temperature of its exhaust pipe using an IR temperature gun. that cylinder was working perfectly fine.

but according to Honda and NGK the secondary circuit is a sole complete circuit. so if i pull out a plug cap the circuit opens. so in that kind of situation how the other plug keeps firing ? does the EMF behave different from battery voltage ? since it's a very high voltage does it still manage to make a spark by jumping through the spark plug electrodes ?

for instance let's say i pulled out the plug cap of plug "A" or plug "B". so the circuit opens. but in reality the other plug of the same coil kept firing. this is what i experienced. could someone explain this please. thank you :)
 

crutschow

Joined Mar 14, 2008
23,526
The spark may be jumping through the wire or coil insulation to ground on the wire you pulled, to complete the circuit.

Note that it's bad practice to pull the plug wire off an operating engine without grounding the wire, since that generates a very high voltage which can arc over and damage the coil (which is perhaps what is happening when you remove the wire).
 

Thread Starter

Autobike

Joined Feb 23, 2018
69
The spark may be jumping through the wire or coil insulation to ground on the wire you pulled, to complete the circuit.

Note that it's bad practice to pull the plug wire off an operating engine without grounding the wire, since that generates a very high voltage which can arc over and damage the coil (which is perhaps what is happening when you remove the wire).
thank you :) yea sorry about that. it's a bad practice.
as you said, the spark may have jumped to the ground through the wire or coil itself. i didn't hear any buzzing noise (what we hear when an arc happens) or visually inspected any spark. but the engine was running at that time. so i think there was a chance which could go unnoticed. since there was no other way to complete the circuit it should ground from somewhere. thx again.
 
Last edited:

shortbus

Joined Sep 30, 2009
7,080
but according to Honda and NGK the secondary circuit is a sole complete circuit. so if i pull out a plug cap the circuit opens. so in that kind of situation how the other plug keeps firing ? does the EMF behave different from battery voltage ? since it's a very high voltage does it still manage to make a spark by jumping through the spark plug electrodes ?
People seem to think the waste spark thing is something new, but Harley used it from the beginning of their twin cylinder engines. The big reason for not having both plugs connected is the flash over in modern day dual plug coils.

While not familiar with Honda's approach to this, there is only one that I know of that HAS to have the second plug fire as part of the circuit, that is in some GM engines that were made right before the Coil on/near plug took over.

Pulling a plug wire was a common thing to clean a partially fouled plug in old Harley's, but you needed to still have the spark jump between the wire and the plug electrode for it to work, the extra gap jump is what did the cleaning.
 

crutschow

Joined Mar 14, 2008
23,526
Pulling a plug wire was a common thing to clean a partially fouled plug in old Harley's, but you needed to still have the spark jump between the wire and the plug electrode for it to work, the extra gap jump is what did the cleaning.
Don't see how having one plug wire jump a longer gap would significantly affect the spark in the other (fouled) plug. :confused:
 
Last edited:

crutschow

Joined Mar 14, 2008
23,526
You add the extra gap to the fouled plug, don't know why or how it works, just that it does.
Okay.
Perhaps that creates a faster rise-time spark which is more likely to fire a fouled plug.
Capacitive discharge ignitions are know to be better at firing fouled plugs because of their faster rise-time spark.
It gives less time for the foul (low resistance across the plug gap) to drain away the spark energy before it crosses the gap.
 

Thread Starter

Autobike

Joined Feb 23, 2018
69
thank you very much for your information @shortbus . learned something new :)
this is the coil which i have in my bike.



actually there was another incident which happened few years back. i think it is also related to this question.
one day i noticed that the sound of my engine was different. so first of all i checked the four exhaust pipes and one was running cold. then i pulled out the plug cap of that cylinder and the other plug cap of the same coil and checked the coil resistance with my multimeter. it was an open circuit according to the multimeter reading. so i removed both plug caps from the HT lines and then put my multimeter directly to the HT lines. then i got the correct resistance reading of the coil ( correct reading without plug caps according to the manual ). so there could be something wrong with the plug caps. after that i tested the resistance of the two plug caps individually and one showed open circuit. that plug cap had been connected to the cylinder which ran cold.
i purchased a used plug cap where the reading was correct and fixed it to my coil. finally i checked the coil resistance with plug caps on and the resistance was ok. since then i've been riding with that plug cap and haven't got any problem.
but according to the circuit if one plug cap shows open circuit the whole circuit opens. so both the plugs should fail. but it didn't happen in reality. only one cylinder ran cold and the other plug related to the same coil ran perfectly fine. so like @crutschow said it could be a short circuit which completes the firing of the other spark plug.

i found this interesting article.



as they say people have experienced a spark only on one side of the coil (only one spark plug fires). according to them (i have marked in red), if the circuit opens both the plugs should fail. reason is it's a single complete circuit. if only one plug fires it should be a short circuit which has not yet been manifested.
to be honest i haven't seen or heard a noise of an arc. but apparently it's the only way which creates a spark only on one side. thank you.
 
Last edited:

debe

Joined Sep 21, 2010
1,142
Another problem on these types of ign systems where the plug is deep in the cyl head. Is spark punches through the rubber/ silicon boot & causes one cylinder to not fire. When this happens on carefull inspection where ive marked, there will be a small black hole burnt through. Its quite a common fault.WASTED SPARK COILS.jpg
 

shortbus

Joined Sep 30, 2009
7,080
i purchased a used plug cap where the reading was correct and fixed it to my coil.
This is one thing, personally I wouldn't do. Maybe to get home if stranded but not as a way of making things right. Used ignition wires are not some thing to be trusted.

Like Debe said they over time will punch through or in some case get enough carbon on them to become conductive and short to ground.
 

shortbus

Joined Sep 30, 2009
7,080
Okay.
Perhaps that creates a faster rise-time spark which is more likely to fire a fouled plug.
Capacitive discharge ignitions are know to be better at firing fouled plugs because of their faster rise-time spark.
It gives less time for the foul (low resistance across the plug gap) to drain away the spark energy before it crosses the gap.
Have never tried it on a newer ignition like a capacitive discharge. My experience is with old Panhead Harley's using points. The trick has got me home more than once.
 

Thread Starter

Autobike

Joined Feb 23, 2018
69
Another problem on these types of ign systems where the plug is deep in the cyl head. Is spark punches through the rubber/ silicon boot & causes one cylinder to not fire. When this happens on carefull inspection where ive marked, there will be a small black hole burnt through. Its quite a common fault.View attachment 184498
this is a very useful info. i didn't know that. thank you.

This is one thing, personally I wouldn't do. Maybe to get home if stranded but not as a way of making things right. Used ignition wires are not some thing to be trusted.
Like Debe said they over time will punch through or in some case get enough carbon on them to become conductive and short to ground.
yea you are right. does carbon build up inside the plug cap ? what is responsible for that ? thank you.

Well, plug fouling has generally ceased to be a problem since the elimination of leaded gasoline. It was the lead deposits that caused the foul.
isn't it the too rich mixture caused by wrong mixture or some ignition problems which makes the plug fouling ? thank you.
 

Thread Starter

Autobike

Joined Feb 23, 2018
69
Yes, that can cause plug fouling of the gap.
But in the old days it was often caused by lead deposits from the leaded gasoline.
i wasn't aware of that. does it make the same dry soot which is created by a rich mixture. google shows some tan color deposit as the leaded fouled plugs. thank you.
 

Thread Starter

Autobike

Joined Feb 23, 2018
69
as a conclusion, considering the facts above and as @crutschow said it should be a short to ground which makes only one spark plug work. thank you all. much appreciated.
 

shortbus

Joined Sep 30, 2009
7,080
yea you are right. does carbon build up inside the plug cap ? what is responsible for that ? thank you.
It's a combination of things. Age of the rubber, condition of the plug, and probably other thing's I can't think of right now. I worked where they made spark plug wires and boots, do you know what makes them a black color? It's something called carbon black, so as the heat and age breaks down the rubber in the boot there is carbon left over.
 

Thread Starter

Autobike

Joined Feb 23, 2018
69
It's a combination of things. Age of the rubber, condition of the plug, and probably other thing's I can't think of right now. I worked where they made spark plug wires and boots, do you know what makes them a black color? It's something called carbon black, so as the heat and age breaks down the rubber in the boot there is carbon left over.
that's interesting. i thought they are completely made out of rubber. thank you.
 
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