Author Topic: KLX 250 plug cap resistance  (Read 5028 times)

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Offline farmer58za

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KLX 250 plug cap resistance
« on: October 22, 2022, 09:19:05 AM »
Hi All
I've just replaced the ignition HT coil on the 1979 KLX. I get a big fat spark off the end of the plug wire, but a much smaller one off the plug, in a NGK cap with a 5k? resistance. If I substitute the cap for a NGK non-resistor type, I get a much bigger spark on the plug.

I know the plug/plug cap resistance choice was a big thing with old Motoplat ignitions, where a resistor type plug would cause hard starting. Is this still the case with more modern (Jap) ignitions?

I don't really have hard starting on the KLX, but conventional wisdom tells me to leave the non-resistor plug cap on the bike.

Any advice from the KLX experts?

TIA

David
Dave's not here man

Offline John Orchard

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Re: KLX 250 plug cap resistance
« Reply #1 on: October 22, 2022, 04:59:19 PM »
I'm interested to hear what people think, as far as my understanding; a resister plug cap waits for the voltage to buildup a bit more before letting it through, hence a larger spark? Do you think maybe the spark is brighter from the lead because it's jumping a bigger gap, compared to the .7mm of the plug?
Johnny O - Tahition_Red factory rider.

Offline pokey

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Re: KLX 250 plug cap resistance
« Reply #2 on: October 22, 2022, 08:47:18 PM »
The resistor cap is a bit of a misnomer and means its sealed against electrical interference and possible shorting from whatever.  its just better sealed than a conventional plug cap with no difference in electrical resistance. The plug has the actual electrical resistor and you can check the NGK website for the details  but as a rule use Non resistor plugs (no R in the model number) as they produce a better fatter spark. but a resistor plug is used in cars to suppress" electrical noise" Hope that makes sense.

now with some of the modern ECU systems you may well need to run resistor plugs and resistor caps and for that consult the manufacturer. never run a resistor plug with a non resistor cap as you will get electrical leakage from the cap and poor performance.



« Last Edit: October 22, 2022, 09:01:20 PM by pokey »

Offline farmer58za

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Re: KLX 250 plug cap resistance
« Reply #3 on: October 23, 2022, 12:34:56 AM »
Thanks for the response Guys. Here are some further comments:

<snip>...Do you think maybe the spark is brighter from the lead because it's jumping a bigger gap, compared to the .7mm of the plug?

The spark on the (very used) KLX plug, was less that I observed at the end of the plug lead (which I expected chance what plug cap was in place). However the spark at the plug was better when I used a non-resistor plug cap in place of the resistor plug cap.


The resistor cap is a bit of a misnomer and means its sealed against electrical interference and possible shorting from whatever.  its just better sealed than a conventional plug cap with no difference in electrical resistance.... <snip>

Not so in my case:  The "5K Ohm" resistor cap measured a resistance of 4.7 k Ohm . The non-resistor cap measured a resistance of zero Ohm.

The KLX uses a non-resistor plug.


« Last Edit: October 23, 2022, 12:42:03 AM by farmer58za »
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Offline pokey

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Re: KLX 250 plug cap resistance
« Reply #4 on: October 23, 2022, 01:02:24 AM »
Sounds like you have a modern style cap and they have a graphite and glass actual resistor to filter the  power. Older resistor caps only had phenolic  body and longer rubber seals at both ends to stop leakage. resistance was handled by the wire. I would think an old KLX running  stock ignition and even old after market wouldnt produce enough charge to  energise  the cap and the plug like a modern ECU that uses coil caps. hence why most old school and racing applications all run NON resistor devices. Why run  a component with more resistance ? Guess your answer is to run a Non resistor plug and non resistor cap.

to work out if you need a resistor cap the manufacturer of the stator needs to be consulted. if it didnt come with one dont put one on it,

Caps come with no resistor  or 1/5/10 K ohms and it needs to be match to the specifications documented  by the stator.

The model number has all the information to match the application.

« Last Edit: October 23, 2022, 01:31:09 AM by pokey »

Offline sleepy

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Re: KLX 250 plug cap resistance
« Reply #5 on: October 23, 2022, 10:53:00 AM »
I'm interested to hear what people think, as far as my understanding; a resister plug cap waits for the voltage to buildup a bit more before letting it through, hence a larger spark? Do you think maybe the spark is brighter from the lead because it's jumping a bigger gap, compared to the .7mm of the plug?

That's an interesting theory regarding the resistor and it holding back for a bigger spark. I heard all sorts of theories on how ignition system work when I was working trying to teach mechanics how early car EFI systems worked.
The resistors in the spark system are there to spot electrical interference with electronic equipment. The resistor works like only turning on your garden tap part way, the water comes through just slower. In an electrical circuit like a spark circuit there is a certain amount of current flow and voltage available from the coil. By adding in a resistor the voltage at the spark gap is reduced and the current is reduced as well. The more resistive elements in the circuit the less spark. They can be resistive leads (5K ohms per foot) resistor caps and resistor plugs, most applications only need one. Cars that still have plug leads usually have resistor leads but may still have R plugs as well, this is mainly needed for the onboard ECU's and radio/sound equipment.
As for as the KLX is concerned I would think it would be fine without any resistors or just a small value cap if it has a wire lead but then it could interfere with close radio or TV's. My wife used to complain that me running a bike in my shed when she was watching her favorite TV show would make the screen go very streaky.   

Offline John Orchard

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Re: KLX 250 plug cap resistance
« Reply #6 on: October 23, 2022, 04:19:57 PM »
I have found many times that if trying to start a bike (usually a two-stroke) with a fouled plug, lifting the cap off the plug, so that the spark voltage has to jump from the cap to the plug, I have found, many times, that the plug will fire, and the engine will start. I presumed the increase in resistance (from jumping the mentioned gap) caused the voltage to be much higher and firing a fouled plug?
Johnny O - Tahition_Red factory rider.

Offline sleepy

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Re: KLX 250 plug cap resistance
« Reply #7 on: October 23, 2022, 05:09:25 PM »
Can't say I've heard of that one and not sure I want to be holding a plug cap off by a small amount while kicking, not that I'm scared of getting shocked just not something I like. Adding a gap in the circuit will force the coil to produce it's best voltage and with a fowled plug might help.
I have been told a few times that on a cold morning you should put in a much hotter heat range plug so as to make the motor easier to start. Some how they believe the hotter plugs gives a hotter spark by some sort of magic. May prevent fowling while the motor is cold. 

Offline Momus

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Re: KLX 250 plug cap resistance
« Reply #8 on: October 23, 2022, 07:06:28 PM »
The air gap trick to clear a fouled plug is one of the oldest.

It got me out of the bush and home a couple of times 50 years ago.
If you love it, lube it.

Offline John Orchard

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Re: KLX 250 plug cap resistance
« Reply #9 on: October 23, 2022, 07:56:03 PM »
This off one of the tech sites got me on the higher voltage at plug with more resistance......

Resistor wires are for radio suppression (noise). Resistor plugs raise the voltage necessary to jump the gap, thus raising the amount of energy imparted into the spark itself. With today's alcoholized gas and lean burn engines, you need higher energy to light the charge. Most of today's autos need at least a .040" gap, if not larger.

there IS a drawback to resistor plugs. It raises the voltage to the point that it can cause insulation failure in older coils. Today's coils are built to handle the extra resistance and higher voltage imparted by resistor wire and plugs. Today's auto ignition systems typically make over 60,000 to 100,000 volts!:eek: Most of our old engines made between 10 to 20,000 maximum. Older systems cannot overcome that resistance. Spark (electricity) will find the easiest path to ground. if resistance is too high, it will jump thru the old type shellac used on older mag and battery type coils, thus shorting out the secondary wiring. On some engines, this extra resistance will also short out on distributor caps, and if the resistor wiring is old, it will also break thru the insulation sleeve on the wire itself.

it is NOT recommended to use resistor secondary wiring on magneto fired engines built before the mid 1960s. it also is not recommended for multi-cylinder gas engines built before the early 1960s as the extra voltage built up could arc between distributor posts within the cap. This is especially true in aircraft and v-12 engines, as the posts are close together. With resistor wiring, the potential between posts may be less than the resistance between the post and the plug to be fired, so the spark will jump the air gap between posts and cause a cross fire. Case in point - the American La-France V-12. Using resistor wires will cause cross fire between cap terminals as the spark will jump to a short wire lead to ground rather than a long lead (resistance is greater as distance to end point increases). On the ALF V-12, the high tension leads can be as short as less than 2 feet to as long as nearly 5 feet long, each! With 24 plugs, you can see why resistance is critical. in actuality, there are about 125 feet of high tension wire for 1 engine!. The electricity will always take the shortest path. On the ALF engines, and on our old timers, you must use a wire core wire, like Packard 440 or an equivalent. You should not use modern carbon core resistor wire. Another magneto that is susceptible to resistance damage is the Maytag 72 D and DA. Putting resistor plugs and or resistor wire on them is a sure way to kill the already weak insulation in the coils on these engines. On most of our old engines, it also is not a good idea to pull the high tension wires to stop an engine. if you must pull wires, pull the low tension side ot short the high tension to ground, to stop an engine.

In short - resistor wiring and or plugs raise impedance - the ability for electricity to flow. This raises the voltage necessary to complete the circuit. higher voltage means higher energy at the spark plug gap, thus a hotter spark. Resistance lowers current flow. A special note is also needed here: YOU must not pull high tension wires off autos to test for spark! Too much resistance, such as an open circuit will damage computer controlled ignition systems, and can damage the coil. On older GM High energy systems (HEI), they can build to over 100,000 volts! When it first came out in the late 1960s, dealer mechanics were given notice that the HEI coils could kill you! A cross body shock from an HEI coil would stop your heart, much as an AED used to remedy heart attacks do today. There were several cases of serious injury and death associated with them, as older mechanics, used to pulling off wires, were getting severely shocked by the new system.
Johnny O - Tahition_Red factory rider.

Offline John Orchard

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Re: KLX 250 plug cap resistance
« Reply #10 on: October 23, 2022, 07:58:15 PM »
Can't say I've heard of that one and not sure I want to be holding a plug cap off by a small amount while kicking, not that I'm scared of getting shocked just not something I like. Adding a gap in the circuit will force the coil to produce it's best voltage and with a fowled plug might help.
I have been told a few times that on a cold morning you should put in a much hotter heat range plug so as to make the motor easier to start. Some how they believe the hotter plugs gives a hotter spark by some sort of magic. May prevent fowling while the motor is cold.


I remember up Reefton Spur one day, we had to tape a pair of sparkplug caps away from the plugs on an RGV250 so the bike would start and he could get home.
Johnny O - Tahition_Red factory rider.