Spark Plugs and CDI- seeking electronics advice


Active Member
Local time
11:56 PM
Jan 31, 2008
South Australia
Debatable thread

I was reading a workshop manual the other day of another type of bike engine.
The recommended spark plug in bold, insisted the plug be a resistor type.
What this means is, when buying a spark plug by it's number, in that number it has to have the letter "R" which stands for Resistor.

I then rang to order the replacement parts and was again reminded that it MUST have the resistor type spark plug or it will blow the CDI again.
I asked why !
Can someome clarify the following reason;

I was told the voltage on the plug lead can infact jump back into the CDI. A resistor spark plug stops this from occuring.

I was wondering due to the number of posts about CDI's having been fizzled, replaced and fizzled again, perhaps the spark plug needs to be the resistor type on our HT's ?

So for example if we use NGK BSHS, then it might have to changed to;

Google search explains somewhat here. Would this apply to the HT CDI ?
look for:
NGK-BR6HS-10-00 $1.99

BR6HS-10 - Plug Gap 1.0 mm/0.039 in
Fits F6 ('01 - '04), F8/T8 ('01 - '04)

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hey bolts thats good research, i esepecially like this controversial bit,

In fact, using a non-resistor plug in certain applications can actually cause the engine to suffer undesirable side effects such as an erratic idle, high-rpm misfire, engine run-on, power drop off at certain rpm levels and abnormal combustion.

Its gives me another thing to blame, as my engine used to do all these things,
and sometimes does a couple of them now. :devilish:

One last thing, I use a NGK B-6L, is this a resistor plug ?

Also, is the standard plug resistor or Non-resistor ???

I'm liking this debate.
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Thanks Will and Large for participating,

I must admit I'm no auto electrical expert, everything we did back then was change-over and cars had a coil, points and rotor button.
Chrysler bought out the then hailed "black box" and a balast resistor. Some Jap. cars had them to. ( I'm going way back pre 1980's)
You knew when a balast resistor was knackerd cos the car would only run with the key permanently on the Starter Motor Key Position then woulkd not run as soon as you let go of the key. Many tried to nurse their cars abck home, only to make matters worse and chop out a fly wheel and starter motor. For us then it meant, "$work$" ! ( removing a gearbox to chnage the fly wheel because the owner knew no better than to have car towed, all because the balast resistor blew it's guts.

Coils then had the letter "R" to signify a "resistor coil", "resistor leads" and something about "resistor plugs", but was never explained why.
We just did as we were recommended to do on tune-ups.
All this stuff had been forgotton over time due to "progress".

Now with this info has come to light ( resitor spark plugs), perhaps there is a point the HT people have made a oversight or don't understand this minor glitch.

I'll get a "resistor spark plug" ( NGK-BR6HS ) next week, hope they are available and see what happens. It does make sense though, the voltage jumping back into the CDI and frying it for no apparant reason.
The information says the R will stop this from occuring.
It might also explain why some of the bike computers don't compute ?

Question then is, will a HT CDI fry with a R plug and the kill switch wired up as per standard kit instructions ?
I think google has answered my question.

from this link, the plug I'm using NGK B6L, shows as a standard NGK plug.

There are other plugs listed on the same page that are called
resistor plugs.

NGK Spark Plugs
DR8EA Resistor Spark Plug

So whilst its listed as a standard plug, the NGK B6L is described as having good electrical insulation, which is what the problem you've described,
as being potentially harmful to CDI.

this is something I will keep in mind at next plug purchase...

EDIT: I found the NGK BPR6HS on this site

it has exactly the same description as the standard plug,
except for saying its superior power construction.

So in my mind they're the same plug.
Here's the depth comparison

# Thread - 14 mm
# Reach - 1/2 in

# Thread - 14 mm
# Reach - 7/16 in

In my opinion, I think they're basically the same.
Is there an actual correct Reach ? Or a reach range that works ?
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Thanks for that research there Will, we have established the R plug IS available, but we need to establish in fact the true technical/ "science" details weather it will make any differance if a R plug is fitted to the HT or no differance at all.