Motorcycle Spark Plug Caps(Important!)

An often overlooked cause of engine faults like Bad performance,single cylinder firing on Twin Cylinder or Multi-Cylinder mis-firing and popping or even a full engine shutdown can be the Spark Plug Caps. It can even fool us into believing there's a carburation fault as the symptoms can be similar to fuel delivery/mixture problems

Most modern Motorcycles/Cars are fitted with,"Resistor" Plug Caps. This is a legacy from the late 1970s when it was found that,"Straight-Through" Plug caps caused timing errors in the then new,"CDI" ignition circuits.By Adding a 5k ohm Ceramic Resistor in-line into the HT wire this cured it.

This was done by making a Spark plug cap with this resistor inside it. It doesn't matter if it's a Straight design or Angled.


Now here's the,"Rub". At the same time or within a few years Spark Plug manufacturers also started making,"R" rated Spark plugs that also contained the 5k ohm resistor in effect,"Doubling up". This means the combined added resistance was now 10k ohm. This has no detrimental affect on the ignition but the,"Resistor Spark Plug Cap" now became redundant. I speculate the only reason the Cap manufacturers now supply them is out of tradition and the old machinery for making Straight-Through plug caps has been scrapped so it would cost too much to,"Tool-Up" to make them.

 You will see the,"R" in the Resistor Spark plug's number like CR8E for my Suzuki SV650

The problem is over time and with older Motorcycles especially that are more open the elements corrosion inside the R-Type Plug Caps increases their resistance because of the increased number of contact points. There are even more contact points in some of these Plug caps as they use very tiny washers between the Spring and the Resistor. This will eventually lead to the symptoms/problems at the beginning of this Blog.

The piece of Card on the Radiator is for temporary protection of the Radiator Fins and my Knuckles whilst I work on the front Plug/Area. To make it easy to get to my Front Plug I have removed the Horn Bracket and moved the horn to the front left of the bike. I can now just remove the 3 Radiator bolts and force the Radiator forward to gain access to the Front plug without having to remove the Coolant.

Horn Moved To Front

Service Check and Solution

Using a Digital Multimeter on a 20k ohm setting test the Spark plug cap. If it exceeds anything north of 8k/10k ohms then it has started to become suspect. I recently serviced a friends Motorcycle that was running rough and his Plug Caps were 17k ohms!

It's Time to Replace the Plug caps as soon as they start to go North of the 8k/10k ohm reading IMHO.

The OEM plug caps for my Suzuki SV650 are expensive hence why you see the Straight long-reach NGK Racing ones which are much cheaper. They come in two colours e.g Red and Black and their code numbers are,

NGK SD05FMR  for the red and

NGK SD05FMB for the black

Note: You can and should also check the Spark Plug Resistance. It should be 5k ohms or below.

I had to make some grommets/skirts out of some old inner-tube as seen in the picture above as the old stock grommets would not fit and especially on the front cylinder of Transverse mounted V-Twins like the SV as the Front cylinder angle lends itself to the accumulation of Water and Road-Crud.They need this protection from Rain/Road water being sprayed up on them.


1: Before screwing the Spark Plug Cap into the HT wire cut a few millimeters off the HT cable so it has fresh copper to go into.

2: Put a Blob of AFC50 into the Brass screw inside the Plug cap/s.

3: Put a Blob of ACF50 on the end of the Spark Plug that goes into the Plug Cap/s

4: Smear a thin film of ACF50 all over the Plug cap/s.

5: If like my SV 650 your Front Mudguard is a,"Shorty" buy and fit a,"Fender Extender". This will help keep road water and debris off the engine and front plug.


6: Also check the Cylinder Drain holes are not clogged!

Final Tip

Check the Service Manual on how to check/test the Ignition coils on the machine as these too fail over the years and as they get weaker they are more susceptible to heat from the engine and can cause similar problems as at the Start of this Blog.

Universal Mosfet Regulator/Rectifier Fitting

SV650( and general) Carb Balancing

Motorcycle Action Camera Helmet Fitting

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June 2020