Plug Caps

Should I use Resistor or Non-Resistor plug caps on my 1982 xj750 Maxim?

 

nobody has any comment? come on now!!

 

Resistor.

 

Sparks Plug Caps:

pc2) Original TD (Tokia-Denso) and aftermarket NGK SPARK PLUG CAPS are available in a variety of styles and configurations. All plug caps include their weatherproof upper and lower rubber end boots.

Okay, before we get going, let's quickly review a little bit about the original Yamaha COILS, PLUG WIRES, PLUG CAPS, and the SPARK PLUGS used on these bikes:

Factory ignition systems are designed to operate properly with a total system resistance on the secondary side (the "going-to-the-spark-plugs" side) of the coils of around 20-30K ohms (ohms being a measure of electrical resistance). Electrical resistance depends on a number of factors: wire size, type of material, length of material, age of material, ambient temperature, etc. etc. In any case, most factory XJ coils and wires combined---BUT WITHOUT THE CAPS OR PLUGS ATTACHED---are specified to have a resistance of around 11K ohms, +/- 20%, at 68-degrees Fahrenheit. Please see the complete list of specifications in the "COILS" section above.

Plug WIRES on factory coils are non-replaceable, at least not without a bit of surgery. Suffice it to say that if your coils measure out of specs for resistance (as described above), they're junk and should be replaced, either with another factory coil or a pair of the HCP245 Dyna aftermarket coils.

We do offer an NGK plug wire in-line splicer (HCP2789) that will allow you to cut off and remove a bad factory wire and replace it with a length of our aftermarket plug wire.

The original spark plug wire resistive CAPS---mistakenly called "boots" by some people---are the hard plastic insulators that fit onto the spark plug threaded stud, and accept the plug wire from the coil on their other end via a simple "twist-on" method------yes, the plug wire end of the cap has a metal screw that bites into and "screws" onto the end of the plug wire, penetrating the plug wire inner copper core and making a mechanical connection.

Plug caps use a small internal ceramic resistor, and the resistance of each cap should be checked with an ohmmeter on a periodic basis. Caps that are +/- 20% resistance from specified levels are considered bad, and should be replaced.


A variety of different size and shape plug caps are listed below, in both their original Tokai Denso brand and replacement NGK brand. The replacement NGK caps come in a variety of original configurations and can be safely used to replace broken, missing, damaged, or out-of-spec original caps. And all original and replacement caps will accept up to 8mm spark plug wires easily.

NOTE: all original and replacement spark plug caps are designed to be used on spark plugs that have their screw-on threaded stud topper removed. And while all of the NGK replacement plug caps are specific to either the 12mm plugs (all XJ550, XJ700-X, and XJ750-X models) or the 14mm plugs (all XJ650, XJ700 non-X, all XJ750 non-X, XJ900RK, and XJ1100 models), the original Tokia-Denso plug caps can be fitted to either 12mm or 14mm plugs, so that XJ550 caps can be physically fit upon XJ650 (etc.) engines, and vice-versa.


It is important to remember that you should strive to keep the ignition coil secondary circuit (plug wire, plug cap, and spark plug) total resistance as close as possible to original specifications, which are:

- 15K ohms per cylinder on all 1986 XJ700-X, XJ750-X, and XJ900 S/SH models.
- 10K ohms per cylinder on all XJ550, 1985 XJ700-X models, 1986 XJ700 non-X, XJ750RL, and XJ900 N/FN and F models.
- 5K ohms per cylinder on all XJ650, 1985 XJ700 non-X, all 1981-83 XJ750 models, XJ750-D, XJ900RK and RL models, and XJ1100 models.

For people who wish to rebuild their otherwise usable original Tokia Denso plug caps, we offer replacement resistors in either the 5K or 10K versions, or non-resistive (0 ohms) replacement rod. Owners who wish to use non-stock spark plugs (such as the popular NGK Iridium plugs or any other replacement plugs) will need to make the appropriate reduction in the secondary circuit total resistive load, which is most easily achieved by reducing the resistance of the plug caps by 5K ohms from their stock specifications.

Just remember to add up the resistance of your plug wires (original wires and all of our replacement wires are 0K ohms, non-resistance wire), the spark plugs (0K for non-resistive plugs, 5K for resistive plugs), and of your plug caps (which can be varied as outlined above) to arrive at the correct total resistance value for each plug!

EXAMPLE: 1982 XJ650 Maxim models originally used the HCP1292 non-resistive (zero ohms) BP7ES spark plugs, along with 5,000 (5K) ohm plug caps on each plug. If you substitute a resistive plug (such as the HCP1293 Iridium plugs, or any other resistive plug) for the original non-resistive plug, then your spark plug + plug cap total resistance will increase from the original 5,000 (5K) ohms to 10,000 (10K) ohms. Therefore, you should reduce the resistance in the plug cap to zero ohms to keep the system at the same total resistance levels as original.


The key to remember is that you simply want to maintain the TOTAL system resistance at whatever the factory rating is. You can't do much about what the coils themselves are rated at, so outside of the coils, you have three choices of where the resistance can be added:

a) the plug wires
b) the plug caps
c) the plugs

For example, the XJ550's require 10K (10,000) ohms of resistance in each wire/cap/plug circuit (outside of the coils themselves). You can achieve this 10K of resistance thru any combination of component pieces; however, it may be simpler to use zero-resistance wires, and then use either:

1) 10K plug caps and 0-ohm (non-resistor) plugs (this is the method that was originally used). This is achieved by using new factory plug caps (which are 10K ohms) or rebuilding the stock plug caps (if you have the originals) with new 10K ohm resistors.

2) 5K aftermarket NGK plug caps and 5K aftermarket resistor plugs.


And finally, always check, with an ohmmeter, the coil primary and secondary resistance, your plug cap resistance, and your spark plug resistance and condition before you start "throwing money" at engine performance problems that seem to be electrically-related!