Question on plug caps

Hello. I have a 1982 Seca XJ650R. I am troubleshooting why it never runs consistently. I tested the plug caps and get a different reading for caps 1 and 4 vs. 2 and 3. Also 1 and 4 seem slightly longer. Question: Should all caps be the same (5000 ohms) or would 1 and 4 have a higher resistance reading than 2 and 3? I ordered new ones but that will result in all 4 caps being exactly the same. Thanks!

 

Yes!


SPARK 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 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.

NOTE: there is a a lot of information below about a sometimes mystifying topic….electricity, and electrical resistance (and how it can make your life better……) below. Please ask questions if there’s anything that you don’t understand in relation to selecting the correct components for your bike!

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 ignition 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 plug 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, one of our aftermarket coils, 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, in cases where the wire itself gets damaged, but the coil itself is still in good shape (physically and electrically).


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 metal 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:



NOTE: "K" is abbreviation for a thousand units, so "5K" ohms = 5,000 ohms of resistance, etc.

NOTE: some models within this era could have been fitted with "noise suppression" plug caps (these caps are evidenced by a metal "shield" wrapped around the plastic cap body) and such caps may have used a higher resistance rating as opposed to their stock ratings listed below (for non "noise suppression" style caps).


5K +/- 20% = 4,000 to 6,000 ohms per cap acceptable range:
all XJ650 models
1985 XJ700 air-cooled models
all 1981-84 XJ750 (except XJ750RL) models
all 1983-84 XJ900RK and RL models
all XJ1100 models
all XS1100 models

NOTE: some sources specify that XJ650 models, XJ750 air-cooled models, and XJ900, XJ1100 and XS1100 models used 5K caps on the outer plug (#1 and #4), and 10K caps on the inner plugs (cylinders #2 and #3). Other sources say that all 4 caps were 5K ohms. And other sources say that 10K caps were used on the outer cylinders (#1 and #4), while the two center cylinders use 10K caps…….whew! Our recommendation is to use the 5K caps on all 4 cylinders.

 

Thank you! Much appreciated.