Servicing the Sparkplug Caps

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Note: Even one bad spark plug. One bad plug cap or one bad HT coil lead or bad coil can stop the engine under load from revving above around 5,000 rpm/60 Mph Approx. For cheap CDI coil replacements (not Transistor Ignition Coils (TI))

Using a Digital Multimeter on a 20k setting a Resistor Spark plug should have a reading of around 5K [5,000 ohms) between the brass plug holder and the Ignition Wire screw.

On the USA based CX/GL bikes, there is a resistor inside the spark plug cap that can cause a lot of problems with the ignition system and the strength of the spark delivered to the spark plug. There is a simple cure for this. Remove all the internal components inside the cap and change out the resistor and the aluminum rod with a length of brass or aluminum rod equal to the length of the 2 parts combined. This is a good process to also just check to see if you have a lot of corrosion inside the assembly also. It can cause a high resistance in the delivery of the spark, and should be cleaned well if you do not want to change out the rods as shown below. I have been running these brass rods for about 4 years now, and have had no issues at all. It works.

Inside the tip you will see a the brass end that clips onto the plug. It's threaded into the cap itself and will need to be removed.


Make sure you use a wide flat blade screw driver that is also thick enough to make good contact in the slot so it does not slip and round off the brass slot.


When you get the threaded cap out you will find the resistor, an aluminum rod, and the tension spring under it, in that order.


Notice in this picture the dark colored resistor. This one came out of a cap that had a lot of corrosion inside it, and was causing a lot of heat and resistance in it. This is a good sign there was a problem with how the bike ran.

Now for the cure. The brass rod is the replacement for the resistor inside the cap. It is the total length of the 2 combined. The total length of the brass rod is 2.170 inch, and the diameter is .190 inch. I used a section of brass welding rod I got from a welding supply shop. You can use aluminum rod also. Before you put these parts back in, make sure you clean out the inside very well. I used a long thin flat screw driver and twisted and cleaned out the contact until I could see clean metal inside it so the spring makes good contact, as well as a thin brass bottle brush to remove all the corrosion. Use a good electric contact grease or a battery anti corrosion compound found at most auto parts stores when you reassemble it.


If you have a rod that is stuck inside, as a lot of them are, due to corrosion, you can drill it out if you have the right equipment. I used a 1/8" drill bit with tape built up on it so it stayed centered and drilled a hole down the center of it. Followed with a long 1/8th bit to get it all the way through until I felt it go through the end and into the spring area. Be ready to stop when you get near the end so you don't mess up the spring. Now I followed it up with a drill bit of 13/64th or a #4, .208 thousandths, to drill out the remainder of the aluminum from the inside of the cap. Make sure you keep it centered so it does not mess up the threads where the brass clip cap threads into. Be careful also, to make sure you keep it centered so it does not angle off and drill out the plastic cap. This worked very well for me while I was getting a cap ready for Blindstitch. He has been waiting patiently for a set to replace his bad caps.


This is the pile left from drilling out the stuck rod from all the corrosion that had it stuck inside.


Be careful and take care to clean all the components well, and you will have a good solid spark delivery to your plugs. Hope this helps you out and cures a bit of your ignition issues. Larry

Here is a cutaway of a plug cap posted by Blindstitch:

Just made this quick mock up of why the mod is important. Any one of those interior pieces can corrode and cause resistance. Larry provided the actual cut in half portion of the cap. The rest is all photoshop and random parts of pieces online.

Resized to 45% (was 1023 x 443) - Click image to enlarge Sparkplugcap.jpg

Some just have a long spring and no aluminum rod. Resized to 45% (was 1023 x 443) - Click image to enlarge SparkplugcapcutcropLongspring.jpg

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