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Spark Plug Cap Maintenance ( ST1100 ) *

Started by KoTAOW, November 08, 2014, 09:08:05 PM

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Submitted by Mike Durkin, aka Space Ghost, STOC #8311


Spark Plug Cap Maintenance ( ST1100 )

Honda used the same spark plug cap for all years of the ST1100   -  # 30700-KAF-010.

While trying to diagnose a no-spark condition on my 1994 ST, I discovered that Honda used the same basic plug cap design on the ST1100 as they did in the 1970’s on the CB750 and many other models.

The plug cap contains a 5000 ohm resistor to cut down on radio interference.  Back in the 70’s Honda was running spark plugs with no internal resistor, so the 5000 ohm resistor was a good thing….except it introduced a potential failure point…and they do fail.

Fast forward to 1990 and Honda, despite using resistor plugs on the ST1100, still felt the need to have a resistor in the plug cap. And yes, it is still a potential failure point.

To remove the plug caps from your plug wires, just unscrew them â€" CCW.

Dissecting the Plug Cap

Going in from the spark plug end, you can un-screw the spark plug terminal with a flat blade screw driver.  The 5K ohm resistor and the spring will fall out.

I’ve seen two failure modes for this design:

1 â€" the resistor deteriorates and increases in resistance (a bad thing â€" less energy to your plugs and your coils have to work harder to drive the spark across your plug gap.

2 -  Arcing takes place at the lower portion of the spring, causing an increase in resistance between the spring and the metal terminal at the bottom of the cap.  Same downside as #1.  The plug cap in the picture above has a good 5K ohm resistor. The spring and the spring terminal showed signs of sever arcing to the point that the circuit was open.  

New plug caps list for $ 28.49 and sell for $ 17.48  at Power Sports Plus.

I bought a used set of coils and caps for my 94 when I was thrashing around trying to fix my no-spark condition.  Caps for the 1/3 coil had 5K and 100K resistance.  Caps for the 2/4 coil had 5K and infinity resistance.  The coils came from a 1998 and the EBay seller was also auctioning off near perfect body panels and a “running” motor. Based on my resistance checks the bike was running on two cylinders and rather poorly at that. Kinda makes you wonder why a 1998 ST1100 with good plastic and a running motor was being parted out?

The bottom line is that the ST1100 plug caps are a potential failure point and they should be checked at least once a year.

There are two ways to check them.   Remove them from the plug wires and measure the resistance from end to end â€" you are looking for 5000 ohms.

The second way is to leave them on the plug wires and measure 1/3 and 2/4 â€" end cap to end cap â€" you are looking for approx 25K ohms  - that’s 15K for the coil secondary and  5K + 5K ohms for each end cap.


Thank You again for your contribution Mike Durkin, aka Space Ghost STOC #8311