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Last Updated: August 07, 2013

Spark Plug Cap Resistor

I recently encountered another problem with my suzuki ignition, that even if i am kicking the hell on the kick starter wont even nudge or start my bike. Ever since i added this ignition-wire-ground-strap , it became a tester of some sort due to the spark jumping on that shield ground, that from there it is telling me, the PLUG CAP resistor is too corroded to accept even an arc to feed the spark plug GAP. It has been three times i replaced my plug cap. so in the end, instead of buying the same crap, i made a minor modification using that plug cap and a resistor of 2.2k used in every electronic circuits.

The picture above represent the dis assembled spark plug cap of my shogun Pro 125 cc, the plug cap housing on top, the screw on the left, next is the built in resistor, then the washer, and spring.
Before i disassemble the unit, i first check the plug cap internal resistance if its functioning properly with a MULTI-TESTER (digital) end to end. At to my dismay, it has no resistance, and that means, the contact of the cylindrical resistor as shown below, have a corroded contact with the screw as shown on the right picture with its dark surface showing how it was filled with carbon deposit.

EFFECT of the plug cap internal resistance failure

From my perspective since i am the one who personally use my SHOGUN everyday, i know how she react and perform on the street. Sometimes it rocks, pulls very quick, with minimal vibration on my PEGS, the bike aint sluggish, the throttle respond to all of its opening, thus it means the combustion is performing well.

WHY is that So?

a resistor with no contact means its open and when it is open it cant conduct current nor electricity to where it is going. In a high voltage application it will act like a GAP thus arcing inside the plug cap, producing black debris or i may say carbon deposit inside the cap. therefore energy LOSS is imminent.

It will also produce lots of RFI that affects Ignition entirely, very obvious on my fuel mileage lately. the bike run sluggish, wont pull, too much vibration.

HOW to dismantle your plug cap

use a long nose plier to squeeze the retaining screw towards the inner wall of the plug cap to loosen the screw retaining washer inside, when loose , you can pull the screw out.

HERE is the resistance check before and after the plug cap REBUILD

Before rebuilding this cap, i measure the resistance to where the ignition wire is fitted and where the spark plug is being inserted, notice there is no reading, an indication of a failed spark plug cap resistor.

After the rebuild, make sense

All of this are based on my experiences with my bike, not all bike will have the same problem that causes your bike to perform poorly. other stuff like carb settings, intake system leaks, weak ignition system may be the causes of your bike, but since i know my other stuff are healthy, Ihave never thought that this plug cap is the culprit after analysing the performance of my bike.

This may be useful to you and hopefully the power and performance of your shogun can outperform other bike out there.

From time to time, the cap must be inspected for cracks, and continuity, If there is no resistance reading, might as well replaced or rebuild, its your choice, mainly this article provides you resources on why sometimes your bike is running weird, and or inconsistent.  plug cap problems seem reasonably common after a few years and make the bike run badly rather than not at all. The performance difference is pretty dramatic, but that's mainly just from fixing the HT problems rather than some magic associated with how I did it. 

If you of course changed your spark plug cap, use OEM.. to ensure you are buying a cap with resistor, and if in doubt might test it end to end if it is indeed a resistance type plug cap...USING a non-resistive plug cap, will definitely give you problems by producing lots of high frequency noises that may affect your entire ignition.