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Resistor spark plug caps?

Points Based Ignition | Electronic Ignition Upgrade
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GG Member
Posts: 124
Joined: Tue Apr 13, 2010 10:45 pm
Location: Costa Rica

Resistor spark plug caps?

Post by GG » Tue May 17, 2022 8:17 pm

Hi guys - I have a very strange symptom on my 1966 CB77 - the right spark plug lasts about 2 weeks and begins to foul at startup. Left side lasts forever. If I remove the cap and create a gap, the bike will run, but putting the cap back on makes it stall. I do not have resistor caps. Should I use a resistor cap? Like I said, the left side gives no trouble!

PS - I have the spark tester tool, and the spark jumps a huge gap. Doesn't seem to be a problem with the coils or wires (wires have been trimmed just for good measure - didn't help).

Bill Green
Costa Rica

deuce_7 Member
Posts: 40
Joined: Sun Mar 18, 2018 10:36 pm
Location: California

Re: Resistor spark plug caps?

Post by deuce_7 » Sat May 21, 2022 4:04 pm

While troubleshooting:

Inspect the spark plug cap for any small cracks, which can divert the high voltage electrical path.

Might be time to replace the high tension (HT) spark plug wires. The HT wire insulation can break down and the wires can wear out, boosting internal wire resistance. Spark plug wire resistance can be tested with a multimeter.

When reassembling the HT wires, insure end terminals are secure and anchored into the HT wire core.

All the above assumes coils are good.

Sounds like you've addressed all of the above. Double check?

How is the compression in each cylinder? Valves adjusted? Timing is accurate for each cylinder?

GG Member
Posts: 124
Joined: Tue Apr 13, 2010 10:45 pm
Location: Costa Rica

Re: Resistor spark plug caps?

Post by GG » Thu May 26, 2022 12:12 pm

Hello! thanks for the reply. Have done most of the checks mentioned. The wires are perfect - virtually no resistance. I do have resistor caps and am looking for some non-resistor. But I don't think it is that, because the bike has always run great with these caps. I'm thinking the condenser is on its way out. I've swapped coils - same results. I can't figure out how to test this condenser with all the different leads! :\ Have a great day!


Geoff Hastings Member
Posts: 313
Joined: Wed Feb 22, 2017 2:59 pm
Location: Epsom, Surrey

Re: Resistor spark plug caps?

Post by Geoff Hastings » Fri May 27, 2022 2:33 pm

It is possible to test a condenser or, to give it the name used in the electronics game a capacitor. Trouble is, you need a capacitance meter and unless you are testing lots it would not be worth it. Cheapest option is to swop it.

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