Use an Ohm meter to check the resistance through your spark plugs when you have them out for cleaning. Non-resistor plugs should be zero Ohms, most resistor plugs at 5000 Ohms. If not close to the above figures then it’s time to replace them – perhaps try a different brand!!!
You are measuring the resistance through the plug from the top to the tip of the central electrode – along the route the spark goes.
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