spark plug & spark plug cap resistor

[Guest]

Can anyone please try to explain me why there's a resistor in a spark plug + one in the spark plug cap?

And in addition: how is it that a spark plug (or cap) can get bad? I seem to have a bad spark plug every now and again, I don't even understand really how they can go bad — it's just a fat copper wire (and a resistor) going from top to bottom...

thanks!

[dantodd]

typically you don't run a resistor plug when you are running resistor caps.

[crookedspoon]

@dantodd: sure, should've been clearer with the question: why is the resistor there at all? Why does the current from the coil needs a resistor before generating a spark. Seems like there'd be more energy to spark without it.

[dantodd]

usually the resistor is there to protect electronic parts, like electronic ignition, from RFI.

[crookedspoon]

that makes sense — didn't think of that! cheers!

[eil]

The resistors cut down on RFI like dantodd said. That's their only function, but it's an important one. A vintage bike will run just fine without them. But good luck trying to use a digital voltmeter or most kinds of electronics anywhere near it. And you'll annoy any neighbors who watch broadcast TV, listen to the radio, or use baby monitors. In rare cases, RFI will set of smoke alarms too.

The resistors/contacts inside spark plug caps can get corroded over time, leading to weak spark at the plugs. On my 850, the resistors in the caps were pretty much shot so I replaced them with pieces of metal (cut-off machine screws) and used resistor plugs instead.

Spark plugs don't normally go bad, but they do erode over tens of thousands of miles. If you suspect a newer one of "going bad", it could be corrosion in your caps or where the wire plugs into the cap.