Thread: Spark plugs - what fails?

Hi,
Camped up the scrub for a while the battery was starting to get a bit low, solar hadn't been enough, so it's out with the trusty Kipor generator to put a few Amps back in the battery to run the diesel heater for a bit of warmth.

Normally it starts after one or two pulls, but lately it needed a few more.
This time? No way was it going to start.

I did notice the low-oil light glow when I was pulling the cord, but only when the generator was tipped back a bit so I could get a decent heave on the cord.

I left it at that with the plan to check it at home.

OK, change oil, clean and check gap on plug. No joy!


Note plug number and drive to shop for new plug, but its not that simple.

Repco couldn't find any reference to the plug. Grrr! So buy a can of Start-Ya-Bastard!

Still no joy!

Check micro switch under the engine on-off switch, check low oil light does not light on cranking, shake oil around to make sure level switch has re-set.
Clean and re-gap plug.
Apply Start-Ya-Bastard as per instructions.

No Joy!

Checking the manual shows an NGK plug is the recommended brand and its number.


Back to Repco - Yay! they have this one on the 'puter, two in stock; but they can't find them!

Local mower shop has one! ($2 dearer, but what the heck)

Put in plug, pull cord, and away it goes immediately.

Now I did check the old plug for spark by resting it against an earth (screwdriver reaching down into the plastic cover) and there was a feint spark there.

So my question is what actually fails in a spark plug?

Surely cleaning it (so resistance is not too low) and confirming the gap is correct (so resistance is not too high) should ensure it is in the Goldilocks zone of spark plug happiness.

What am I missing here?

cheers

As suggested above, the usual failure mode for plugs is electrical leakage. This is most likely due to soot, lead or combination deposits on the conical bit inside. But almost impossible to see in many cases is a cracked insulator, which, if the crack is hidden inside the steel body, is going to be impossible to see. This could be due to external damage, or thermal stress plus a manufacturing fault.

John