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Friday, 20 September 2013

Plug problem

Anyone had a problem like this?
I've got a white Traveller that I bought about five years ago, but have never ridden. It was so cheap at the time that I couldn't pass it up, and over the years I've tidied it up. Because I haven't ridden it, I start the engine every two weeks or so to circulate the oil around the engine. I usually run the bike for about five minutes, or until the fans cut in.
Back in June when I was servicing the silver Traveller before its holiday, I also serviced the white bike, and thought I would try an 'Iridium' sparkplug in it (NGK DPR9EIX-9).
When I fitted it I checked that the gap was correct (0.9 mm), and the bike started and ran normally. Since then I've started and run the bike 4 or 5 times, so it's only run for 20 - 25 minutes since the new plug was fitted. However, last week it wouldn't start and only 'popped' a bit and wouldn't 'catch'. I tried three or four times, even spraying 'cold start' (Di-ethyl ether) into the airbox, but the battery would flatten without it starting.
I had time this week to have a look at it and took the plug out. The gap had increased to 1.5 mm! Here's a photo of it (cleaned up a bit) next to an ordinary plug.

If you position the plugs with the outer electrodes against the ceramic, you can see that the inner electrode has eroded away.

So why has it eroded so much after such a short period of running?
Interestingly, way back in 1979 when I had a Yamaha SR500, I fitted one of the 'super plugs' of the time. I think this was Palladium rather than Iridium, but it had a similar narrow central electrode.
A couple of days after I fitted the plug I was riding through town when the bike backfired then ground to a halt. I couldn't get it started again so pushed it about half a mile home. I took the plug out to find that the central electrode had completely vanished!
It's strange that so many years apart, the only times that I've used plugs with narrow electrodes made of exotic materials, they have failed the same way in a very short space of time. In both cases they were in Yamaha single cylinder engines, and both were the manufacturer's specified plug for that bike.
Any ideas?


  1. Have used these plugs in all my bikes for years including a Skorpion, and never seen anything like that, the centre electrode looks ok but the other looks to have opened up, not sure how that could have happened.

    1. It's hard to see on the photo's but it looks like the tip of the earth electrode is missing, as it seems to not cover the centre electrode, or it's just been bent out of shape.

  2. Could be a counterfeit plug perhaps. Ebay purchase?

    My Skorp's running an Iridium and has been for the last five years, there's been some hard miles in there and it's not missed a beat.

    1. Good Point! I hadn't thought of that. I bought the plug a couple of years ago from Ebay, so it could have been counterfeit.