Jump to content
Sign in to follow this  

Have you ever heard of a spark plug failure?

Recommended Posts

I recently added a 2002 RM125 into my stable of bikes. The stock spark plug retails for $35 something dollars. We opted to replace it with the regular NGK for around $4.

This particular motor has had quite a bit of work done to it by a very reputable company (WMR) www.wmr1.com and can really scream for a tiddler. It is actually a 134cc "cheater" motor. Don't worry I am a vet. , so no, I am not cheating.

I was doing some recent riding and it was running fantastic after fattening up the jetting due to the cooler temperatures. After about 15 minutes of riding, the bike lost power without warning and rolled to a stop. It would not fire back up.

The motor felt like it had much less compression than usual when kicked through. A top-end teardown revealed that there had been a failure of the sparkplug. The porcelin around the inner electrode (underneath the bent strip of metal) had broken off and came loose in the top end. Porcelin is very hard and gouged the top of the piston, the head and hammered the gap in the plug closed.

I called WMR and was told that while this is uncommon, it was because of the cheaper plug.

It is no big deal, I am not out for blood over it, the top end probably needed some freshening up anyways. WMR has been fantastic with offers to fix things.

I just wondered how many other people had encountered this situation. I have been riding bikes for 30 years and never had it happen before.

Thanks for your time and responses. :)

Share this post


Link to post
Share on other sites

I understand where you are going with this but ask yourself how would the bike run great for 15 minutes if the plug were too long?

The plug was a correct replacement, just a cheap version.

Share this post


Link to post
Share on other sites
I understand where you are going with this but ask yourself how would the bike run great for 15 minutes if the plug were too long?

The plug was a correct replacement, just a cheap version.

...OK then does the seat have a taper to it or is it a flat seat? maybe the sealing washer could have been left out and the piston is just barley "kissing" the plug. Or maybe the plug was dropped and the porcelin was fractured to begin with.. :)

Share this post


Link to post
Share on other sites

I have had a similar thing occur on a car motor but that was caused through the head gasket leaking and the cooking coolant over pressuring the cylinder. The ceramic broke off and the metal around the base of the plug was worn and eroded away.

Down here the plugs cost about $125 (Aussie). The cheaper plug (even in the same brand) can't take the demands of the motor and are not specified to be fitted for the reason you discovered.

If you can find an old mechanics workshop they usually have hidden away one of those old spark plug cleaners. It used sand/grit to sand blast the plugs back to new. I used a local bloke for all my racing plugs when I had a 2 stroke and it only took a couple of secs to do each one. His machine had a viewing chamber where you could insert the plug, pressurise it and put a spark through the plug. The difference in before and after to the sparks was amazing.

Plugs generally foul right down deep in the bottom where the ceramic meets the metal body. Not where we try and clean them at the exposed electrode. Don't toss those plugs. They're too expensive and you can get years out of them by cleaning them the old fashion way.

Share this post


Link to post
Share on other sites

The inner porcelin insulator around the electrode is much smaller on the $35 dollar plug than on the $4 one. The plug was definately not dropped and there are no markings on the top of the piston directly under where the sparkplug electrode tab is located. The piston and head gouges that I spoke of earlier were nearer to the sides of the piston and actually caused a minor ring failure(thus resulting in the loss of compression). I was very lucky not to damage the cylinder. The surface area where the break occured shows two distinct fracture lines. The piece could not have come off if it had been broken off like a doughnut ring as the spark gap is not large enough to allow it to pass from off of the

I am assuming that the higher compression of the motor may have cause the porcelin piece to cavitate and break off in chunks. Just my theory.

I surely won't go cheap again. :):)

Share this post


Link to post
Share on other sites

yes,its very common.ive seen it a few times.ive seen the electrodes come off as well.thats why they spec's the high dollar plug.for whatever reason the harmonics of those motors was strange.

Share this post


Link to post
Share on other sites

Huh? where are you getting the $125 from, a stock replacement for that much i doubt it. I paid $30 for an Iridium the other day.

Share this post


Link to post
Share on other sites
Huh? where are you getting the $125 from, a stock replacement for that much i doubt it. I paid $30 for an Iridium the other day.

That's what a dealer quoted me. It's not just an iridium plug but a specialised unit for that bike (low volume = high price). I havn't got an RM but I was buying some plugs for varied bikes and when I asked the price for the RM plug I soon changed my mind. The dealer said they were about $125 and I didn't push for an exact price. That scared me enough. If you can get them for $30 I'll have one.

Share this post


Link to post
Share on other sites

You can actually get them for $21 through Rocky Mtn. atv/cycle supply. Still, twenty plus dollars before shipping :p:) .

:) It is cheaper than rebuilding a top end every 15 minutes though.

Share this post


Link to post
Share on other sites

I have seen this quite often, especially in 4 stroke nitrous motors. Detonation can cause this to occur. It was probably a defective plug. I have never seen any real damage though from it. It usually just breaks the porcelin up and does'nt do any harm. If it was a new plug I might contact the manufacturer and see if they will step up and pay for part of the motor.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×