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Spark plug life & failure


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Is anyone having issues with plugs on their TRS?  My bike (2017 280RR) would not run again last night at the trial.  The previous trial (2 weeks ago) it was fine.  I have fitted a new plug this afternoon and it runs fine again.  The "old" plug is less than a year old and replaced a failed plug that had also cost me a day's riding.

As I was loading up the van to go home another club member with a 250RR returned to the car park with a defective plug.  He said his bike "eats them".

I have ordered some more for stock and will make a habit of carrying one in the van.  Just wondered if this is a common issue or not?

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I would guess the bike runs a bit rich, the plugs always seem a bit "oily".  Last time round I changed the air filter as I thought that might be a factor.

I'm afraid two stroke is a bit voodoo to me.  But this is a good place to start - thanks for that.

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1 hour ago, faussy said:

NGKs are useless once oiled

I guess I would have to agree,  but what is any better?  Please spell it out cause we wanna know!  I have had NGK's fail on me at the worst times- like just before a ride and they look perfectly fine- maybe a little chocolate brown but if they get wet - flooded carb or whatever it's hard to get them firing reliably again.  By the way, NGK is phasing out the non- resistor B7ES and going to a B7 RES which I am running now- I really can't tell any difference with the performance and the price is still cheap enough.  I pulled the new B7 RES out to look at it and it was growing a metal "whisker" off the grounded tab that sits over the center firing pin, however, the whisker was not pointing towards the center pin at all- very strange.

BTW I'm running Amsoil 2 stroke "Saber" oil- full synthetic at 50:1

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9 hours ago, Tillerman6 said:

...  I pulled the new B7 RES out to look at it and it was growing a metal "whisker" off the grounded tab that sits over the center firing pin, however, the whisker was not pointing towards the center pin at all- very strange.

I remember my dad teaching me that whiskers were the issue with spark plugs.  He had a bantam to ride to work on as a young man.  I have owned four stroke bikes since my youth and spark plugs are not something I have given much thought to until now.

Many thanks to everyone that has given their feedback.  I think I will get fitted one of the knurled mixture adjustment screws and experiment a little.  I bought a couple more plugs off eBay but the ones that failed came from a specialist trials shop.  I have been running the BPMR6A from NGK and could not readily see an alternative.  But now I have a little knowledge to work with.  (And yes, that is supposed to be a dangerous thing I believe)

Edited by ChrisCH
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My 2 Penny/Cent:

If you use the right plug and right carb or electronic injection setting the plug will last very long.

In my personal experience older plug models are more tolerant / can be used in a broader heat range,

Plugs without R gizmo (inbuild resistor) last longer.

My oldest plug still working is mounted in my Bultaco Sherpa 199b, with is a nice pink colored LODGE plug from 1981,
which is has been used for 3500 km.

The plug seems to have been mounted when the bike was new and is still working nice, only minor wear and shows
with a clean big crisp spark, the plug even survived the engine top- end rebuild thus a complete piston, crank and rebore!

I have bought 10 of these old LODGE plugs so the next 200 years the bike is saved!

I only use standard NGK plugs without resistor and if in need I have changed the heat range to be aligned with the current
riding adjustments, thus have hotter plugs mounted to my SWM and the Montesa but a colder to my TY.

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I've been riding trs bikes since early 2017, had a new 17 rr and now have a 19rr. In all that time I've changed one plug. It didn't need changing I just bought a spare in case so fitted the new one and the one I took it is still in my rucksack.

As others have said mixture is probably the issue

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I would also be suspicious of the spark itself, as already said modern plugs don't work very well when fouled. My 350 AJS when running on a Lucas racing mag,(Which was in good condition) would go through a couple of plugs a year, I could always tell when the performance was dropping off. When I changed it to a BTH electronic mag I could run the same plug all year without even cleaning it. The BTH mag clearly had a stronger spark which could cope with a dirty plug. I proved it using an inductive pick up and viewing the spark trace on a laptop based oscilloscope. The Lucas spark was less accurate and weaker. Then when the condensor started to fail I could make the spark trace disappear by applying 100psi of workshop air to the plug. If you have access to this kind of kit it would be good to compare the HT trace of your bike to a similar one that doesn't eat plugs.

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On 5/27/2021 at 10:58 AM, lineaway said:

Back in 2018 my son went through 2 plugs on the first loop. It was a 20 mile loop. As soon as we removed the ball out of the check valve on the gas cap, he never has changed the plug again.

 The sticking ball can be hard to identify. By the time you change the plug, the ball could have released and you instantly think the plug fixed it. Get rid of the ball and that can no longer add to any problems. Besides it`s a free fix. Just pull out the inside rubber out and remove the ball. Some removal of plastic might be needed. Good luck.

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On 5/27/2021 at 4:00 PM, cascao said:

Main plug failture root cause is rich carb

That makes sense in a way- the fuel is ice cold and the spark plug is fairly hot, so the ceramic insulator takes a beating from the temperature changes I would think if there is excess fuel being sprayed onto the business end of the spark plug.  I did not have another failure on my newest NGK plug, but I noticed that it was dark brown and more or less wet when I pulled it out today- I ordered a new OKO carb yesterday which will hopefully solve both problems. I have recently been letting the bike run the carb dry when shutting down for the day.  I don't think this will change the plug readings towards a false "rich" wet looking plug. 

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