When should you replace a spark plug?

[Black Sheep]

I rarely replace spark plugs. If it's working and the electrodes aren't particularly eroded, I just check them and put them back. However, some service schedules say to replace plugs routinely. Subaru and a Mantis cultivator being my examples.
Any thoughts or advice on this? 

[Catz]

When i was a despatch rider i ran a set of NGK's on my old VT500 until they literally packed up and that was after about 23,000 miles, so i always replace plugs after no more than 10,000 miles now.
I was waiting for the day and so always carried some spare plugs in the top box.

[Colsbeeza]

BS, Unless they are playing up, I just clean and inspect on an irregular basis, whenever I feel like it or I am not happy with bikes performance, or some misfiring etc. I usually check them after any carb changes - jets etc. If they can be cleaned and show no significant erosion on the electrodes and no damage to the insulation, I will run them forever. Practically - 10,000 miles is about right.
I have had variable life from NGKs, but Champions OK.
Col

[CheeserBeezer]

The problem with spark plugs is the fact that our engines weren't designed to run on the fuel available nowadays. A spark plug will last maybe 20,000 miles in a modern engine. Unfortunately, many of us are running rich mixture or putting additives in the fuel to get the bikes to run right. If this is the case the plug insulator nose gets coated with stuff which will ultimately allow the spark to track, rather than jump the gap. Cleaning spark plugs is rarely effective so I find myself changing spark plugs every 2000 miles, or even less. In my opinion, the answer depends on how well your bike copes with modern fuel and to what extent the mixture has been altered to get your bike to run OK.

[WozzA]

    You stated a Subaru and a Mantis cultivator...   I'm thinking the wife's Subaru runs with Iridium plugs..    My old Falcon ute also had iridium's in & clocked well over 200,000kms in 20 years without a problem..

[groily]

I replace when the earth electrodes start to get a bit thin - which means far too infrequently probably!
They don't seem to soot up too much on mine luckily, running on 98 juice (which now is E5 here).
Think I only ever had 2 sets on my A in many years - Bosch and then a pair of B6HS.
The alloy heads hereabouts have always run happily on B7 or B8ES, occasionally over the years N3-5 with or without a C. The baby LE Velocette has been running tiny NGKs (I forget the spec) since 2009 - but hasn't done that many miles. The electrodes on them are like new still.
My XJR Yamaha started to misfire about 4 years ago after about 35K km and I had to replace the Denso plugs on that, which isn't a trivial 5 minute job.

Net of which is, I'm not big on replacing, but am quite big on periodic checks and gapping (on oldies).
Not sure whether I think plugs do better on mag-fired, trad battery / coil fired, or modern electronic systems.
(Modern cars, just dunno. Dunno where the plugs are even!)

[Rex]

I get that.
On my old Audi A6, when the costs and sensibility of dealer servicing became nonsensical for an older car, I had to look on Youtube for a plug change tutorial given the hidden-away location of the plugs, with coil packs etc on top too.
Easy job when you see how, but the old plugs were probably the factory original three-electrode type, and the central electrodes were severely waisted through erosion.
New plugs were fitted, but made not an ounce of difference to the starting or running of the old girl.

[chaterlea25]

Hi All,
My Super Rocket is running NGK's they do not seem to last long ?? I am convinced it is a fuel quality problem as it has shown up since E5 now E10.  The bike will be running absolutely fine, stop for a break or coffee and then no start. Replace plugs and it fires up instantly . No its not a mag issue. Other riders over here report the same .
I do not remember ever changing the plugs in my Wife's 2004 Mini Cooper, maybe once? Sold that with 140k miles on it
Our petrol 16v civics needed plugs reasonably often to keep fuel consumption  at it's best.. for the last 10 years cars and van are diesel

John

[muskrat]

G'day BS.
 I run NGK's in all my bikes but never put ethanol fuel in them, 98 only. The only time they play up is in the HD if I leave the choke on too long and soot up. I clean them every service and change them every second or third service, about 10-15,000 miles.
Cheers

[sean]

Hi All,
My Super Rocket is running NGK's they do not seem to last long ?? I am convinced it is a fuel quality problem as it has shown up since E5 now E10.  The bike will be running absolutely fine, stop for a break or coffee and then no start. Replace plugs and it fires up instantly . No its not a mag issue. Other riders over here report the same .
I do not remember ever changing the plugs in my Wife's 2004 Mini Cooper, maybe once? Sold that with 140k miles on it
Our petrol 16v civics needed plugs reasonably often to keep fuel consumption  at it's best.. for the last 10 years cars and van are diesel

John

I run champions in mine there are a lot of fake NGK copies go on u tube there  are several examples even NGK has a thing on their web site showing how to identify the fakes ....I dont think it would be profitable for China to copy the champions as they are only used in lawn mowers etc these days with most vehicles running resistor type plugs.

[BSA_54A10]

You can run them till the centre electrode wears away level with the insulaor nose  if you can bend the ground strap down that far to keep the gap correct .
As already mentioned they tend to foul and once fouled they can only be burned clean as the centre electrode no longer is glazed.
What I have found is if I turn the fuel off and starve the engine off the last time I stop then the plugs burn clean in the last few very lean stokes so the engine starts first kick next time
Current plug has been in there for 5 years so done about 50,000 miles   

[Black Sheep]

So I am probably right in my parsmonious way of running plugs until they obviously need a change. Back in 1970/1971 Champion N3s in my Norton would only last 10 - 20 miles. Any other equivalent was fine. Too cold a plug for the bike I expect but in these days I was into 'How fast would it go?'
The Subaru was given a set of Iridium plugs at the 20,000 mile service. Thought they would last longer than that.
The Mantis cultivator service pack includes a replacement spark plug so I fitted one anyway despite the original looking perfect. . It now starts first pull rather than 6th or 7th. Hmm.
Anyway, thanks for indulging my musings incurred whilst trying to keep the scorched remains of my garden alive.

[Slymo]

I think if your sparkplugs are the problem then your sparkplugs aren't the problem they are in fact a symptom. Since we stopped putting lead in petrol the life of a spark plug is pretty close to indefinite. They will eventually erode away but we are talking hundreds of thousands of miles especially with the relatively low voltage sparks produced by a K2F or a battery and coil. If they do foul then you have a mixture or spark generation issue and replacing the sparkplugs is only a temporary fix. Replacing a set of sparkplugs on an engine that is running and starting well as preventive maintenance i reckon is just an increasingly generous donation to your local parts shop.

[BSA_54A10]

Well Slymo I would argue that point.
How many of you have heard that a plug was "bad out of the box"
Well having actually made spark plugs I know that never happens unles you are buyig unbranded supermarket plugs
Every plug is tested as it is being made.
Not 1 in 1,000
Not 1 in 100
Not 1 in 10
Each & every plug get around 20 tests before it leaves the machine and then it is graded according to the results of those test .

However modern fuel ( in Aust at least ) is not petrol
It is a light fuel oil blended with whatever volatile scrap solvents that are available at the time
Stick your fingers in it and have a feel, it is greasy
Modern "fuel" is conductive at compression presures
Thus in a dead cold engine the droplets of fuel will condense on the plug and form conductive paths if the engine dose not fire .
Further more this conductive film can not be washed off easily so it stays on the plug allowing it to short

[Slymo]

Well Slymo I would argue that point.
How many of you have heard that a plug was "bad out of the box"
Well having actually made spark plugs I know that never happens unles you are buyig unbranded supermarket plugs
Every plug is tested as it is being made.
Not 1 in 1,000
Not 1 in 100
Not 1 in 10
Each & every plug get around 20 tests before it leaves the machine and then it is graded according to the results of those test .

However modern fuel ( in Aust at least ) is not petrol

 
It is a light fuel oil blended with whatever volatile scrap solvents that are available at the time
Stick your fingers in it and have a feel, it is greasy
Modern "fuel" is conductive at compression presures
Thus in a dead cold engine the droplets of fuel will condense on the plug and form conductive paths if the engine dose not fire .
Further more this conductive film can not be washed off easily so it stays on the plug allowing it to short

Hence my suggestion that it could be a spark generation issue. The carbon left by modern unleaded fuel (her in NZ at least) is super conductive as you say and is the death knell for any plug. I had an 860 Ducati with an original Ducati Electronica  spark generator that was failing and fouled at least a dozen plugs before I replaced it. My point which aligns with yours I think is that replacing a working spark plug as preventive maintenance is a needless expense and if spark plugs keep failing it is likely mixture or mag.