Spark Plug Replacement Interval

Anyone have an idea why Honda recommends a spark plug replacement interval of only 8K miles ?? I have never heard of a replacement interval that low for any automobile, but I don't have any idea what other motorcycle makes/models recommended interval is.

BTW, my ST1100 started running poorly after I had only about 10K miles on the plugs ( standard type ) and new plugs cured the problem. I have about 8K miles on those new plugs, now.

As a side note, I think I got a bad tank of gas ( water ? ) last Monday on an IBA SS1K ride. It sounds like it may have been running on only 3 cylinders at lower speeds, but ran much better at higher RPM's. I added 8 oz. of dry gas ( isopropol ) and after about 40 miles ran much better. I filled the tank today and added 10 oz. of home brew Seafoam. Took it for a 25 mile ride and it runs great now. I suspect I may have had a partially plugged low speed jet.

Comments are welcome, especially about the spark plug replacement interval. There must be a reason why Honda has that recommendation.

No offence intended Jim, but recommendations on replacement parts are intended to bring customers into dealers for maintenance services and, like yourself, many will change out good parts just because the manual says so. I use the OEM NGK plugs, have for over 20 years and I run them for 30,000 miles usually and only change them then because I feel I've had my money's worth from them, not because of any noticeable reduction in performance. I do take them out every so often for a cleaning and gap inspection though. Air filters go about the same distance, depending on how clogged they appear to my trained eye. I think Honda says 12,000 miles for them.

So the short answer is mo' money for the selling dealers.

Odd, that. It's inspect at 8k miles and replace at 16k for the St1300.

I'm now waiting to hear from Uncle Phil & John.

At a tech event a few years ago I was helping a guy who had 75k on his ST1300. Turned out the air filter and plugs had 75k on them too! The air filter had half and inch of oily crud on it and the plug gap was like 080. But the bike ran like a watch. Since the dealer was only a couple of miles away we got a new air filter but they didnt have the right heat range for the plugs. So we cleaned and gapped the plugs and put them back. Bike ran just as good as mine. Probably still is!

Jim Van said:

Anyone have an idea why Honda recommends a spark plug replacement interval of only 8K miles ??

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Any manufacturer recommendation is based on an average of circumstances of some sort. Sure the cynics will say they're only cash grabs. Some may be. But they do have a warranty to protect so there may be actual testing involved.

My take is that most motorcycles operate at generally higher RPM than most cars and may be ridden much harder than most cars. I don't know that manufacturers take into account the amount of miles over time compared to cars. Maybe they do.

Also some bikes fire the plugs on the exhaust stroke or at least other than the compression stroke. I don't know if any cars do so.

So maybe it's a cash grab. Maybe Honda could say 10Kmi since that's what you got out of your plugs before experiencing a loss of performance.

Or maybe it's a sleazy marketing thing where Honda wants your plugs changed (not inspected?) at 8K so there's no loss of performance noticed in the first place to besmirch their name.

I don't know. Inspect/clean/replace as you see fit and don't think about it. And iridium plugs last even longer (don't know if they ship with any cars/bikes but maybe).

ST Gui said:

And iridium plugs last even longer (don't know if they ship with any cars/bikes but maybe).

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My 2001 Accord had iridium plugs OEM and the manual said to change them at 192,000 km (IIRC), so go figure. Changed those when I got to that mileage, which took about 10 years!

Bush said:

My 2001 Accord had iridium plugs OEM and the manual said to change them at 192,000 km (IIRC), so go figure.

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Nothing to figure there. And as I think of it though without direct knowledge of plug type my FORD manual says 100Kmi for a plug change. So long intervals for most cars/trucks is probably much longer than I thought (whatever that was).

Bush said:

My 2001 Accord had iridium plugs OEM and the manual said to change them at 192,000 km (IIRC), so go figure.

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ST Gui said:

Nothing to figure there.

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What I meant by that was, it is quite reasonable to change out an iridium plug at well over 100,000 miles, but for Honda to say the ST1100 needs new plugs every 8,000 miles is ridiculous, regardless of what type plug they are.

Got a set of iridiums as a WeSTOC door prize many years ago and installed them. Put a replacement set in a couple of years ago, after many tens of thousands of miles. Just because... The old ones looked fine. It'll be a long time before I even think about plugs again. FWIW

John

A couple of years ago I had trouble starting my 1990. I had spark, I had fuel, and air. I asked my mechanic what to do, he said put in new plugs, I said I've got a strong spark, he was adamant, and I did as I was told, installed them and it started right up. When I bought my 97 two years ago, it wouldn't start, installed new plugs and off she went. I read that even though you may have good spark the ceramic insulator fatigues and affects performance of the plug. That may be the reason Honda suggests replacing them at the stated intervals.

It is my best guess this "spec" was written back in the late 1980s before iridium plugs were the norm and plug replacement/inspection is super easy.

I suspect the 8K replacement interval has something to do with the plugs being only 10 mm in diameter. There isn't much space between the insulator and spark plug body.

I had a set of Autolite iridium plugs in the bike at one time and checked the gap at around 12 K miles, and the gap was about .040" due the center electrode being eroded. I have a set of Denso iridium plugs and I'll try them and see what they look like after 10K miles.

But if I'm going to pull the plugs every 10K miles to check, I might as well go ahead and replace them with standard plugs. Standard plugs are cheap.

kiltman said:

A couple of years ago I had trouble starting my 1990. I had spark, I had fuel, and air. I asked my mechanic what to do, he said put in new plugs, I said I've got a strong spark, he was adamant, and I did as I was told, installed them and it started right up. When I bought my 97 two years ago, it wouldn't start, installed new plugs and off she went. I read that even though you may have good spark the ceramic insulator fatigues and affects performance of the plug. That may be the reason Honda suggests replacing them at the stated intervals.

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This is what I needed to know !!

Jim Van said:

This is what I needed to know !!

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I suspect it's more what you needed to hear.

On my Honda motorcycles I have taken to replacing the plugs and air cleaners as a set at 25,000 mile intervals. The air filters are probably over due. The plugs still look good with a little larger than should be gap, Could probably clean and re-gap the plugs and put them back in, but I feel better putting in fresh plugs.

BTW my CB 1100 calls for pulling the plugs every 8K too. My first set were replaced (as well as the air cleaner) at 25K as I am wont to do.

I also use the iridium plugs in my 2 STs- I have no intention of changing them for lots of miles- when I had my Acura (and yes, I know bikes will run at higher rpms overall and 'wear' them more quickly) the interval for changing them was 100,000 miles. They're not terribly expensive (maybe $35-40 for a set of 4) and then it's one and done. I'd recommend iridium plugs and leave them in very long term.

My car has 12mm plugs and I change them every 50K miles. I think the 10mm plugs makes a difference.

Bush said:

I suspect it's more what you needed to hear.

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Mr. Bush :
It sounds like you don't want to believe the klitman's experience ( twice, no less ! ).

My 97 was offered for sale in a not running condition. The price was advertised $1350 Canadian. It was a California model with ABS, Corbin seat and PIAA driving lights as well as having a GVI top box. All I cared about was would it turn over. The battery was dead, I boosted it and it turned over. That's all I cared about as I suspected that all it needed was a good battery, carb cleaning, and new plugs. I ended up getting it for $1150. When I got it home I pulled the plugs, inspected them and made sure they were clean and properly gapped. It wouldn't start. I took a gamble put a fresh set of plugs in it and it started up right away. I was a happy camper. I then invested in changing fluids etc.
all this is anicdotal. I was just throwing it out there. YMMV