Spark Plug Inspection and Maintenance

  • August 13, 2009 8:28 AM PDT

    In addition to replacing the spark plugs at the manufacturer's recommended intervals, spark plugs should be removed, inspected and cleaned (if necessary) every 5,000 miles.  The tips below can be used on all spark plugs whether they are on your motorcycle, car, boat, or event your lawn mower.

    If the plugs exhibit normal wear as indicated in picture #1, you can simply clean and re-use the same plugs. If the spark plugs exhibit extreme wear or any of the wear features pictured below, the plugs should be replace and the necessary repairs adjustments should be made to the motor.

    To clean the plugs simply remove all debris with a fine piece of sand paper, rinse with gasoline or engine cleaner, check the plug gap (adjust if necessary) and re-install.


    Normal Spark Plug Wear

    This plug has been running at the correct temperature in a "healthy" engine. Operating in such a desirable environment results in deposits that will be light tan or gray in color with most grades of commercial gasoline. Spark plugs demonstrating normal wear should still be changed at the manufacturers recommended intervals.



    Carbon Fouled Spark Plug

    Carbon Fouled Spark Plug

    Basically, soft, sooty carbon deposits, as shown , have a dry, black appearance. Often indicates bad ignition wires, an incorrect heat range spark plug or an over-rich air/fuel mixture caused by a clogged air cleaner filter element, or a faulty choke. Fuel injectors that malfunction can also lead to this condition.

    Other less common causes include weak ignition system voltage or poor cylinder compression.


    Pre-Ignition Spark Plug Wear


    As shown , this condition produces melting of the center electrode, and somewhat later, the ground electrode and insulator. Usually one or a combination of several engine operating conditions are the prime causes of pre-ignition. It may originate from glowing combustion chamber deposits, hot spots in the combustion chamber due to poor control of engine heat, cross-firing (electrical induction between spark plug wires), or the plug heat range is too high for the engine or its operating conditions.


    Overheated Spark Plug Wear


    Note the dead white or gray insulator nose which appears "blistered." Electrode gap wear rate will be considerably in excess of that normally expected. This is often caused by overadvanced ignition timing, poor engine cooling, a very lean A/F mixture, a leaking intake manifold, or the use of a spark plug too hot for the application.



    Spark Plug Mechanical Damage

    Mechanical Damage

    Mechanical damage to the firing end, as shown, is caused by some foreign object in the combustion chamber or the plug nose is too long for the engine (incorrect plug selection). Since small objects can travel from one cylinder to another (because of valve overlap), the other cylinders should always be checked to prevent reoccurrence of damage.



    Oiled Fouled Spark Plug Wear

    Oiled Fouled

    A spark plug shorted by excessive oil entering the combustion chamber is shown. This is often caused by piston rings or cylinder walls that are badly worn. Oil may also be pulled into the chamber because of excessive clearance in the valve stem guides, or badly worn valve stem seals.



    Broken/Cracked Spark Plug Insulator

    Broken/Cracked Insulator

    Typically caused by improper handling of the spark plug prior to or during installation. Broken insulators can also be caused by severe detonation. Major causes include a lean air/fuel mixtures, ignition timing advanced too far, and insufficient octane rating of the gasoline.


    This post was edited by Lucky at March 2, 2017 10:00 AM PST
    • Moderator
    • 1364 posts
    August 13, 2009 10:21 AM PDT
    Very nice . Great maintance tip
    • Moderator
    • 18965 posts
    August 14, 2009 12:02 AM PDT
    DITTO Thanks for the posting.
  • g
    August 15, 2009 6:54 AM PDT
    got new plugs a few months back ,my kawasaki dealer was wanting £40 for 4 plugs .got them from a lawnmore site for for about £10 ,same things .
    • 844 posts
    August 20, 2009 5:07 AM PDT
    Good stuff. One thing to mention on spark plugs, is how the altitude effects the bike and spark plug wear. If your Harley Davidson, or any motorcycle for that matter, is carburated and tuned for lower altitudes, you will almost definitely experience the Carbon Fouled Spark Plugs seen above. Really no way around it unless you have the carb re-tuned. So my suggestion is to carry some spare plugs and a spark plug wrench if you are planning on some mountain riding.
  • September 12, 2009 1:09 AM PDT
    To answer your question should you clean them-NO.if you leave just a tiny pit of crap on the plug and put it backin you are putting that crap directly into the cylinder and that is not good.Take um out and look at them if you want to but don't clean them in my opinion.
    • 0 posts
    January 31, 2010 11:52 AM PST
    Also keep in mind that most heads now are aluminum. Let the engine cool all the way down before removing them. Apply just a touch of antiseize to the threads when installing them. Don't over torque but above all do not under torque! If they do not seal, then gas's get by the threads and get carbon in them and next time you go to pull them out they bind and it you force them out you will take the threads with them.

    Also if you have a newer bike with Platinum plugs and fuel injection, you only need to do this every 18-20k.
    • 513 posts
    January 31, 2010 12:20 PM PST
    If you have dirty plugs and are having misfire due to say wet weather and water getting sucked in through your carb(older bikes here) if you carry one of those metal nail files wimmen usually have you can whip out the plugs and give them a quick clean that will get you home.
    I only ever had to do it once and that was about a mile from home after three hours of monsoon type rain.
  • February 5, 2010 1:37 AM PST
    Using premium fuel how many miles do plugs last in a late model Harley?
    • 5417 posts
    February 5, 2010 1:44 AM PST
    Well the manual says to inspect every 5,000 and replace every 20,000. With a well tuned bike and good clean fuel, I imagine 20,000 would be fine. I normally change mine when if I am going on a long haul and they have more than 10,000 on them, so I have never really pushed it to the limit to see how far you can go.
  • July 1, 2011 7:09 PM PDT
    My bike has had the same plugs in it since 1972,points and condensers too.
    Its a Honda CB 500-4 K1 model.
    44,000 miles and no visible wear,or lack of performance,will change everything at 50,000.
    Keeping your oil changed is the secret,and using good filters at every change(2500 miles) for my preference.
  • July 2, 2011 4:08 AM PDT
    I look at plugs like I look at changing the's cheep and easy to do so a little overkill (changing too often) don't much hurt the pocket book and darn sure can hurt the bike!
    • 5417 posts
    July 6, 2011 3:45 PM PDT
    Yep, I agree there Russ. That's why I typically change mine right before the riding season or any really long trips.
    • 3006 posts
    July 7, 2011 11:30 AM PDT
    It seems to me like the best way find out if your bike is running properly,is pulling the plugs every so often,Am I taking a big risk doing that? I have even stopped on roadside to read them & clean /reinsert..
  • July 7, 2011 12:14 PM PDT
    blurplebuzz wrote...
    It seems to me like the best way find out if your bike is running properly,is pulling the plugs every so often,Am I taking a big risk doing that? I have even stopped on roadside to read them & clean /reinsert..

    The only problem I see with doing this on the side of the road is if you happen to break something while performing this check your now stranded!! and number 2 and the big reason is that your motor would be hot and I lhave always thought that you wanted to change out your plugs with a cold engine, so that the metal was not expanded and you would not strip or destroy the threads on your cylinders when you removed the plugs. I dump a set of plugs into my bike just before a long trek and about every 10k miles if I am not doing any long riding. Fuel injected bikes run lean and clean and spark plugs are usually not one of the problem areas for newer bikes anymore.
    • 3006 posts
    July 11, 2011 1:44 PM PDT
    Never had a plug break due to the engine being hot,usually I wait about 15 minutes before yanking it,being a metric watercooled bike it seems to cool down a lot quicker, I imagine air cooled motors have a lot longer time before its safe to pull a plug.I also have always carried an extra set in the bags,just in case I find one that isnt working or gives me a problem.A riding buddy has had the same set of plugs in his bike for over 20000 miles,and when he pulled them he said they looked almost brand is a metric same model as mine.
    • 5417 posts
    July 12, 2011 4:41 AM PDT
    Out of habit from the old carb days I still carry a set of plugs with me. Back then with my bike tuned at sea-level, seems like every time I would get up in the high altitude (7,000 feet or so) I would foul plugs. Even though I still carry them, I have never had to replace plugs on the road since I've had EFI.

    Like wheels said, I am not a big fan of opening anything up or taking anything off (including plugs) just to inspect them while on the road. You just never know what may happen - damage a wire, drop a part, break a part, etc - and normally the tools and spare parts you have on the road are limited.
    • 1161 posts
    December 21, 2011 12:08 PM PST
    Just what I want to replace a long with my wires. Thanks for the advice!
  • December 31, 2011 3:22 AM PST
    When in doubt, change 'em out!
    • 1855 posts
    December 31, 2011 4:23 AM PST
    A bit of side note. My '98 FXDWG experienced popping and cracking when ever I rode out to the Black Hills. Most specifically when not at cruising speed; coming into towns, traffic, etc. Changing plugs wouldn't have any affect actually. Changing the needle valve would but none of that is necessary IMHO. On this particular EVO, all that was necessary was to pull out the enrichener (choke) a bit. Once out on the road at cruising speeds that may or may not be necessary. However, darn good idea to change plugs on the return trip to lower altitudes.

  • January 3, 2012 5:09 AM PST
    Mukuni carbs are great for the changing altitudes because of the little enrichener nob I had the HSR 42 on my 94 Wide Glide and boy did that make a ton of difference going through New Mexico on those high Mesa areas and making the trip into Taos.
    • 834 posts
    March 6, 2014 1:48 AM PST
    Are spark plugs lasting a lot longer in new motorcycles or am I just lucky. I just pulled my plugs that have 18,000 miles on them and they look like new and the bike is still performing great. Should I change them anyway
  • March 6, 2014 2:48 AM PST
    Mine foul about every 1K, but I run my low speed jets rich to help cool the engine in summer. Like bikercrap they're cheap and to easy
  • March 6, 2014 2:52 AM PST
    My old CR480 went through plugs every 30 min, but then again it went through piston and rings every 3hrs and rear tire every 2 months
    • 3006 posts
    March 7, 2014 5:32 AM PST
    I just recently changed my plugs,after putting about 20,000 on em.Being on a budget doesnt help when it comes to riding my scooter.LoL.Mine were wore out,and I had cleaned & regapped them 3 times.My ride is due for a major overhaul,would love to tear it down and do some mods yet the funds are not there rite now.