Revisions on 06/04/2014, 05/21/2017, 06/24/2018, 02/14/2021, by the author (Snowbum)
Numerous problems can occur if your engine’s spark plugs are not correct or not working properly; or, there are problems such as tight valve clearances, improper carburetor mixture adjustments or jetting, needle settings or worn needles and worn needle jet, worn engine items creating excessive oil burning, ETC. Spark plugs are ‘read’ by looking at the combustion chamber end of the spark plug. You can look at the color and deposits from combustion and determine a fair amount…with some practice. It is not just the condition of the normally near white or light tan or slightly orange color of the central electrode insulator, but the condition of the outer and central electrode and the color and look to the metal on the flat end portion of the spark plug, which is about 1/8th inch wide. Once you gain experience [it doesn’t hurt to ask SEVERAL supposedly experienced mechanics to confirm your analysis], you will find that much can be learned from a look-see. Spark plugs are looked at to determine if they are worn to needing replacement; to see if the engine seems to be operating correctly; and to get some idea of specific problems. Reading spark plugs is somewhat of an art, requiring experience. Experience is especially needed with modern unleaded fuels, most of which contain alcohol …and other ingredients …that make reading spark plugs more difficult, as the ‘old colors’ do not always apply. SOME things never change, however, so, read on. You should have a working knowledge of spark plug heat ranges, proper torque setttings, whether or not to use an antiseize compound on the spark plug threads; and what the proper spark plug caps are for YOUR motorcycle.
An in-depth article on spark plugs, problems with newer Bosch Spark Plugs; and chart of all three Bosch spark plug numbering systems; chart of Bosch versus NGK; and a listing of NGK spark plug caps, and much more, such as use of antiseize compound, etc., is here:
https://bmwmotorcycletech.info/sparkplugs.htm
Unless you have a very good reason for doing so, the stock factory heat range spark plug or equivalent, with the factory specified gap setting, is preferred. Some spark plugs have poorly made threads, some have extra grounding parts, some are not made well in other areas. DO NOT use multiple grounding electrode or split grounding electrode spark plugs, the advertising hype is just that, hype! In general, use of FINE WIRE spark plugs is also not a great idea (those are often iridium or platinum at the thin central electrode tip).
WARNING!…Bosch has been adding resistors to its non-resistor spark plugs, and this is NOT a good thing for your Airhead motorcycle. Bosch paper boxes and the actual spark plug inside may not agree about the resistance. Basically, do NOT use any spark plug that has an R in the model number, and see the spark plug metal body for that, do NOT depend on the box the spark plug came in. See the author’s website article for more details. The latest Bosch numbers are not informative without a chart or table to tell you what the number means.
The heat range of a spark plug, for practical purposes, deals mostly, but not entirely, with the white [when new] ceramic insulator inside the spark plug; its length before it contacts the metal shell, its material, and the material of the center electrode. Mechanics [aka “Wrenches”] typically think of heat range as being primarily controlled by the length of this insulator, and that is, for the most part, correct. Exposed to the hot combustion gases, the insulating ceramic heats up. It must get hot enough to burn away any deposits of small quantities of engine oil and combustion byproducts, yet not so hot that it retains so much heat that it is destroyed…that CAN happen…one CAN see ceramic tips that are fractured, have teeny bubbles, or otherwise damaged.
You do not want the tip to be excessively hot for another and VERY important reason: a glowing red tip can pre-ignite the mixture before the spark occurs….that can destroy an engine quickly. If the air-gasoline mixture is pre-ignited before the proper desirable time, the heat and pressures involved will…or can be …monstrous. This effect, under the poor circumstances, is called detonation, and can DESTROY an engine VERY rapidly. If you should see wee tiny round balls (usually) of shiny, or nearly shiny aluminum metal, inside the spark plug, you have probably been experiencing detonation, and that is aluminum from ….perhaps …your PISTON!……and this condition needs immediate fixing! There is another form of detonation, called pinging (or pinking), usually caused by wrong ignition timing or poor grade of gasoline. It can cause substantial harm if it is highly repetitive. This is a rather metallic sound, whilst the prior mentioned detonation is not heard, or is a much duller almost thudding sound. In general, pinging is heard well below 4000 rpm; and detonation usually occurs above 4000 on our Airheads. Pinging tends to occur not only from excessive engine spark advance timing; and/or poor octane of the fuel; but as one goes lower in altitude, the atmospheric pressure, and the amount of oxygen available in the air, increases, and cylinder pressures rise….making for higher combustion pressures. Carbon deposits make it all worse. Heavy use of the throttle at too low an RPM is called LUGGING, and can also cause detonation at lower RPM.
If the spark plug is too “cold” in heat range designation, it will accumulate deposits, probably not fire every time it is supposed to, cause the engine to lose power and get poor gas mileage, and possibly eventually short-circuit from deposits …and/or stop working properly. It is entirely possible for a slightly marginal ignition system, or very slightly problematical spark plug, etc., to waste fuel and give you NO symptoms. Note that a spark plug can act like a too cold spark plug if the fuel/air mixture is much too rich, this happens from worn carburetor parts rather too often.
A spark plug is called a hot plug [or, a hotter plug] when the insulator (or other design feature) is such that center electrode is or acts as if it is longer than some other spark plug, and that ‘other’ plug is usually taken to mean the stock factory plug or whatever was in the engine previously. This ‘hotter plug’ operates with a higher relative insulator temperature. A spark plug is called a cold plug [or, a colder plug] when the insulator is effectively shorter, and the spark plug insulator operates at a lower insulator temperature. There is seldom any reason to not use the manufacturer-specified spark plug for an engine. Substitutes for most spark plugs are available from other manufacturers than the original, and for the Airheads, the NGK plugs have proven to be excellent; and the Champions and some others have proven to be iffy, particularly the quality of the metal threads.
GENERALLY, for many manufacturers, a number in the spark plug model numbering, goes UP as the plug gets HOTTER. This was NOT so with the early Bosch numbers, that look something like W225T2; and, is not so for NGK plugs (older and presently). The later Bosch system with numbers such as W5DC, conform to the ‘larger number is hotter’ method. Now there is another Bosch numbering system using 4 digits, which are hardly very informative, compared to the older numbering system, which may still be in effect on the boxes (W5DC; W6DC; W7DC….and so on). A certain number in one manufacturer’s model number does NOT necessarily mean it has the same heat range as the same number in another manufacturer. In fact they almost always are different. When BMW specified more than one manufacturer, in your owners or service manual, they TESTED those plugs in their engines. If YOU substitute a manufacturer and plug BMW did not specify, and do that substituting from the new plug manufacturer’s cross-reference guide, you MAY find the spark plug is just a bit colder or hotter than the original; or, is otherwise not what you expected or wanted. In some situations, the substitute spark plug may cover, in one number/style/type, a wider or narrower heat range in itself. Some clever engineering went into broadening the effective heat range of any particular spark plug. The Bosch W5DC covers a rather wide range, same for W6DC, W7DC, etc. The widening of the usable heat range for any one model number of modern spark plug is small, but it IS noticeable, compared to very old style spark plugs. For Bosch, adding the C in the number meant a slightly wider heat range, although the C was simply announced as ‘copper’ (which transmits heat better than many other metals).
There is a fair amount of science involved in designing a spark plug, which appears on first glance to be a VERY simple device…in fact, there is a lot more to them than one might expect, and it is not my intention to delve deeply into the subject here. There MAY be occasion for SOME of you to want to use a different heat range plug & have the knowledge on how to do that.
The exposed tip (often called an ‘extended tip’) that BMW specifies is the correct style. Your Airhead MUST have an extended-nose type plug to perform properly. It is best NOT to have a resistor type plug (use of a resistor plug can degrade engine performance). Another type of spark plug that does not work well some of the time is the so-called ‘platinum’ or ‘iridium’ plug [with its very small diameter thin central electrode tip]. I also recommend against Bosch’s Platinum +4 plugs, with an extended nose. There is NO spark plug by ANY manufacturer, that has more than ONE ground electrode, or a split ground electrode (forked) that is any better than the standard plug, and such sales-pitched plugs are likely to be WORSE in your Airhead. This is especially so with the 4 grounding electrode plugs. There is a REAL engineering reason for the particular position in the combustion chamber of the spark plugs BMW specified; one is the effect on the incoming fuel/air charge…which is masked by multiple grounding electrodes…so, do not use sales-pitched plugs.
The BMW combustion chamber shape, the gas mixture swirl, and more, NEEDS the extended tip style of plug with ONE ground electrode. If you substitute, you are on your own as your own engineer, & performance will likely suffer, sometimes that is so mild that you do not notice except when refueling. BMW designed the combustion chamber so that the combination of mixture swirl and spark, and many other things, happens properly, together. In every instance I have tested multiple electrode spark plugs, either the engine responds a bit poorly, or, there is no response change I can feel, but fuel mileage goes DOWN, from faint misfiring. The incoming fuel charge is used by BMW to cool the plug tip area. This does not happen well at all with non-extended tip spark plugs. The shape of the combustion chamber, angle of the valves, ETC., all contribute to the nature of the swirling gases going by those plug tips. If the plug tip is not in the mainstream, so-to-speak, or is shrouded by grounding electrodes, things don’t work as well.
It is important that the Airhead spark plugs be installed with crush washers. The manufacturer’s prefer you use a fresh crush washer each time…not really 100% absolutely necessary, but does give more repeatable results IMHO. It is fairly important, unless you have very educated hands, to use a torque wrench….and to torque to proper specifications. If not torqued to [proper specifications, the heat transfer is not consistent. If overtorqued, you can damage the heads. BMW has used 16-18 foot pounds in most literature for its spark plugs. While I personally think that is too much, and 14-16 is more correct, what IS true is that BMW specs are for NON-oiled (NEVER oil spark plug threads!!), NO antiseize compound, on the threads. Once antiseize is used, you must CONTINUE to use it, as it works itself into the aluminum alloy of the head threads. Under NO circumstances should you use such a high torque as 18 footpounds with antiseize. My personal specification for the standard 3/4 inch length 14 mm spark plug installed in an aluminum BMW head, with a faint amount of antiseize compound, is approximately 15 footpounds. Use of a quality nickel antiseize paste is HIGHLY recommended by ME…although its use can be controversial, and is well-treated in my own website’s spark plug article.
For those with the 14 mm thread of 1/2 inch reach [length] BOTTOM spark plugs (dual plug heads conversion), the torque should not normally exceed 12 foot pounds with antiseize. There is a tolerance on this value, that depends on several factors. Primarily the lower torque is due to the thinness of the metal in the lower spark plug hole area and the lower amount of threads contact area, as the area is 1/2 inch, instead of like the top stock plugs which is 3/4 inch. Some folks use the 3/4 inch reach 14 mm spark plugs on top and bottom, with spacers on the bottom plugs. Some have welded a spacer to the bottom of the cylinder head, and that works really well (some thread the spacer to match the head threads, that is especially GOOD, provided it was begun from the combustion chamber side after the welding of the spacer), and normal torque can be used, and I recommend 15 ftlbs with faint antiseize compound being used. Some folks use a one-step HOTTER plug for the BOTTOM plug, due to its tendency to run slightly oilier/cooler. Some fewer folks have bottom spark plugs of a smaller thread size than 14 mm, but the selection is not great, and one must torque exceptionally carefully, with even less torque being used.
If you have dual-plugged heads do NOT insert the longer top spark plugs into the lower threaded holes without a spacer of some sort as mentioned. That, and overtorqueing at any time, WILL cause $$ damage.
NEVER EVER oil the spark plug threads to act as a lubricant…for normal installation….as the oil will carbonize from the heat….and you will have thread damage eventually.
HOWEVER: If the threads are carboned-up for any reason, and an old spark plug does not remove relatively easily…or, worse, seizes somewhat during the unscrewing, then apply a very thin penetrating oil or even kerosene, numerous times, let sit overnight, to soften the carbon, and then remove the spark plug carefully, perhaps even rotating inwards a bit, then outwards, adding lubricant…..then with plug removed, clean the threads, and remove the oil. Reshape the threads if needed. The best penetrant is 50% acetone and 50% automatic transmission fluid. It is proven better than fancy $$ penetrants. My testing shows that a top quality substitute is KROIL.
TEST RIDE AND ANALYSIS PROCEDURE:
It is best to begin with brand new spark plugs. Install them properly torqued. The ignition system MUST be in good condition, with resistor caps measuring OK, coils not having an open secondary winding, ignition wires fully inserted into coil towers, proper timing is set, valves set properly, and so on. Whatever carburetion jetting, needle, fuel level, etc., is in use…..don’t bother with road testing as described unless those carburetor parts are in good condition! One of the purposes of the Test Ride and Analysis Procedure is to get the carburetion jetting within a reasonably close range. There is no reason do do this except in special circumstances. However, there are times one needs or wants to change the carburetion jetting a small amount, and spark plug testing is or can be part of the procedure. NOTE: NEVER bend a ground electrode (to set gap) after the spark plug has been in use, even once!
1. You need to find a road that will allow you to do the testing. Yes, an adjustable dyno would be really nice…but they and their usage is costly, so it is the roads for most of us! You need to take your spark plug torque wrench with you; and, probably a regular spark plug wrench for removing the plugs…and, some thin gloves to allow you to handle the hot spark plugs. Try not to drop a plug…once dropped, even if it looks OK, it should be thrown away! DO NOT keep a dropped plug for ’emergency spare’. Finding the right road conditions may not be particularly easy! …especially if you live in a flat area.
For a really good evaluation, you need to do test runs separately in the 1/2 throttle, 3/4 throttle, and full throttle positions. With full mechanical carburetors, often referred to as slide carburetors, this is self-explanatory. With CV carburetors, and, yes, they also have slides…but also have a butterfly valve tied to the throttle…….. the throttle on the handlebars only indirectly controls that carburetor ‘slide’ assembly. Because of this, the rpm’s, and load must be considered rather a bit moreso than with the pure slide carburetor. The rpm’s need to be considered on the full mechanical slide carburetors too, but it is somewhat less critical for them. After many years of doing these tests, on both types of carburetors (and with fuel injection bikes too), I think that on ALL Airhead models the tests need to be done at ABOUT 5000 to 5500 rpm (R45 and R65 models at 6000 to 6500). Lower than 4500 does not work well. There is nothing critical about the rpm, but you need to be above a certain point on the horsepower curve, and what I recommend will work fine. Properly done, with the rpm being combined with the rpm setting, you are entirely operating on the main jet, or, only slightly on the slide needle jet and needle position at full throttle, and on the needle and needle jet at 3/4 throttle and at 1/2 throttle.
Keep in mind what you need to do: You need to find a road, hopefully half a mile long (better, a mile or more), which has such a UPward slope, that you can HOLD the selected throttle position (approximately 1/2, 3/4 or full) at APPROXIMATELY the rpm noted and desirable. Obviously you must select a transmission gear that allows this. The longer the road stretch, the better the spark plug readings. You CAN NOT do this test properly unless you can HOLD the throttle position. A nice long mountain pass with a goodly uphill gradient is my favorite.
2. Find that long hill, experiment with the gear selection and throttle, etc., until you can do the three tests thusly:
Ride at the specified approximate throttle setting and rpm for the mile or so (half mile is OK WITHOUT backing off the throttle, NOT EVEN ONCE!). At the end of the distance, snap the throttle full off and pull in the clutch and shut off the engine, trying to do these things all at the same time. Coast to a stop,…[in gear with the clutch pulled in if you have to]. …try to not let the rear tire drive the engine, hence the clutch being pulled in, as letting the movement on the road rotate the engine, with the throttle off, will tend to mask the spark plug readings.
3. Remove the spark plugs and take a look at the 1/8th inch wide metal circle at the end of the plug. I recommend all four of the plugs (on a dual-plugged model) be removed and looked-at, at least for the first test. There should be no sign of any melting/erosion of that outer metal electrode nor central electrode nor any insulator damage. Since you already know that your ignition timing and advance unit, valves, etc., are OK, any sign of plug problems is cause to suspect pre-ignition or detonation. The metal area at the end of the spark plug should have a very light blackening or a bit grayish. If a touch of dull black carbon is on these areas, you are in the ‘OK to slightly rich’ area. Slight to serious richness (really dark carbon) can come from many different reasons, which include very high float levels…or leaking float needles; a slide needle set too high, too large a main jet by far (at full throttle and maybe 3/4 throttle), etc. The performance is usually better with just a touch of light gray-black. If shiny black, that is oil fouling…likely from worn rings or bad valve guides. If the plug metal (and insulator) looks like new, you are probably running too lean.
If running really excessively lean, the bike may perform badly, and may well improve during a test run if you turn the enrichener (choke) ‘on’ “slightly”, seeing what effect that has, which can be a good test. Excessive leanness can come from not just the needle/needle jet setting or even size of needle, but from float level too low, leaking carburetor-to-head adaptors, too small a main jet, insufficient fuel flow from clogged tank screens or filters, etc. If really quite lean, the bike will probably buck some, that is, surging. The needle and needle jet normally wears and causes richness…but the aluminum needles tend to wear the needle such that the notch is broadened, and the needle sits lower, which is leaner. I have seen them hung up richer too.
4. NOTE that reading the ceramic center insulator is difficult, and requires practice, and the colors shown in various spark plug books may be wrong….gasoline additives vary a lot these days! Look at the ceramic insulator surrounding the center electrode. The condition of that insulator will tell you about the heat range of the spark plug (you had previously torqued the plug properly, didn’t you?). Analyzing heat range is a bit of an art, as is analyzing the metal in the previous step. Practice and checking with quite experienced wrenches is the best way to learn. The insulator should be almost white, but not like new. Over many miles, the insulator tip will gain some color, and the color is rather dependent on the make and type of gasoline. But, using a new plug, and the throttle test outlined, will not show that much color. The end of the insulator, and perhaps to about 3/16 inch or so inwards from the tip, should have a touch of color, this depends on the type of oil and the compounding of the gasoline, so color can vary. Typically it is a very light tan, but could be slightly grey-black, or even faintly yellowish or orangish. These colors all will be, if the mixtures are correct, rather subtle. Under NO circumstances may the insulator be dead white and/or blistered (blistering, or worse, teensy aluminum balls, if present, and sometimes one must use a magnifying glass to see it, is a sign of the coming serious damage you will soon experience). The center electrode should look good and certainly no signs of melting or eroding.
NOTE: An in-depth article on spark plugs, problems with the Bosch Spark Plugs; and chart of all three Bosch spark plug numbering systems; chart of Bosch versus NGK; and a listing of NGK spark plug caps, and much more, such as use of antiseize compound, is here:
https://bmwmotorcycletech.info/sparkplugs.htm
snowbum (#1843) HTTP://BMWMOTORCYCLETECH.INFO/INDEX.HTML
Revisions:
Updated 02/14/2021, by author
