Spark Plug Caps for Points and Electronic Ignition; cautions, etc.
On the early Airhead points type models, spark plug caps contained a resistor of 1000 or 1200 ohms. Using a resistor that is part of the cap is a better method than using highly variable (between manufacturer’s especially) “resistor” type spark plugs, in which some of those resistors may be coils. Resistor caps reduce Radio Frequency Interference, and help form the proper type of electrical spark. Later Airhead points models and all electronic ignition models used 5000 ohm caps, to additionally reduce radio interference and to also protect the 1981+ electronics ignition.
The spark plug cap resistor has other purposes:
It reduces spark plug tip and ground electrode erosion, reducing gap change over the life of the spark plug, and does this by reducing the electrical current flow. It does NOT reduce the voltage. Works in a complicated electrical way with the coil to produce a quality spark at the spark plug for good fuel mixture igniting. Note that if the resistance was too large a value, then the EFFECTIVE spark ENERGY (combined voltage, current, and with SOME time effect) will be lessened too much. This means that using resistor spark plugs WITH resistor caps is a BAD IDEA. Same for using 10,000 ohm caps.
On the electronics ignition models, the spark plug caps, whether NGK or BMW, should be nominal 5000 ohms, and NEVER use 1000 ohm caps, as you WILL damage the Hall element in the canister, eventually.
There is a benefit to using the 1000 ohm caps on any of the POINTS models instead of 5000 ohms.
In 1981 BMW installed a full electronic ignition, and the 5000 ohm resistance type spark plug caps are used for all the previous reasons; AND, they are a NECESSITY to stop a type of harmful electric current/field, which can flow through the wiring as well as being radiated. Thus if 1000 ohm caps…or no resistance caps…was used on these later models, that could almost surely eventually damage the Hall element in the canister, or even the electronic ‘black box’, called a module, located under the gas tank. The damage could show up immediately….or, much later perhaps at a very inopportune time. The 1981+ models have much higher coil output than earlier models to deal with the leaner mixtures used, so the 5000 ohm caps are hardly a problem and do NOT reduce performance, in fact, performance is increased overall, with the 1981+ ignition. The 5000 ohms of resistance lowers the radiated electrical field which could otherwise get into the wiring on the bike making its way to the Hall device in the electronic timing canister in the timing chest, ruining it, and, again, this may not show up immediately, and THAT is the most common failure, usually occurring because the owner or uninformed mechanic removes a spark plug cap while the engine is running. Removing a spark plug cap from the spark plug was a common method used during carburetor synchronization on the PRE-Airheads models, and BMW WRONGLY kept that method in the early Airheads owners booklet, and some other literature, at least for awhile. The same problem may well occur with the Dyna III ignition unit, and definitely is also a problem in the same way for the Boyer Brandsen Microdigital ignition.
The Hall device canister (1981+), as well as the module under the tank, are all expensive devices from BMW, yet usually very reliable. Because of the sensitivity to damage, it is NOT ACCEPTABLE, under ANY circumstances, to remove the spark plug caps nor wires when the engine is running (for safety, take this to mean any time the ignition switch is ON), and, as noted above, a common use by old BMW mechanics in the magneto era….and even into /5 and later era (wrongly on them). You CAN short circuit the spark plugs or the output of the spark plug cap to the cylinder head fins on ANY model, if that is the way you want to balance your carburetors, but beware of open circuits here.
BMW does NOT use carbon resistance type of high voltage wire to the spark plugs, NOR, the often heavily advertised and touted spiral metallic type. These types should NOT be installed! Use either the stock type wires; or, you could use high quality silicon insulated stranded copper wire.
I recommend NOT EVER using resistor spark plugs with the stock resistor plug caps. You COULD use non-resistor caps and resistor plugs, but it is VASTLY better to use the original type parts; or, use NGK spark plug caps. Any use of non-resistor caps with resistor plugs is fraught with possible problems.
Bosch has mostly discontinued the NON-resistor spark plugs we used in our Airheads. Bosch of Germany makes good spark plugs with nice threads and thread coatings (NGK makes a quality plug with excellent threads too, but Champion’s threads are SUSPECT). Be SURE that if you are purchasing Bosch spark plugs that there is no R in the model number as stamped on the spark plug (DO NOT depend on the model number on the little box). Be sure, R in number or not, that the plug is NOT a resistor type. Note that Bosch is going to a all-number spark plug identification number. That is not helpful. If not made in Germany, inspect carefully, the threads, etc.
If you use 5000 ohm caps WITH resistor plugs on any Airhead model the spark may be too weak, the bike might seem to run OK, but in marginal conditions, such as very cold, or slightly marginal ignition components, even a dirty plug, excessively lean mixture, etc….the bike might be hard to start or misfire at higher rpm….and give poor fuel mileage…and cause excessive carbon deposits in the cylinder head and on the piston top. Under the worst conditions serious mechanical damage could occur. Bottom line: Use NGK spark plugs if in doubt; or, MEASURE your Bosch spark plugs and be sure they are NOT resistance types. Example: WR7DC is a resistor plug, W7DC is not. Pay attention to what is stamped into the metal body of Bosch spark plugs, NOT what it says on the small box the spark plugs come in. If you find an all-numeral number on the spark plug, MEASURE the spark plug resistance, be sure it is zero.
The author’s website has an article, #32, with much greater in depth details on the spark plug caps and the spark plugs, with charts; details on use of antiseize compound, torques, and lots more is covered. I RECOMMEND you read that article.
https://bmwmotorcycletech.info/sparkplugs.htm
Warning! Once a spark plug has been run in an engine, even just once, even for a short time, the ground electrode metal changes its granular structure slightly. NEVER, EVER! adjust a spark plug gap on a used spark plug! ….on rare occasions the ground electrode has come off, inside the combustion chamber, and caused engine damage.
The list of Technical articles can be found near the bottom of the page: https://bmwmotorcycletech.info/index.html
Revisions:
updated 06/23/2018
updated 02/14/2021
Snowbum (#1843)
