Dual-plugging the BMW Airhead motorcycle:
I responded to some questions on and off the Airheads Mailing LIST about dual plugging and the automatic advance unit (ATU). I commented on setting timing and the types of modifications made by others to the ATU. I have had dual plugging on some of my R100RT bikes, but did not think it always of great value for the average rider. The result was a lot of misapplied and mistaken comments from Members of the Airheads LIST. I got other interesting replies, almost all being off the List.
HERE, I will try to make definitive comments on dual-plugging and the automatic advance units. FIRSTLY, I advise you to read the more comprehensive and more-up-to-date article on dual plugging on my website:
https://bmwmotorcycletech.info/dualplugging.htm
In my estimation, dual plugging is, or can be …an minimally to moderately expensive real answer & solution to some things that might be problems.
A dual plugging conversion will reduce over-all heat and likely heat peaking areas in the combustion chamber, due to more even distribution of the combustion heat. Thus, dual-plugging is easier on valves and valve seats, especially in the R100 models which run hotter than the lower displacement Airheads. The valve seats will NOT likely tend to warp towards the spark area with dual plugging. There WILL be better performance, particularly in the later LOW compression-ratio Airhead models, and probably better fuel mileage, particularly in the older HIGHER compression-ratio models. Dual-plugging usually allows a lower octane grade of gasoline to be used, saving you money, and this can be substantial. You may obtain a very slightly higher fuel mileage by using regular grade of fuel instead of premium grade (where compression ratio allows) and this FACT is NOT well-known. Dual-plugging conversely allows higher compression ratios and thereby more performance; if that is what you want. That can be done by piston change or by milling the head or barrel base, or a combination of these. If a coil fails (its secondary winding is the area of usual failure); …then you can ride home on one coil if dual-plugged. In emergency you could probably substitute any two tower coil, from a Japanese bike. Dual-plugging may pay for itself over the long term in mileage and other things.
There are disadvantages to Dual-Plugging:
You now have two coils (yes, I know some stock BMW Airheads have two coils, some have one), each firing both cylinders (that is the proper setup for dual-plugging). The cost can be somewhat high for the conversion and parts, depending on when the conversion is done, and how. It is cheaper to do the dual-plugging at the same time that one does a ‘valve job’. The cheapest is when you also have a 1981 or later bike that already has the BMW electronic ignition, as it can be retained. For them, you need only a pair of coils and the machining work, and some wires. Obviously, you have twice as many plugs to change, whenever you do that, and they are NOT necessarily the same part number. You have more ‘stuff’: you have two different (from stock) ignition coils that likely are not easy to find if one fails on the road (granted, the Accel coils seem to last forever, but you cannot easily use a stock BMW twin tower coil because they are 12 volt, and you need two coils; yes, some have used the 12 volt two tower coils in primary windings parallel connection). Coils seldom fail…except for the gray plastic cased twin tower coils BMW used on some models…and you won’t be using those particular coils for dual-plugging.
Here, I get into some more technical details:
The automatic advance unit will be a modest compromise (more on this later in this article). Slightly off-idle performance may or may not suffer a tiny amount, but that is easily and cheaply curable with an idle jet change. Accurate carburetor synchronization will be slightly more critical, as the butterfly valves will be slightly more closed at idle rpm. You have to be careful not to over-torque the lower plugs if they are the 1/2″ thread length spark plug (if not using the same as the top stock plug which is 3/4″ thread length, which can be done by modifying the head for a welded-on aluminum spacer). That is: If a threaded aluminum adapter (think of it as a very thick washer) is welded to the head, then possible over-torquing is not nearly as critical as the spark plug thread length can be the same as the top plug. In many instances the lower plug heat range will be or should be 1 step hotter; something that is not well-publicized. When you dual-plug an Airhead, it is more often done with the same diameter plugs as came stock, that is, 14 mm. BUT, the lower plug fits in somewhat thinner metal, and therefore a 1/2 inch THREAD REACH plug is used, instead of the stock 3/4 inch ….unless a welded washer/spacer is added. NOTE that some have simply used an aluminum spacer, not welded …not a great idea, for it will likely have poorer heat transfer, and it’s NOT a strong regarding the head threads. It is relatively easy for those who are mechanically hamfisted, to strip out the threads in the NON-welded washer type of modification. The STOCK specified torque for the regular 3/4 inch plugs is 16-18 ftlbs in some books….and I think THAT IS TOO HIGH!….and 14-15 is adequate for 3/4″ reach plugs; less for 1/2″. For a 1/2″ reach plug in 14 mm, perhaps 12 foot pounds or a bit more. DO NOT USE OIL to lubricate spark plug threads, OIL will COKE/CARBONIZE and help destroy threads when the spark plug is removed. I believe that one should not install spark plugs (all of whose threads are made of steel) into aluminum alloy heads, without using ‘antiseize’ compound, which, since it acts as a type of lubricant, means that the torque applied must be LESSENED from stock. Once the plugs are so treated, the compound works its way into the aluminum of the cylinder head and is difficult to remove, so I recommend always continuing with the antiseize use …and the lowered torque settings. If you do not have a torque wrench (why not?), use NEW gaskets (crush washers), torque to JUST crush the gaskets almost fully, although one must be extra careful. I HIGHLY recommend NEW crush washers EACH time a plug is removed and installed, and HIGHLY recommend the use of an ACCURATE torque wrench! The recommended torque values when antiseize is used is typically about 80% of the clean and dry values.
If the bottom plug is used with the welded aluminum washer/spacer you can use a 14 mm x 3/4″ plug, the same as the top plug. If the bottom plug is smaller than 14 mm, then be very cautious about exceeding 10 ftlbs or a bit more. I highly recommend using a 14 mm 3/4″ reach bottom plug, same as the top plug, and installing a welded spacer, with the spacer also threaded. If modifying heads already modified for dual plugging, then the threading should be done by having the tap insertion from the inside, so that the threads all match up.
For those with the BMW electronic ignition, which was stock from 1981, be sure you use stranded copper aftermarket wires and NGK LB05F caps….which are 5000 ohms nominally. If you do not use the proper wiring and caps you will likely destroy the Hall Element located in the ‘canister’, also known as the bean can. The damage comes from re-radiated energy, and that is reduced properly by using 5000 ohm caps (which was standard for 1981+ anyway).
POINTS bikes can use 1000 ohm nominal caps, NGK LB01F caps, for a very slight advantage, compared to 5000 ohm caps. Use of a points amplifier is helpful….just be sure can handle the coil currents.
NGK makes a variety of resistor caps, in various angles, and resistances. There is an article on my website that gets rather deeply into spark plugs and spark plug caps:
https://bmwmotorcycletech.info/sparkplugs.htm
If you are contemplating doing a dual-plugging conversion on your later airhead with the ~8.2 compression ratio pistons, be advised that if you shave the head, every .005″ you shave will up the C.R. approximately 0.1 point. Approximately 9.6 is the upper limit for today’s premium gasoline (USA). I recommend against that high a CR. On a good clean combustion chamber and good running in-tune engine, you can use about 9:1 on regular gasoline which is typically 87 octane…while not a recommendation, this usually works out about right. On the eighties USA-shipped Airheads with 8.2 CR, a bump to 8.8 is easy and worthwhile, and a bit more is also usable. You can use 9.5 CR and Regular gasoline in some instances, and certainly can with premium fuel. Use of rather high CR is possible with a camshaft change, and attention to details.
Old style platinum spark plugs MAY foul easier, and their gap arrangement is not the best for Airheads. I do NOT like those platinum plugs, and some have failed mechanically, not just electrically, and a mechanical failure of the end center electrode internal hidden ‘rivet’ is dangerous to your engine.
Your spark plugs MUST be the extended-nose types. NO specialty spark plugs of ANY kind are as good as the stock cheap Bosch OR NGK NON-resistor spark plugs. NOTE that Bosch started, in 2007, to phase out NON-resistor spark plugs, and the Bosch box may not indicate what is actually inside the box. Do NOT use resistor plugs in Airheads! BMW dealerships may still stock NON-resistor Bosch plugs. NGK plugs are GOOD. Avoid brands other than Bosch and NGK. AVOID special plugs! Besides the nose and gaps being shielded a bit from the input charge in many instances …some brands have POOR quality control on the threads, and will be damaging to the heads…and there are other good, known, reasons. Save your money and purchase Bosch STANDARD or NGK STANDARD; non-resistor, extended nose spark plugs. Use STOCK spark plug gaps. Note that with UNleaded fuels, the spark plugs will last a LONG time ….and if the engine operates cleanly, 30,000 miles is NOT unusual.
When dual-plugging is installed, it has a specific effect on the combustion process, as it usually has TWO flame fronts in each cylinder. This EFFECTIVELY DOES THE SAME THING AS ADVANCING THE TIMING OF THE IGNITION. BUT, it does not do this evenly, IN EFFECT, for all engine speeds; in some ways it is similar for one plug stock situations, but this is way technically nerdy to explain fully, so I will not, here. Suffice it to say that the change to now having two flame fronts DOES THE MOST EFFECTIVE ADVANCING AT LOWER ENGINE SPEEDS. The two ignition timing modification methods (one or the other) done for dual-plugging, are COMPROMISES. The effective advancing of ignition by having that second spark plug diminishes as RPM goes up, and at around 5000 or perhaps even 6000, the added sparkplug has much less effect. In fact, it is possible that the SINGLE plug ignition is MORE efficient at VERY high rpm’s due to more turbulence in the combustion chamber….but you don’t normally ride constantly at such rpm, and this is more a point of discussion, and hardly of much importance in the real world, even at that rpm. One of the beneficial effects of dual-plugging, no matter which method of ignition modification is done regarding the ATU, is that pinging, particularly on the R60/5 and R60/6, but affecting all Airheads, is LESSENED, often to zero.
There are TWO schools of thought on the ignition timing and advance for a dual-plug installation. I will get into both of them; but more in depth on my own website article on dual-plugging:
For some, the method done is to RETARD the idle timing to approximately the OT mark at idle rpm. This has the effect of making the spark occur more accurately for a dual-plugging. These folks leave the maximum timing point alone, which will be somewhat retarded then, but that can be OK, and works well due to the EFFECTIVE advancing with the dual plugging. One can also modify the limits on the automatic timing unit to give a few degrees more maximum advance, this helps slightly. One can NOT go too far …the canister-ATU or nose ATU has just so much room for that modification. Instability has NOT been any problem. In MY opinion, you might ALSO want to consider moving the rpm point of maximum advance, up the scale a bit. That works well for todays fuels, and can allow a slightly higher compression ratio as well. I suggest that the rpm for maximum advance might be moved from 3000 to 3300 or even 3500 rpm. It is my opinion that this will mostly eliminate pinging problems from the use of regular grade gasoline and 9:1, & might allow 9.5+- via milling or 9.5 pistons on REGULAR fuel, even at sea-level. This is easily (?) accomplished by using one or both slightly stiffer springs on the advance unit … AND/OR shaving the weights to make them lighter. It is easier to do on the NON-canister model, using a timing light and your bike tach.
For the canister models, bending the ears very slightly outwards is tricky, but can be done without disassembling the unit. You would still use OT as the timing mark. Moving the OT mark a degree or three farther advanced (higher in window) will help only a wee bit…somewhat depends on your bike. The extended timing modifications help only a slight amount. Stock advance units (about 25 or 26 degrees available) go to full advance at about 2900-3200 rpm. As noted, moving it a few hundred rpm higher may help persistent pinging problems with higher compression engines. That comment also applies to the R60/5 and R60/6 which have such mild valves camshafts that the EFFECTIVE dynamic cylinder pressure is conducive to pinging (pinking); which is MUCH reduced by going to dual plugging. Very early /5 advance units quit advancing at about 2000 rpm, and should not be used with todays poor gasoline’s unless you know what you are doing.
For the other school of thought, the timing, at idle, is set closer to, or actually to, the stock S mark. This method does not match the low-idle rpm spark advance the engine would prefer, but it works well enough. The maximum advance is limited, using sleeves or other means and some folks wind an old set of advance springs onto the advance unit’s pegs to increase the diameter. Some may also raise the rpm for maximum advance towards 3300-3500, particularly so with higher compression ratios.
Both schools of thought/methods, SEEM to work OK.
NOTE!!……not often mentioned is a small problem that some have when dual-plugging. Dual-plugging causes the engine to want to idle faster; assuming one left the carburetors as they were before the dual-plugging was done. Thus, one backs off on the carburetor idle stop screws, to reduce idle rpm. This makes the butterfly (CV carbs) come closer to being closed, and since it is at that position that the critical idle ports are in close proximity to the bottom of the butterfly valve, the synchronization and adjustment of the carbs becomes a more critical, so one must take some extra care to do it correctly. That includes careful installation and alignment of the butterfly. But, there is another effect. It is my belief that this effect occurs on the CV carbs as the idle passageways are in the LIQUID circuit. The result, if this happens to you, could be, not necessarily will be, a SLIGHT stumble when JUST turning the throttle in a rather gentle takeoff from a stop. The cure, besides a careful synchronization and idle mixture adjustment, is to INcrease the size of the idle jet. I usually go from the stock size of 45, to 50 or 55.
There is a lot more information on my website, link is below, on dual-plugging, and the Airhead ignition systems, including articles on theory, spark plugs, caps, ETC.
https://bmwmotorcycletech.info/index.html
09/14/2020: copy from Snowbum website, and place here on airheads.org. Snowbum did extensive editing for clarifications.
02/11/2021: Clarifications
