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. Although I have had dual plugging on one of my R100RT bikes, and have had it on others, I did not think it necessarily always warranted 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, to say the least, almost all being off the list.
HERE, I will try to make some definitive comments on dual-plugging and the automatic advance units. FIRSTLY, I advise you to read the more comprehensive and likely more-up-to-date article on dual plugging on my website:
In my estimation, dual plugging is, or can be ...an minimal to moderately expensive solution to some things that might be ...or ARE ....problems.
Dual plugging DOES:
Reduce over-all heat in the combustion chamber, likely due to more even distribution of the heat; and, thus is probably easier on valves and valve seats, especially in the R100 models which run hotter. The valve seats will not likely tend to warp towards the spark area with dual plugging. There likely will be better performance, particularly in the later LOW compression-ratio models, and probably better gasoline mileage particularly in the OLDER HIGHER compression-ratio models. Dual-plugging almost always allows a lower octane grade of gasoline to be used, saving money. It is also likely that with dual-plugging you will obtain a slightly higher fuel mileage by using regular instead of premium (where compression ratio allows); this FACT is NOT well-known. Dual-plugging 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. Dual-plugging may pay for itself over the long term in mileage and other things.
You now have two coils (yes, I know some BMW Airheads have two coils, some have one), each firing both cylinders. If a coil fails (its secondary 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.
There are other disadvantages to Dual-Plugging:
The cost can be somewhat high for the conversion and parts, depending on when and how. It is cheaper to do the dual-plugging at the same time that one does/needs a 'valve job'. The cheapest is when you have a 1981 and later bike that already has the BMW electronic ignition, as it is retained. For them, you need only a pair of coils and the machinging work and some wires.
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 a jet change. Almost for sure the need for an accurate carburetor synchronization will be a bit more critical, as the butterfly valves will be a wee bit more closed at idle rpm. You have to be careful not to