Technical Tips – Page 8 – Airheads Beemer Club

Diode Boards & Grounding Wires (including information on testing diodes)

Diode boards generally fail by having one or more diodes ‘open’ internally, “open’ means they do not exist electrically. Rarely do they short-circuit. There are small diodes on the diode board with two types of functions. Early boards, such as on the /5, did not have as many diodes as later boards, the later boards had a stator windings center-tap, which used the extra diodes to improve output wattage. Failure of any of the small diodes will cause strange output problems. Usually, a diode board failure is at the solder joint of one or more of the six large power diodes; or, that diode opens. It is quite rare to see shorted large diodes on the board. If a large diode solder joint becomes lousy or opens or the diode itself opens, the alternator will still produce current, but it will be quite considerably lower in total wattage output. The typical result of one large diode opening (or, solder joint failure) is that the battery will show near proper voltage during riding, but when the headlight is turned-on, the voltage will sag rather considerably.

Diode board testing and information about repairing the faulty Wehrle-manufactured ones (I’ve seen a few Bosch boards with the same problems with non-bent-over large diode leads and soldering failures) is covered in depth in the June 1999 issue of AIRMAIL, an article by the late great BMW Guru Oak Okleshen. Airhead owners may want to read the article. The Oak-recommended & difficult to do board modification could be done, with considerable effort, for reasons of solder joint failures, more likely if you have a R100 engine, which develops more heat (worse if an RS or RT). I’ve had good luck with a much simpler method, which is the removing of the ‘paint’ with gel type paint remover (often it has no effect though and abrasion is needed). I always enlarge the solder pad area first, whether the gel remover works or not, by careful scraping with a sharp Xacto thin blade hobbyist knife (don’t remove copper material), & then re-soldering using a high temperature solder (50-50 plumber’s solder and rosin flux) and a extra hot soldering iron. No re-painting or re-coating is needed, but certainly could be done. My repair method is vastly easier to do because drilling the PC board & adding wires & soldering per Oak’s method is quite difficult & best done by total disassembly, which Oak did not get into and is a huge PIA! Yes, it can be done with long tweezers or forceps, but that is quite tricky. I cannot recommend Oak’s method, and I won’t even do it myself. I’ve also seen several Bosch branded boards with this problem, perhaps Wehrle made them? There have been some other Bosch and Wehrle labeled products that made me think either could have manufactured them.

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Diode Boards & Charging

Many are intimidated by electricity, and the amount of electrical problems with Airheads can be a sore point. Most often problems are due to lack of regular maintenance, particularly cleaning and tightening various connections, checking alternator brushes, changing the battery before a catastrophic battery failure; and general old age and mileage. This is really not much different from other motorcycles. If properly maintained, the Airheads electrical system works fine and is reliable. If, however, the bike is ridden mostly in stop and go city traffic, the charging system may not keep up, and the battery might require nightly re-charging. The Smart type of chargers are especially convenient for that purpose.

The stock charging system starts producing usable amounts of electricity at ~2000 rpm, but upwards of 4000 rpm may be required for enough output to take care of lights (including extra lighting), ignition, heated clothing, accessories, and have electricity left-over for re-charging the battery. Aftermarket alternators of higher output are available that fit inside the timing chest area.

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Bosch Metal Can Mechanical Voltage Regulator, Cleaning & Adjusting

The first part of this article covers the basics for this 12 volt VR unit, as used on BMW Airhead motorcycles before BMW changed to electronic ignition in 1981. This article applies ONLY to the mechanical voltage regulators. These were used on the Airheads from 1970 to ~1980.

At the end of this article are some addendums. #1 is a description and schematic diagram. #2 is for the BMW /2 (6 volt) motorcycle’s VR. I suggest you DO read that section, even if you do not have a /2, as it has a slightly different way of explaining the servicing (included in this article).

Voltage regulators, both mechanical & electronic, in metal cans, were installed as stock items on early BMW Airhead motorcycles. There is nothing wrong with the mechanical type for the early BMW Airhead Motorcycles, except that mechanical types DO wear, over time & mileage. In an emergency an old mechanical VR might do OK in any Airhead model, although the electronic ignition could possibly (not necessarily will) miss-fire occasionaly with a mechanical regulator. Mechanical regulators wear mostly from their constant points vibrating/buzzing/arcing. Electronic regulators will substitute for the mechanical regulator on any Airhead model, directly, with no problems. All the mechanical regulators were inside of tall metal cans. Later voltage regulators were electronic; but the early electronics versions ALSO were in metal cans, but they were shorter in height. The next series were in smallish plastic cases, often with black bodies and red tops. There are aftermarket electronic regulators available, some are adjustable, & most 3 male spades type of voltage regulators from European cars (that the Airhead cable plug fits into) could also be substituted. The stock METAL can electronic regulators are also adjustable, there is an article on this Airheads website about how to go about that, since the adjustment is sealed. There is also a bit more of an article here: http://bmwmotorcycletech.info/boschelreg.htm

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Alternators – Aftermarket

There is nothing wrong with the design of the BMW Airhead motorcycle’s stock Bosch alternators. Like most anything they will age, and/or they may be abused, they may fail, although not in excessive numbers. They are often never maintained until there is a failure. Wattage output was adequate for most riders when the motorcycles were originally sold by BMW. The modest electric output may be of concern if you have lots of accessories ….such as heated clothing, heated grips, extra headlights, etc. I am not for or against aftermarket alternators. I am reporting only facts and testing information and conclusions. You may well do fine with the stock system. But, if you have special needs, such as large city stop and go commuting; or need more watts ….perhaps you are approaching the stock alternator limits (which decreases reliability) ….there is plenty of information in this article. The /5 Airheads had a 180 watt Bosch alternator with a stator diameter of 105 mm where it fit into the engine case. It can be upgraded easily by using an EARLY /6 alternator stator (if it is 105 mm, and NOT 107 mm), and any pre-1985 (approx.) rotor (2.8 ohm rotor not recommended) and any /6 or later diode board. Reliability can be improved for ANY Airhead alternator, stock or aftermarket, and this includes a stock /5 model, or a /5 upgraded to the 280 watt 105 mm Bosch; OR EVEN AN AFTERMARKET ALTERNATOR; ….by using the later, better ventilated, front metal engine cover. If you have an RS or RT model, and the front fiber-glass-like cover is NOT louvred, even more reliability will be had by changing to the louvred type, or putting attractive holes into yours.
Refer to: http://bmwmotorcycletech.info/altcapability.htm

This article includes information that is far beyond what advertisements for various alternators provide. Charging a battery in a vehicle like your Airhead is NOT as simple as you may think; nor, is interpreting the alternator ratings, typically simply shown as wattage. This article presents a huge amount of information; and, also presents information that helps you select an aftermarket alternator. I suggest you read this article completely through to get an over-view; then refer back to the areas you are interested in. If confused, please inquire on Snowbum’s favorite forum for inquiries; an E-mailing LIST, the Airheads List, as hosted by Micapeak.com. Input will help others, rather than my answering individual inquiries, which are discouraged.
http://lists.micapeak.com/mailman/listinfo/airheads

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Alternators – Stock

BMW uses 3 phase electricity generators called alternators on all models since 1969, & some before that date. Why 3 phases? 3 Phase is more efficient at converting rotational energy to electricity. 3 phases enables the same or much more output in a smaller, lighter package, even less steel laminations are needed, & possibly more charging at lower rpm & possibly less horsepower drain on the engine. 3 phase is usually better for vehicles with radios & other electronics, as the system is, in effect, using an A.C. generator at higher frequencies than single phase. That means that with the battery acting like a monstrously large capacitor (which it DOES), alternator whine noise & other problems is MUCH reduced in a 3 phase system, compared to single phase. There are various other advantages that 3 phase offers. The 3 phase rotor MAY have less inertia, thus better vehicle acceleration, assuming diameter is kept small & overall inertia similarly, thus there may be a lowering of torsional stresses on the shaft & components. That last item can be critical on some engines where the rotor is affixed to the crankshaft & can be considered an extension of the crankshaft.

A 3 phase alternator is more compact & efficient … up to 1.73 times electrically more efficient.

At the link here, scan down the article, are sketches of the waveforms & some additional detailed information on single and three phase systems. http://bmwmotorcycletech.info/boxerelectrics.htm. The sketches will help you understand 3-phase.

Ø is the symbol universally accepted in electrical circuits for ‘phase’. A 3Ø system has few drawbacks. One is that the diodes circuitry used for rectification is more complex & there are more diodes; another is complexity & more labor in manufacturing.

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Alternator, Rotor, Stator, Brushes, etc.

Rotors and Stators:

This has been a confusing subject to many. Anton Largiader’s website article with text & photos will explain explain some things; or, some differently. I have minor disagreements with his article, BUT….it does have some useful information & has some photos you should look at. I have the most detailed and pertinent information here in my own article. Read Anton’s article here: http://largiader.com/articles/charging

The original /5 BMW Airhead motorcycles had a 180 watt alternator with the end that fits into the motor timing chest cavity being 105 mm in diameter. SOME 1974-1975 /6 bikes, & possibly a few barely into 1976, were made with the 105 mm cavity. These /6 bikes with 105 mm stators had 280 watt alternators, so it is possible to upgrade a /5 from the 180 watt alternator to a 105 mm 280 watt alternator with the appropriate parts changes and the parts WILL FIT. These changes are stator & diode board, at a minimum. If your /5 rotor measures ~7 ohms, which was the original value, then I recommend it be changed to a next version rotor of ~3.4 to 4 ohms; the electronics VR is optional, but recommended. ALL /6 and later stators had a center tap on the stator windings, & some small diodes were added to the diode board. If you do not use the /6 or later diode board, the output will be less. I suggest NOT using the last version of the Airhead rotors, which were ~2.8 ohms. For all other /6 and all later Airhead motorcycles, the cavity and stator were 107mm.

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Valve Settings

A couple of list members have asked me about valve setting procedures, Here’s how I go about it:

The bike should be cool. Pirsig, In Zen and the Art of Motorcycle Maintenence, writes how he gets up before dawn and sets his valves before the sun clears the horizon and hits the bike. This is a bit anal, in my opinion. The machine just needs to be cool to the touch. Allow an hour or more if this is your first time, with practice, you will get the time down to 20 minutes or so, but, for now, make it a leisurely, unhurried proceedure.

Remove the valve covers (you might want to have a pair of cover gaskets on hand as the gaskets can stick, tear or become rock-hard over time). If they are in good, clean shape, they can be left in place and reused. Have something underneath to catch the couple of tablespoons of oil that will fall out when the covers are removed.

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Valve Seat Recession – A Definitive Reply

A considerable percentage of BMW Airhead motorcycles have had various valve and/or valve seat problems, up until the 1985 models. This article will try to explain the background, and why the various problems developed, and why some have problems and some do not. Note that air-cooled motorcycles are more subject to ‘top-end’ stresses, than water cooled motorcycles.

Tetraethyl lead, TEL, is dangerous if absorbed through your skin when in pure form and much less so in diluted form, and it is a form of lead previously used extensively, highly diluted, in gasoline. Way back when it was still in use (still is in some parts of the World) it was sold by the Ethyl Corporation, and such gasoline’s had a nickname: Ethyl.

This compound has TWO major effects. First, in sufficiently high percentage, it can raise the octane value a fair amount, very important in WWII aircraft engines, especially those that were supercharged; some were both supercharged AND turbocharged. The original purpose for using tetraethyl lead in gasoline was specifically for raising octane. Some of those aircraft engines required 145 octane gasoline. Usage for the purpose of raising octane was carried forward for decades for cars, because it was cheaper, even when the lead was sold to the refineries expensively by the then Ethyl Corporation, than using only refining methods to increase octane. Many decades ago, premium (higher octane) gasoline’s were simply called “Ethyl”. Usually those gasoline’s had MORE TEL (Tetra Ethyl Lead). There is a lot more to this story, this is a simplified version of lead usage.

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Starting your Airhead motorcycle, including in cold weather

Cold weather starting is nearly always done, & properly so, by using 100% full choke and manipulating the throttle a bit during cranking, as the engine begins to start. Have the clutch lever at the handlebars pulled-in during the cranking to reduce loading of the starter motor by the transmission with its cold thick oil. Helps the battery too.

Many manuals, including the factory Owners Manuals will say to not touch the throttle. In my experience, that is wrong. I have found that most Airheads require some throttle manipulation upon starting in cold weather, and often in mild weather.

As soon as the engine is running, reduce the amount of choke as soon as you can, yet if you need to, and you likely will, keep ‘some’ choke on, until you have smooth running, including when riding. Too quick a reduction may result in the engine dying and needing a restart. Typically the choke lever is returned to ~half-way within half a minute. Even in the coldest weather, the choke lever should be returnable within a few minutes to the half-way position, and not long after to full off …or, nearly so. For very cold weather, try to keep the rpm between 1200-1500 during non-moving time until some decent warmup is had.

Never blip the throttle to high rpm when starting, this is particularly very bad with a cold engine and wear will be high. In some situations you can break rings or collapse an oil filter with a quite cold engine. Generally, you can start an engine & take off modestly, using quite moderate rpm, after 30 seconds to 2 minutes of high idle rpm (1200-1500), if the temperature is down to as low as 40°F or so. I suggest using modest throttle when taking off, and not going over 4500 rpm, preferably not over 4000, until the engine is warmed some, which takes a couple of minutes.

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Rocker arms, shafts, bushings/bearings. Rocker arm breakage. Valve gear. Pushrod tubes. Various parts of the cylinder heads

BREAKAGE OF ROCKER ARMS:

This is a rare occurrence, usually happens under racing conditions, with very high rpm, and/or high lift cams, increased valve spring strength, insufficient spring clearance at maximum lift, etc. However, it did happen, but rarely, on stock early Airheads, in non-racing use, and only in the /7 era, as far as I know. If you look closely at a rocker arm, immediately next to where the ADJUSTOR screws into the rocker arm, the web width there on the suspect rocker arms is about 7 mm wide. BMW made a production change and the rockers with the factory change had a 11 mm width. There was no change in the rocker part number. It does not appear to be a formal re-call by the Factory. I am not listing all the rocker part numbers, except two, because:
1. No new rockers are available that are 7 mm.
2. It is easy to confuse yourself over the SEVERAL part numbers used for the rockers. The original part numbers, long gone now, were 11-33-1-262-403 and -404.
3. You can measure your rockers.

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