If you have the Wherle ELECTRONIC regulator, refer to this article. If you have the Bosch MECHANICAL regulator, refer to this article.

This is a how-to article, the purpose of which is to make your Bosch metal can [electronic version] easily voltage adjustable. There are numerous ways to increase the voltage, which is the usual reason for wanting to adjust a regulator. The method that follows is THE best for THIS type of regulator. Don't attempt this modification if you can't work on small parts comfortably.

Parts needed:

1. rubber grommet....no special size, from Radio Shack or ? (optional). You MAY want to use a plug to seal that grommet's hole, or?
2. a few Q-tips, some acetone or similar strong solvent.
3. Small jewelers screwdriver.
4. Small drills (optional).
5. 100 ohm potentiometer...in case you ruin the existing one; then you'll need a small soldering iron and piece of 60/40 electronics solder. If you need this potentiometer, you must match its physical size and mounting to the existing one.

The electronic regulators work OK on all years of all airheads. From 1981, the airheads had electronic ignition, and the voltage regulators were electronic. TWO basic types of electronic regulator were used:

1. Wherle: flat plastic case, usually red cover. The circuit is simple compared to the Bosch electronic.
2. Bosch: A metal can regulator, looking VERY similar to the earlier mechanical model, but the can is not as tall as the mechanical model.  There were also flat case types.

This article deals ONLY with the #2 METAL can electronic regulator, above.

Modification:

1. Remove the regulator from the bike. There are two allen head screws, and a plug to disconnect.
2. Remove the tape from the regulator that keeps the top can from coming off the base.
3. Wiggle the top can off the base
4. Inside you will find a printed circuit board with all the components mounted upside down...that is, underneath the board. On the end of the board OPPOSITE the plug/socket end, you will find a small electronic device called a pot (short for potentiometer). This is the factory adjustment point. This is a ROUND part, with a tiny screwdriver slot in the middle.  It is usually sealed with a drop or two of a hard-to-remove white paint.  
5. Try to remove the paint with repeated soakings of a q-tip in something like acetone. This can take some time. If you are successful, the pot will be usable as is, and can be adjustable. If you are not successful, soak the paint, and whilst still wet with acetone, you can VERY GENTLY pry a tiny bit at the paint, to free up the lower adjustment fingers metal ring. Do NOT pry much. If you ruin the pot, it will have to be replaced, with any common 100 ohm pot...that fits, or can be made to fit.

   I have found it best to unsolder the three legs of the pot and remove it, and soak it in acetone for a short while....which ALSO gives me SAFE access to ENLARGE the very tiny stock factory hole for the adjustment, on the board. If you are VERY careful you could leave the pot in place, and drill out that hole by hand...don't ruin the pot. **You don't HAVE to drill the tiny hole larger, but you will need a very VERY tiny jewelers screwdriver for the adjustment, if you do not!

6. Assuming the pot is operational, or the replacement is installed, and the board hole enlarged for access by a more reasonably sized jewelers screwdriver, you have an optional step: That option is to drill the thin metal cover in the appropriate place for access to the pot from the outside. You don't have to do this, you could always adjust the regulator mounted on the bike withOUT the metal cover, then replace the cover. It is up to you. If you DO drill the cover, please DO clean up the hole so it is smooth, and DO install a rubber grommet. The purpose of the rubber grommet is to act as an electrical insulator, to keep you from using a metal tool (jewelers screwdriver) to adjust the pot and then accidentally short the pot's metal wiper to the metal can...a no no.
7. I will assume here that you drilled the hole, cleaned its edges, installed a grommet. Check your work, and install the cover, be sure the jewelers screwdriver will fit the tiny pot slot and rotate the pot. If you are worried about the existing pot being OK, you could use an ohmmeter on the pot and prove that the pot is about 100 ohms between the ends, and that the wiper, with respect to either end, works properly.
8. Reinstall the cover tape and install on bike.
9. Adjustment of the regulator is by the jewelers screwdriver, making SURE the metal of the jewelers screwdriver can NOT contact the metal can. The screwdriver MUST be the right size, or you will ruin the pot. Be gentle in any downward pressure.
10. The voltage setting depends on your usage of the bike, and the type of battery, and the temperature of the regulator when making the adjustment. The regulator is temperature compensated and DESIGNED to INcrease its voltage as the temperature of THE REGULATOR goes down.

Some adjustment guidelines:

Be sure you have an ACCURATE digital voltmeter connected to the battery terminals themselves. I will assume all your wiring and connections in the bike are in good clean solid condition, and your brushes and diode board are in good condition too.

I shall assume a 70°F day, engine started and regulator adjusted within a FEW minutes...so the regulator has NOT warmed up from engine heat, and that you HAVE the battery fully charged to begin with...with only the starting-up drain from the battery since it was charged.  After a minute of engine operation, increase rpm to about 3500-4000 rpm for a minute or so..or, to the point that the voltage is no longer rising from recharging the battery: 14.1 volts is optimum, range 13.7-14.4 volts.

For 47°F   13.8-14.4
For 93°F   13.6-14.2
For 117°F   13.5-14.1

Those doing commuting, stop and go, city driving, probably should use the higher end of the above ranges, those doing long rides should probably use the MIDDLE of the above ranges.   The AGM type batteries (VRLA, Panasonic, ETC.) tend to prefer a bit higher voltage, about 0.2 higher than the higher limits shown above.

For the more nerdy....and for those having an adjustable power source, and wanting a 'bench' test. An APPROXimate adjustment method:

I use an old headlight lamp as a load device here, but almost any lamp or 5 to 20 ohm resistor will work (power resistor of 10 watt rating is OK for short term testing). Connect one side of the lamp to the terminal marked DF, and the other side of the lamp to the power source negative. Connect the D+ terminal to the power source positive. Connect the power negative to the D- terminal. Monitor the voltage of the power source (that is, from D+ to D-). Raise the voltage slowly. As the voltage rises, the lamp will start lighting up. Continue to raise very slowly. The lamp will suddenly go OUT. Note the voltage JUST BEFORE the lamp goes out. Repeat the test SEVERAL times to find the exact voltage. Adjust the pot to where you like the voltage, install the regulator into the motorcycle as noted, and check to see if any final adjustment is needed.

The author's website has further information; and,  in article #19, there is an article on a modification to the GEN lamp circuit, to ensure charging if that lamp burns out, and may even give a slight improvement at the lower rpm. 

http://bmwmotorcycletech.info

12-07-2007
Snowbum #1843