Ted, the owner of the bike commented that “the idle had changed recently”. I grabbed a can of carb cleaner and sprayed a small amount at the connection between the carburetor and the head. This rubber piece can wear out and create an air leak. Sure enough, a dose of the carb cleaner caused a decrease in RPM for a few moments. The cleaner is more volatile than gasoline and the bike will actually run on the stuff.
With the engine turning at 3,000 rpm, one would expect a bit more voltage than the meter is indicating. The red “Gen” lamp would light up and then disappear as we turn the ignition on and off. I started looking for bad connections. Judging from the spider webs, it had been a long time since anyone had been inside the instrument cluster! No loose connections there so it was reassembled and but back in place. The on-off behavior of the lamp continued.
I had done a timing chain replacement on this same bike a couple of
months ago. I had replaced the brushes, so I immediately zeroed in on that area. All looked well, and the ohm-meter indicated the rotor was okay. There clearly was no charging happening as the voltmeter showed 11.99 volts. Since I had plenty of new and used charging system parts on hand, we started substituting the diode board and the voltage regulator. Neither one of these changed anything. I then pulled the instrument cluster to see if the bulb was bad or was
loose in the “bulb holder” fixture. These parts are now “NLA” so I have gotten pretty good at repairing torn foil or loose posts. As the cluster came apart on my bench, it was pretty obvious that the spiders living in there had not been disturbed for quite a while. We lashed up a test circuit with a lamp and some jumper wires and found everything to be in good trim and all connections nice and tight.
Ted’s bike has one distinctive feature about it. The gas tank was painted with a sort of surrealistic pattern of eyes and faces. The artist had signed it and when Ted did a bit of research on the name it turned out the art had been done by the fellow who created the Teenage Mutant Ninja Turtles! Ted was going to have the tank painted with a proper BMW color and pattern but decided to keep the odd artwork once he knew the story behind it.
At this point, I relearned an important lesson—be sure of your
test instruments! The VOM I was using “just didn’t seem right.” I have learned to keep a pretty big stash of common sizes of batteries in the basement. I ran down, grabbed a couple AAA cells and put them in the meter. Ah Ha! The meter behaved VERY differently and started giving steady
values. With the renewed instrument, I went over the various test points of the bike’s charging system. I now was reading the rotor as “open.” The best I can figure, the rotor had an “intermittent” open circuit. This happens from time to time. The rotor spins as fast as the crankshaft and can develop this kind of maddening condition. Replacing the rotor remedied the problem. With the new Emerald Isle diode board and a new electronic voltage regulator (replacing the original 1975 components), the bike’s voltage was steady at thir- teen volts with the headlamp on. Ted decided to keep the new parts in place and have me hang on to the old stuff as spares for him at any future time of need. I know I’ll now be more attentive regarding my battery-powered test meters.
September 2015 BMW OWNERS NEWS 53
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