Airhead Auto Advance Refurb
As part of the frame swap after the crash, I disassembled the auto advance unit and cleaned it up. Of course, I put it back together again afterwards. This page contains a series of photos I shot along the way, along with an occasional comment about what they show.
This is the auto advance unit in it's natural element. Since this is a GS, I found a fair amount of dust and grunge in there with it. A few spritzes with a can of contact cleaner took care of the worst of it.
I removed the two green screws and gently pried the cover off. Pretty clean inside!
Removing the two phillips head screws exposed by removing the cover allows the spacer they hold to come off revealing the top bearing mount for the mechanical advance mechanism. I marked the orientation of the mount with a Sharpie marker.
After I lifted out the bearing support I found a real big snap ring. I used my fingers to gently pry it from its groove, but I marked its orientation with the Sharpie first!
This is the shield that triggers the Hall Effect device. Another pair of marks preserve its orientation, though it's not really important to do so.
The dogs that drive the auto advance have a spring wrapped around the base to retain the pin that secures the dogs to the mechanical advance shaft. I had to take it off before I could drive out the pin.
To go any farther with the disassembly, the pin that holds on the driving dog must come out. BMW machines a groove in the auto advance housing so the pin will clear, and a punch can reach the top end.
I used a flat end punch on the pin, and a copper vise jaw to support the dogs.
There are two washers under the dogs. The metal one goes next to the housing on re-assembly.
Before removing the guts, I released the strain relief bracket from the side of the auto advance hosing. Removing one screw lets the bracket slide out the open end of the housing.
There's another screw on the outside of the housing to remove, and then the mechanism is free to come out of the housing. Note the orientation of the threaded tab to the screw hole.
There are two more washers on the bottom end of the mechanical advance shaft. They might stick to the bottom of the housing.
Here's the mechanical advance by itself. I cleaned up the pivots and lubricated them with some TriFlow, and made sure that they swung in and out freely. As the motor increases in RPM, the arms swing out. As they do, the black pads on the inboard end of the arms bear on the bottom of the plate that holds the Hall Effect sensor, rotating the plate and causing the sensor to fire sooner. This is what advances the timing.
This shows the mechanical advance re-attached to the Hall Effect shield. The Hall Effect device is the black thing with the wires coming out of it. It triggers when the missing part of the shield swings past, sending a pulse of electricity to the ignition control unit.
As they say in all the manuals, assembly is the reverse of disassembly. Not much to go wrong, really, I did have to pay attention to alignment of the threaded tab, and to how the metal and fiber washers were stacked on the shaft. Other than that, piece'o'cake!