12 BMW 1977 R100RS Install Dyna III Electronic Ignition-Refurbish Automatic Timing Unit (ATU)

I replaced all the engine electrical components and wiring and you can read about that work here:

Condition of ATU and Original Dyna III Ignition

When I removed the engine electrical components, I found the points housing was heavily corroded and the timing advance unit, also know as the automatic timing unit (ATU), was also corroded and rusty. It looked like water got inside the points housing and sat for awhile.

Oil and Corrosion Around Automatic Timing Unit (ATU)

Oil and Corrosion Around Automatic Timing Unit (ATU)

Points Housing Full of Crud

Points Housing Full of Crud

ATU is Rusty

ATU is Rusty

The bike has a Dyna III electronic ignition installed. The Dyan III uses a sleeve with a magnet that mounts on the ATU cam tube. It interfered with the left pickup scoring the sleeve and left aluminum shavings on the pickup mounted on the pickup plate. Also, the pickup plate is corroded. The Dyan III instructions caution that if water gets inside the points housing, it can damage the printed circuit traces on the back of the pickup plate.

Dyna III Magnetic Rotor Shows Abrasion From Left Pickup Magnet

Dyna III Magnetic Rotor Shows Abrasion From Left Pickup Magnet

Aluminum Shavings on Left Pickup Magnet

Aluminum Shavings on Left Pickup Magnet

Back of Dyna III Points Plate is Corroded

Back of Dyna III Points Plate is Corroded

I disassemble and refurbish the ATU to get rid of the rust and install new advance springs as the original ones are rusty.

The Dyna III electronic ignition uses the stock ATU but has it’s own pickup plate that replaces the stock points plate. It’s easy to install the stock points plate and points should the Dyna III fail at some point. That’s an advantage to this design. I decide to replace the Dyna III completely with a new unit.

Parts List

These are the parts needed for this work. The tension springs are what is know as the “softer” springs. This means full advance occurs a bit earlier than with the older “Heavy Duty” springs (part# 12 11 1 357 546) which are still available.

Part # Description Qty
Dyna-D35-1 Dyna III Electronic Ignition for BMW R 1970-1978: Euro Motoelectrics 1
12 11 1 357 627 TENSION SPRING (to 09/78) 2

Disassemble Automatic Timing Unit

The original Dyna III sleeve is secured to the ATU cam with two set screws. For the stock ATU, this sleeve would not be there, but all the other parts of the ATU would be.

ATU with Dyna III Trigger Magnet Sleeve Mounted to Advance Cam

ATU with Dyna III Trigger Magnet Sleeve Mounted to Advance Cam

I remove the Dyna III sleeve from the ATU camshaft.

Old Dyna III Trigger Magnet Sleeve Removed From Advance Cam

Old Dyna III Trigger Magnet Sleeve Removed From Advance Cam

The ATU centrifugal advance arms are mounted on a pin, secured with a C-clip.

C-clip Secures Centrifugal Advance Weight Arm On Post

C-clip Secures Centrifugal Advance Weight Arm On Post

After removing the C-clip, I use a screw driver to gently pry the advance arms off their posts. I have to twist the arms outward a bit so they come free from the cutout they abut on the top of the cam sleeve assembly.

Removing Centrifugal Advance Weight Arm From Post

Removing Centrifugal Advance Weight Arm From Post

There is a fiber washer underneath the arms.

Centrifugal Advance Weight Arm Removed Showing Fiber Washer

Centrifugal Advance Weight Arm Removed From Center Plate Showing Fiber Washer

Centrifugal Advance Arms Removed Showing Advance Cam And Center Plate

Centrifugal Advance Arms Removed Showing Advance Cam Assembly On Top of Center Plate

I use a small screw driver to push the advance return springs off the posts. These are very small and hard to find if they wander off, DAMHIK. 🙂

Removing ATU Advance Spring From Camshaft Plate Post

Removing ATU Advance Spring From Camshaft Plate Post

ATU Advance Spring

ATU Advance Spring

Here is how the camshaft assembly mounts in the center plate.

Removing Cam Shaft Unit from ATU

Removing Cam Shaft Assembly from Center Plate

Here are the main parts of the ATU; top is the camshaft assembly, below it is the center plate it mounts in, on the left and right are the centrifugal advance arms.

ATU Main Components Disassembled

ATU Main Components Disassembled

Remove Rust From ATU and Clean

I soak the ATU parts in Evapo-Rust over night.

Removing ATU Rust by Soaking in Evap-o-Rust Over Night

Removing ATU Rust by Soaking in Evapo-Rust Over Night

The parts come out blackened. I use steel wool to remove the black residue. Then I wire brush the parts with a stainless steel brush, apply Auto-Sol metal cleaner and scrub with a toothbrush. I apply Auto-Sol metal polish and buff with a blue shop towel. The parts look much better and the rust all gone.

Cleaned and Polished ATU Components

Cleaned and Polished ATU Components

Reduce ATU Full Advance

Since this engine has heads with dual plugs, the ignition timing changes from the stock settings. Since there are two flame fronts expanding across the cylinder it takes less time to burn the mixture. This bike has the 6 Deg “S” timing mark with a line above and below the “S” mark. These lines are 3 Deg from the “S” mark so the top line is 3 Deg BTDC and the bottom line is 9 Deg BTDC.

Tom Cutter at Rubber Chicken Racing Garage advises retarding the timing at idle by 3-4 Deg, so I can use the line above the “S” to set timing at idle. The stock advance range is 28-29 Deg and that should be restricted to 24-25 Deg of range which reducs the range by 3-4 Deg.

Here are the stock and revised timing settings for my bike with dual plug heads.
My R100 Stock Settings
“S” on My Flywheel:       6 Deg BTDC
Advance Range:     28-29 Deg BTDC
Full Advance:          34-35 Deg BTDC

My R100 Dual Plug Settings
Line Above “S”               3 Deg BTDC
Advance Range:     24-25 Deg BTDC
Full Advance:          27-28 Deg BTDC

Tom told me he reduces the advance range by installing old advance springs over the pins of the cam assembly plate. The thickness of the springs reduces the advance range by about 4 degrees. Since I bought new return springs, I’ll use the old ones the same way.

But, I’m curious about how much thicker the pins on the center plate have to be to reduce the movement of the cam by 4 degrees. As shown below, the pin on the top of the cam assembly moves across the diameter of the large hole in the top plate. So, the diameter of the large hole represents 28-29 degrees of timing advance.

Cam Pin Inside Hole in Center Plate-Distance it Moves Equals 29 Degrees of Advance

Pin on Cam Assembly Plate Fits Inside Hole in Center Plate

I measure the hole in the top plate and it’s close to 1/4″ or 0.250″. The diameter of the pin is 3 mm or 0.118″. Therefore the total distance the pin can move across the hole is 0.250-0.118 or 0.132″. If that much movement results in 29 degrees of timing advance at full advance, then each degree of advance is 0.132/29 or 0.004″. Now if the full advance needs to be reduced by 4 degrees, then the distance the pin moves in the hole has to be reduced by 0.004 x 4 or 0.016″.

When I measured the wire thickness of the return spring, it’s 0.016″ +/-. How cool that the advance spring wire is just the right thickness to reduce the full advance of the ATU by exactly what is required.

I clip off the loop on one end of a spring with wire cutters. Then I can grab the loop on the other end with pliers and pull the spring over the pin of the cam plate. It’s a tight fit.

Cut Off One End of Advance Spring (Spring on Left)

Advance Spring Loop Cut Off on One End of Advance Spring (Spring on Left)

Installing Old Advance Spring on Cam Plate Pin

Installing Old Advance Spring on Cam Sleeve Pin

When I get the spring past the groove in the pin, I clip off the other loop with wire cutters. I test fit the cam in the holes of the top plate and adjust the position of the spring so it bumps against the inside of the hole. I remove the cam sleeve and put a drop of super glue on each spring to ensure they don’t slip. The coils are quite tight on the posts, but I figure some insurance is a good idea.

Old Advance Springs Installed on Cam Plate Pins to Limit Full Advance

Old Advance Springs Installed on Cam Plate Pins to Limit Full Advance

Assemble ATU

I assemble all the parts of the ATU; as they say “Assembly is the reverse of disassembly.”.

First, I install the fiber washers, then the advance weight arms–and per the Dyna III instructions, the metal straps are creased in the middle–and then the C-clip is installed to secure the advance weight arms on their pins. Last, I use a small screw driver to push the ends of the new advance springs into the grooves on the pins.

Fiber Washers Installed on Advance Weight Arm Pins

Fiber Washers Installed on Advance Weight Arm Pins

Advance Weight Arms On Pins and Under Metal Straps

Advance Weight Arms On Pins and Under Metal Straps

Slight Dent in Straps Over Arms and C-clip Installed on Pin

Slight Dent in Straps Over Arms and C-clip Installed on Pin

ATU Assembled and No Longer Rusty

ATU Assembled and No Longer Rusty

Add New Seal to Points Housing, Drill Drain Hole

To help keep water out of the points housing, I add a new seal strip to the groove in the points housing. I put a dab of silicone seal at the two ends and at the sharp corners at the bottom. Then I trim the excess off with wire cutters.

Install New Points Housing Seal

Install New Points Housing Seal

I also drill a small drain hole at the bottom of the housing so should water get inside, it will drain out and not corrode the Dyna III pickup plate.

Drilling Drain Hole in Bottom of Points Housing

Drilling Drain Hole in Bottom of Points Housing

Drain Hole in Bottom of Points Housing

Drain Hole in Bottom of Points Housing

Installing Dyna III Electronic Ignition-ATU Cam Sleeve and Pickup Plate

Here are the components of the Dyna III electronic ignition I purchased from Euro Motoelectrics.

Dyna III Electronic Ignition Components

Dyna III Electronic Ignition Components

The black box at the top left is the electronic ignition module. I don’t install it now but will do that when I install the main wiring harness and the coils. I install the pickup plate at the top right and the sleeve with the magnets at the lower right next to the ATU.

The sleeve has a bushing on one side. This goes next to the advance weight arms when the sleeve is slid on the ATU cam sleeve.

Bushing on Sleeve

Bushing on Dyna III Sleeve

There is a “D” shape to the hole in the ATU cam sleeve. You can see it at the 5:30 position in the picture below.

Orientation of Flat of the "D" - At Bottom of Hole

Orientation of Flat of the “D” – At 5:30 Position of Hole

The Dyna III sleeve is oriented so the two set screws are on either side of the flat of the “D” shaped hole. I position it so the set screws are about equidistant on either side of the flat and tighten the set screws. In the picture below the Allen Wrench is about even with the center of the flat of the “D” shaped hole (I tilted the wrench a bit when I shot the picture). The two set screws need to be snug, but I don’t over tighten them or I’ll dent the ATU cam making it unusable with the stock points.

Flat on ATU Cam is Centered Between Set Screws

Flat on ATU Cam is Centered Between Set Screws

Filing Lower Slot of Pickup Plate To Allow Full Adjustment

As I tested installation of the pickup plate, I noticed that the bottom slot in the pickup plate was not wide enough to allow full rotation of the plate in the clockwise direction. I filled the hole on one side until I get full plate rotation.

Pickup Plate Rotates Fully Counterclockwise

Pickup Plate Rotates Fully Counterclockwise

Pickup Plate Doesn't Rotate Fully Clockwise

Pickup Plate Doesn’t Rotate Fully Clockwise

Pickup Plate Rotates Fully Clockwise After Enlarging Bottom Hole

Pickup Plate Rotates Fully Clockwise After Enlarging Bottom Hole

The wires of the pickup plate don’t have the tough outer sleeve that is used on the stock points wire. The sleeve protects the wires from chaffing when the metal clamp on the bottom plate screw is secured over the wires. Out an abundance of caution, I put some heat shrink tubing on each wire where they will be clamped and cover them both with a larger diameter of heat shrink tubing. The last thing I want is a frayed wire under the clamp cause problems on a dark and stormy night 🙂

Adding First Shrink Tubing

Adding First Shrink Tubing

Adding Second Shrink Tubing

Adding Second Shrink Tubing

I route the red and white pickup wires through the black tubing, route them behind the diode board. I push them though the “D” grommet and orient them so they are behind the large engine electric harness.

Routing Pickup Wire Through Rubber Sleeve

Routing Pickup Wire Through Rubber Sleeve

Routing Pickup Wires Behind Diode Board

Routing Pickup Wires Behind Diode Board

Routing Pickup Wires Through "D" Grommet

Routing Pickup Wires Through “D” Grommet

Pickup Wires Behind Engine Wire Harness

Pickup Wires Behind Engine Wire Harness

I like to replace the slot head screws used to secure the points plate with M4 x 10 Allen head screws. These are much easier to get to when adjusting the timing.

New Allen Head Screws To Secure Pickup Plate

New Allen Head Screws To Secure Pickup Plate

To avoid water being trapped in the points housing, I drill a small hole at the bottom so it can drain out should the grommet on the rubber points wire tube or the edge seal fail.

Drilling Small Drain Hole in Bottom of Points Housing

Drilling Small Drain Hole in Bottom of Points Housing

Location of Small Drain Hole

Location of Small Drain Hole

Here’s the Dyna III sleeve with magnet and pickup plate mounted in the points housing. From this angle, it look just like the stock points assembly.

Dyna III Sleeve and Pickup Plate Installed in Points Housing

New Dyna III Sleeve and Pickup Plate Installed in Points Housing

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