After digging through various on-line resources concerning timing an engine with dual plug heads, I found it all a bit confusing. Part of the confusion is due to the changes BMW made from 1977 through the early 1980’s as they reacted to vehicle emission standards in the US. Gasoline was changing with the removal of tetra-ethyl lead so octane was becoming lower. The higher compression (9.5 : 1) design of the R100S, RS and RT engine introduced in model year 1977 meant detonation, or pinging, could occur without higher octane fuels.

Resources

This is a list of resources I used in compiling this information.

• Bob Fleischer: Flywheel F, OT, S, and Z marks
• Bob Fleischer: Ignition timing & ATU. Single spark plug BMW Airhead motorcycles
• Bob Fleischer: Dual Plugging BMW Airhead Motorcycles
• Bob Fleischer: Ignition
• Bob Fleischer: Butler & Smith SI: Ignition
• Tom Cutter, Rubber Chicken Racing Garage: Email Communication

Flywheels and Markings

BMW changed the markings on the flywheel to retard ignition timing to avoid pinging. Instead of 9° before top dead center (BTDC) of ignition advance at the “S” mark and 34° BTDC at the “F” mark, they reduced both by 3°. Therefore, the “S” moved to 6° BTDC and the dot above the “F” mark shifted the same amount to 31° BTDC. Said differently, all the flywheel marks were moved the same amount so both the “S” and “F” were retarded by 3°.

Here is what I found out about flywheels. I think this is accurate. It is based on material Bob Fleischer published including the Butler & Smith SI. Based on my bike, the “11-22-1-263-788” flywheel had the retarded markings; “S” at 6° and “F” at 31°

(From Bob Fleischer & Butler & Smith)

Part No / Description	Teeth	Bolts	Reuse?	FT-Lbs	“S”	“F”
11-22-1-256-966 /5 & /6 to 1974 (I think MY1975)	93	10 mm	NO	42-45	90	340
11-22-1-262-070 /6 1975-1976 (I think MY1976)	93	11 mm	YES	74+ 52-55 76: 75-80	90	340
11-22-1-263-788 1 /7  1977-early 1978 (non emission)	111	11 mm	YES	75-80	60	310 *
11-22-1-336-380 1978-80 w/ Emissions	111	11 mm	YES	75-80	60	310
11 22 1 337 284 ’81+ Clutch Carrier	111	11 mm	YES	75-80	60	310

Can not find this part number in MAX Fiche. Bob Fleischer indicates this was a 9 ° “S” and 34 ° “F” mark. Bulter & Smith SI (link below) shows 11-22-1-263-788 Part # for /7 in 1977. I ASSUME that 11-22-1-336-380 superceded “788” at some point.

* Butler & Smith SI indicates ALL /7 models, starting in 1977 had “S” at 6 ° BTDC, advance range was 25 ° & “F” was 31 ° ————————————————————————————————————————————————-

This is the Butler & Smith SI (below) from Bob Fleischer’s web page on flywheel markings. The SI includes information in a table about the timing advance unit (ATU) with partial part numbers and different flywheels showing the part numbers and location of the markings.

[SOURCE: Bob Fleischer: http://bmwmotorcycletech.info/F,OT,S,Z.htm]

These flywheels have a mark above the “S” and below the “S” approximately 3 degrees either side of the “S”. These represent the allowable difference in ignition timing between the two cylinders.

Here is my flywheel after I added paint marks to the “OT”, the three marks around the “S” and the “F” mark to improve visibility when I time the engine or set the valves.

I cut a strip of graph paper ruled in millimeter increments and measure the existing marks on the flywheel.

I measured the diameter of the rim the timing marks are on (213 mm) computed the circumference (pi x 213 mm) = 669 mm and divided that by 360 to determine the mm / degree (1.85 mm/°). This shows that the markings are:

• Line Above the S:           3°
• “S” line:                           6°
• Line Below the S:           9°
• Center of Dot Above F: 30°

NOTE:
Although the math is correct, the acceptable difference in the measurements can be +/- 2 mm and still be quite acceptable for the timing the ignition.

Adding a Dual Plug Full Advance Timing Mark to Flywheel

I add a full advance mark at 25° BTDC to the flywheel. That way I can time at full advance which is the recommended method of setting the timing.

Automatic Timing Unit (ATU)

With dual plug heads, the time required to burn the fuel mixture is less than with a single spark plug. Since there are two flame fronts moving outward in concentric circles with each spark plug at the center of a circle, it takes less time to burn the mixture. As the mixture burns, the temperature inside the cylinder rises and as the cylinder continues to rise it further heats the mixture by compressing it. The combination of these two processes means it’s much easier for the mixture to ping if the ignition timing is not retarded as there is more heat build in less time since more of the mixture is burned.

There are different ATU models. This is the model I have in my bike, the Bosch (0 232 002 012).

This table is a compilation of information I found on-line from Bob Fleischer and includes his link to the Butler & Smith SI on the subject of ignition timing (above).

Bosch ATU [Sources: Bob Fleischer and Butler & Smith SI]					Flywheel      Markings
BMW Part# /  Bosch #	Applies To	Springs		Adv  Range	“S” Mark	“F” Mark
12-11-1-351-571 0 23 2 002 005	/5	12 11 1 356 546 “Heavy Duty”		31 0	9 0	40 0
12-11-1-353-639 0 232 002 007	/5	12 11 1 356 546 “Heavy Duty”		25 0	9 0	34 0
12-11-1-356-286 0 232 002 010	/5 To 9/74	12 11 1 356 546 “Heavy Duty”		25 0	9 0	34 0
12-11-1-357-626 0 232 002 012	/6 to 9/74	12 11 1 357 546 “Heavy Duty”		25 0	9 0	34 0
12-11-1-357-626 0 232 002 012	/6 After 9/74	12 11 1 357 627 “Soft” To 09/78		25 0	9 0	34 0
12-11-1-357-626 0 232 002 012	R100RS To 9/78	12 11 1 357 627 “Soft” To 09/78		25 0	6 0	31 0

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Reference (1) above contains BMW part numbers to Bosch part number listings for the ATU and advance ranges. Reference (2) contains the link to the Butler & Smith SI (see above.)

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The advice I received from Tom Cutter of Rubber Chicken Racing Garage for the R100RS is to retard ignition timing at the “S” by 3 degrees and retard the timing at the “F” by an additional 3-4 degrees. To retard the “S” timing just requires adjusting the ignition timing. It may require filing the screw holes that secure the timing plate to get more adjustment range if the stock points are used. To retard the full advance timing the distance the automatic timing unit (ATU) rotates is reduced. This means the dot above the “F” no longer represents full advance. Since the recommendation is to set the timing to achieve the desired full advance, I want to put a mark on the flywheel above the dot so I can use it instead.

Adjusting the ATU Full Advance

Tom Cutter’s suggestion is to use old ATU springs to reduce the full advance. The ATU is composed of a number of parts as shown below.

The “T” shaped part at the top center is the cam assembly with the cam shaft and top plate. Note the two pins extending from the top plate. Underneath it in the center is the center plate with three holes. The center hole is “D” shaped and mounts the ATU on the camshaft nose. The pins of the cam assembly top plate fit inside the other two holes. As the centrifugal weights, the two curved arms on the left and right, move outward they rotate the cam assembly so the pins move from one side to the other inside the holes in the top plate.

You can see how I modified the ATU here:

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

Revisions

2019-01-17 Fixed broken link to article on modifying ATU.