Ignition timing & ATU.
Single spark plug BMW Airhead motorcycles;
installing and adjusting points.
Automatic timing units (ATU) for both single-plugged
and dual-plugged Airheads.
This article is INTENDED to be used with these articles:
I HIGHLY ADVISE you read all of the above articles!
You may want to additionally read this one:
Copyright, 2013, R. Fleischer
Points removal, replacement, adjustment, maintenance. Amplifiers/boosters.
NOTE: See section on points amplifiers in:
It is extremely common to hear about a poor running engine, and often a hard-starting engine...the cause being nearly completely closed-up points. Other common causes include closed-up valve clearances. There is more information in this article, below.
When I install brand-new points, I set them slightly wide, about 0.019" for all models, and I am VERY careful to lubricate (sparingly!!) the cam and the felt (no felt on canister models). Since I have to remove the ATU (on models up through 1978) to replace the points, I also lube the shaft, and inspect the ATU. If you do NOT lubricate the POINTS CAM, the points rubbing block will wear rapidly, causing rapid closing of the points. Lubrication is done SPARINGLY!
On the 1979-1980 models, the points are located in a canister; things are a bit different; but, you still must have a faint trace of lubricant on the points cam.
I've even heard those rubbing blocks (especially models up through 1978) make squeaking noises due to lack of lubrication! You can sometimes hear this even with the outer engine cover in place. Bosch has several lubricants for use at the points cam, and at the ATU shaft. I prefer to use the original specified lubricants, but I WILL use almost anything in a field repair....any good high temperature grease.....even something like BMW #10 red grease will help. I still have the same tubes of my original Bosch greases, that I purchased decades ago. The Bosch numbers were: Ft 1 v4 for the felt; Ft 1 v22 (or 26) for the ATU. DO NOT over-lubricate the cam nor felt! You do NOT want the lubricant to fling off and contaminate the points. That can cause burning and misfiring. The Bosch lubricants do NOT tend to fling off, compared to common greases.
Regarding the ignition points for 1970-1978 Airheads: BMW has, at times, shipped badly made points sets, made in China. The rubbing block is too long (wide), and you cannot get proper timing, etc. I suggest the Norris points from such as Beemershop, etc. if you cannot get your points properly adjusted and timing set properly. Pre-1979 cam tips:
Rotating the points plate clockwise will retard the spark.
1970-1978 Airheads points are located on the forward end of the engine's valves opening camshaft in a small engine cavity. The lower points screw head is very close to the points spring, and if a wrong screw or washer is use, the screw can contact the spring and the points are then shorted, and you get no ignition. This is seen now and then right after someone replaces the points and can't understand why the engine will not start. There can be another reason and this reason shows up immediately...or later!....the wire from the points cavity to the rest of the motorcycle fits through a wire-protecting rubber grommet, and this grommet often gets displaced from its slot when working in the area. DO NOT replace the outer cast metal front engine cover until the wire and grommet are in proper position, or you can cut through the wire and cause a short circuit when replacing the cover. The typical result is no ignition, or ignition works only at some temperatures. That fault might not occur right away.
It is important that not only the grommet be placed properly, but that the O-ring in the irregular shaped groove outlining the cavity be in good condition. Otherwise dirt & moisture can get to the points.
CLEAN the points by rotating the engine until they are closed (so are pressured by the points spring blade) and then opening them by hand ONLY A SMALL AMOUNT, just enough to slide a piece of paper between the points. Absorbent paper of some sort, but not a linty paper towel. I use a non-glazed business card or common printer paper, etc. Put a couple drops of a fast drying solvent on the small piece of paper first. Acetone is fine (NOT wife's oily based acetone). That will eliminate any oil film layer on the points. You do not want to file the points because that removes the special metal layer on them.
If you have to file, because there is a pronounced tit and valley on the points, then do it only lightly, and use a fine grit points file or thin flat jewelers file. Keep the tool square to the points and do not pressure the points much.
Be sure to lubricate the felt and cam, SLIGHTLY, with any good high temperature grease. Check the points every 5,000 miles for gap, clean and refresh the grease, and check timing.
Be sure the points cavity area is clean and dry; pay attention to the rubber grommet and wire as you put the front cover back on the bike.
I will assume the seal for the cavity is good/intact.
On BMW boxer Airhead PRE-1979 motorcycles, there is no canister ignition assembly. ONLY the 1979 and 1980 Airheads had ignition points in a canister; and the engine camshaft that drives the canister mechanism was quite different at its nose. The discussion here is for PRE-1979 models.
The pre-1979 ATU (Automatic Timing Unit) is mounted to the forward end of the engine camshaft by one nut (and a waverly washer). The ATU needs to be secure from rotating in relationship to the engine camshaft to which it is affixed, so the ATU has a D shape hole, which mates to a mating D shape on the forward end of the engine camshaft. It is the TIP (threads and D shape) area of the engine camshaft that is under discussion here:
The forward part of the camshaft, that nose area, which is the threads and D shape, can be found bent at times. The amount of bending is NOT visible to the eyeball. There is a way to fix it, with a small brass hammer. The bending may be only a fraction of a thousandth of an inch, to a few thousandths. The result is, as the ATU rotates, the points do not typically have the exact same GAP and thus not having the same timing point, for both ATU timing lobes. This will result in a dual-image when using a strobe lamp on the flywheel. This results in vibration, often at one or more narrow ranges of RPM. This type of problem is often WRONGLY interpreted as carburetors being out of synchronization. The Dyna dual-pickup electronic ignition will eliminate most double-timing, but the camshaft TIP is usually fixable by slightly and VERY carefully using a SMALL brass or SMALL dead-blow hammer on the tip. ONE OTHER cause for this sort of timing irregularity is some advance unit WEAR, and even irregularity in the factory grinding of the cam lobes. BMW at one time recommended 'stoning' those lobes to equality. Don't bother...AND...it IS TRICKY TO DO! But, a rather poor ATU needs fixing. If the dual timing images are over ~3/16" apart, you could consider fixing the problem. NOTE also, that a worn timing chain, worn chain guides, & wear on one or both timing sprockets, will ALSO cause double images & the same results. Quite noisy chains are indicative.
The brass hammer 'fix' must be done carefully. You must NOT crack nor knock off the threaded cam tip. Use of a dial indicator on the smooth side surface of the cam tip is how run-out is measured. Do NOT try the brass hammer fix unless you KNOW what you are doing, have a dial indicator & know how to use it!!
If a cam tip is broken off, and this usually happens not from the above brass hammer work, but from someone OVERTIGHTENING THE NUT, the Boyer ignition can be used, and was popular for this. Other methods include several types of ignitions that mount to the alternator area, and thus are crankshaft driven, which CAN be a very accurate ignition method.
Broken camshaft tips can often be center-punched, drilled, and tapped for threads; all using a guide made on a lathe. You can also contact: Paul Tavenier, firstname.lastname@example.org
Exact details on how to center-punch, drill, tap, etc., the broken end of the pre-1979 camshaft tips is located HERE:
Pre-1979 cam tips:
When setting the ignition timing, the points gap MUST be set FIRST. Rotate the points plate CCW to advance the timing. For the canister, that means the canister is rotated. Points wear is almost entirely that of the rubbing block, and the points themselves. Do NOT ever 'file' the points, unless it is more or less an emergency in-the-field problem;....the points have a rather thin layer of something like tungsten (?) on them, and the filing can remove that hard layer. I'd not hesitate in the field to file them if they have large tits showing. Just file squarely. Don't open the points quite wide with your fingers when installing or filing, ...you can take some of the tempering spring action out of the points leaf spring....and then the points may 'bounce' at high rpm, causing misfiring of the ignition.
On the ATU, in depth; and, on irregular timing images (dual images, etc):
Fatigued springs on the ATU are easy to see with your eyeball. You don't really have to even remove the ATU. At rest (engine off), the springs must exert a SMALL amount of tension to keep the weights in the fully closed position. You don't want "advancing" to begin until the engine rpm rises a fair amount, typically a few hundred RPM above idle. You can move first one weight, then the other weight, a very small amount.....and see if the spring(s) have enough tension. If totally zero tension in the closed position.... and worse if zero to none on slight weights opening, you need springs. Check BOTH weights springs.
Further tests are probably advisable:
Use a strobe light on the flywheel markings, and have the engine at idle rpm. You can slow the engine further, and you can also raise the idle. For slower, it MAY be helpful to have a buddy slip the clutch and hold the brake, and maybe have the front tire up against a wall, that way you don't have to fiddle with carburetor adjustments. I will be explaining this.
There is one place that test is iffy:
The very earliest ATU's (very early /5 mostly) started their advance at ~800 rpm, which is/could interfere with the higher normal idle rpm that you may use, especially since that is best. IMO....idling the engine around 800 is not a great idea, as it can reduce chain and sprocket oiling and speed up the need for timing chest parts as the chain is jerking around more at low idle rpm. So, if testing a very early ATU-equipped bike, be sure to slow the idle down (see prior paragraph) to be sure that you have the ignition timing set correctly for a slow idle rpm. Many an early /5 had its springs or ATU changed. BUT....from the /6 days, the ATU's got modified, and for our purposes here, it is the springs-change that is most important. Every ATU has a Bosch number stamped into the outer plate....and there is a cross reference chart, later in this article, listing them. The Bosch number is NOT A BMW NUMBER. The springs could have been already changed, and without a new number engraved (typical).
For the /6, or a ATU with updated springs, the rpm at which advancing began was boosted to around 1550 or so....and it is easy to see, and you don't have to slow the engine below a decent regular idle (~1025 rpm). TEST: raise the rpm and nothing happens to the timing mark until ~ 1550.
The final test is to find out at what rpm the advancing quits. Early spring models are at around 2000 rpm, and /6 and later and early models with the stiffer springs, are at about 3000 rpm.
The change to 3000 rpm springs will reduce pinging problems. In fact, with today's lower octane 'regular' gasoline, going to 3500-or tad higher, can be even better....might allow more use of lower octane fuel....but, be cautious. The power difference is very minimal during acceleration from idle. Going to 3500 is usually done by using the 3000 rpm springs and then hand-filing the advance weights a bit (same amount for both of them). DO NOT STRETCH THE SPRINGS!
Split or dual images are very common. Small distances between the dual images are not of importance. As the distance gets greater, more engine vibration and unbalance will be noted. Specifically identifying what the problem is usually means some careful work, but some of it, such as measuring run-out on the camshaft tip area, is not difficult. The camshaft tip can be re-bent, but I advise against it, unless you have practiced, know what you are doing. You do NOT want to break off that threaded tip area by whacking on it, which is the wrong place. Adjustment amounts made are small....on the order of 0.001" to 0.002". Mostly, best to leave things alone. ALL models of ALL stock Airheads can have dual images!...but, obviously, those after 1978 do not have the old points at cam tip design, but the points (or electronics pickup) is in a canister, and that type of ignition is more precise, but still not perfect, because there are places the drive to the ignition can become irregular, such as the chain, sprockets, chain guides, etc.
There are TWO lobes on the timing cam in the ATU for the points models, and on very rare instances these lobes are not precisely ground by the factory as the same. This takes some sleuthing, but on a practical basis, the performance difference is small, showing up as a split image but a slight additional motor vibration.
For the 1970-1978 models, in a few instances, the D-shaped-hole in the middle of the ATU gets worn enough that the ATU is not well-clamped in position. ATU's are, unfortunately, $$$$. NEVER EVER over-tighten the nut, AND, DO use a waverly washer.
Worn chain guides, worn sprockets (but seldom any sort of worn chain) are going to give multiple images. Once the ATU is ruled out for weak springs or badly worn pivots, and the shaft is not running-out excessively; or, you want to try this anyway, there is a sort-of test to hopefully get some ideas about the timing chest parts. Have the valves, ignition, carburetors, all properly adjusted FIRST!
1. Go for a ride, 10 miles or more. The idea is for a full-engine case, etc. warmup.
2. Put the bike in neutral and on the center-stand.
3. TIE the on-bars clutch lever full back. This will let the transmission be totally UNpowered by the engine, as sounds in the hot transmission can mask, particularly for the inexperienced, the sounds you will listen-for.
4. Start the engine, and let it idle. A slower idle is best, but anything under 1200 is usable.
5. LISTEN to the engine.
6. Raise the rpm, keep listening.
7. Find the rpm in-between just a SMALL amount above idle, and a very small amount faster than that. This will require tiny manipulations of the throttle. As you raise the rpm maybe 100 or 200 rpm, and then back it off the same amount, there is a sound change. Engine rattling noise changes will be quite noticeable with worn parts.
8. Then listen at a slow idle.
With a bit of experience and practice, you will be able to identify the source of the rattling noises. NOTE that if the clutch lever was NOT pulled back and tied back, you might be hearing the VERY COMMON hot-oil transmission rattling, and be confused.
MORE...and some different ways of explaining things:
There are TWO lobes on the Automatic Timing Unit (ATU). On the models up through the 1978 model year, the points and timing unit are mounted at the nose of the camshaft, and NOT in a canister like the later models. The forward part of the camshaft, that nose area, of NON-canister models, can be found bent at times. The bent amount is NOT VISIBLE TO YOUR EYE. There is a way to fix the bent tip, with a small brass hammer, but that is not the main point of what I want to say here. When the tip is bent, the bent part may be only a fraction of a thousandth of an inch, to a few thousandths; then, as the ATU rotates, the points do not typically have the exact same GAP and TIMING POINT for both ATU timing lobes. This will result in a dual-image when using a strobe lamp on the flywheel and will result in vibration, often at one or more narrow ranges of RPM. In some instances, the effect at higher rpm is one of points bouncing, due to the irregularities. BOTH of these types of problems are often WRONGLY interpreted as carburetors being out of synchronization. The Dyna dual-pickup and dual-adjustable electronic ignition will eliminate most double-timing, but the camshaft TIP is fixable for the use of the stock ATU and points (or, points with amplifier). ONE OTHER cause for this sort of timing change is some advance unit WEAR, and faintly slight irregularity in the stock grinding of the cam lobes, but mostly it is pivots wear and springs wear. BMW at one time recommended 'stoning' the lobes to equality. Don't bother! But, a poor ATU needs fixing, as does a bent cam tip...if excessive. NOTE that a worn timing chain, chain guides, and wear on one or both timing sprockets will ALSO cause double images and the same sort of typical results. Quite noisy chains are a giveaway. You can determine if the problem is the ATU or the chain, guides, and sprocket(s), by very carefully rotating the engine very slowly in one direction and checking the timing via a light or meter connected to the POINTS, and comparing the results between lobes, to the flywheel markings. This can even be done on the canister points models. Mostly, the technique will show up irregularities in the cam tip and/or ATU cam.
If the 1970-1978 cam tip is bent, the points gap will vary for the two lobes. The brass hammer 'fix' must be done carefully, not knocking off the threaded cam tip, and use of a dial indicator on the smooth surface of the cam tip will tell the story....and if you have fixed the problem. Do NOT try the brass hammer fix unless you KNOW what you are doing, have a dial indicator and know how to use it!!
In general, due to the mini-disaster that can happen if you break a points cam tip, I recommend
Points, Condensers, Amplifiers/boosters, ATU, Dwell:
If the capacitor (aka, condenser) (which is probably rated at 0.2 microfarad) fails (not all that common, actually), the ignition can either quit, or the points starting burning up at a fast rate. Points will last about 12,000 to 25,000 miles withOUT an amplifier or booster; and MUCH longer with one. You DO have to maintain a slight lubrication on the cam that drives the rubbing block on the points if you want the points to last. You MIGHT have to file the points very lightly. Contrary to my own advice, on a few of my touring airheads that had points (and NO amplifier/booster), what I actually did was to, yes, very LIGHTLY file, one light pull-through, the points with a very thin diamond coated blade (300 grit) every 5000 miles when I set the valves. I checked the valves first, on a cold engine, before checking the points, so I did not have to wait for the engine to cool after setting timing. I checked the cam lube, checked the gap, checked the timing at high rpm....rode the bike, and synch'd the carbs.....this was standard at every 5000 mile interval. Points amplifiers or boosters are a very nice addition to the motorcycle, as points can last for many tens of thousands of miles with just cam lubrication, and a booster and no filing, just checking gap and timing. You eliminate wear of the points themselves (you still have to replace them eventually, for rubbing block and other wear, but they will last a LONG time, if you keep the cam lubed) by using some sort of points amplifier. By keeping the capacitor in its normal place you can, if the amplifier fails, convert to the stock system in a FEW minutes! Dyna still makes points amplifiers, and are probably others, including Boyer, and there is the Velleman kit, and others. See: http://bmwmotorcycletech.info/Ignition.htmThere are full electronic ignitions available too, from such as Boyer-Brandsen and Dyna....and there are even newer styles that attach to the crankshaft at the alternator, but if these types fail on a tour, you are less likely to fix it nearly as easily as with the simple points amplifier equipped bike. If you install a Dyna pickup unit, in place of the stock points, follow Dyna's advice about a drain hole.
There is an aftermarket sleeve tool available, that some folks like, it allows the old style ATU to be removed and the points adjusted without the fun of small feeler gauges in cramped places. Note that there is a SMALL feeler gauge tool that used to be shipped with the early bikes, those that had ATE brakes had an additional blade in that tool.
See: http://bmwmotorcycletech.info/tools.htm for a complete description of the above feeler gauge tool (photos), and the sleeve tool.
Here is a photo of the aftermarket-available sleeve tool:
BMW modified the automatic ignition advance unit (Automatic Timing Unit, ATU) ~ the end of 1972. Changes were made in the early years to the mechanical advance unit and to the flywheel markings. The stamped part number on your automatic advance is a Bosch number. Not only was the cam profile of that automatic advance unit changed from 110° to 78° dwell with a sharper shape, but the springs were changed, not all these changes were done at the same time. Same 'phase-in' was done for the flywheel remarking, incorporated with the 1978 3 degree change in the camshaft gear drive. DWELL, RATE of advance, and RPM at which the advance stopped advancing, and flywheel markings were all changed over the years.
SOME few flywheels do NOT match the camshaft sprocket gear, a rather rare situation. Determination is made with a degree wheel on the crankshaft, piston stop on the cylinder, etc.
Below is the background technical information on the automatic advance units used on the NON-canister bikes (ALL prior to 1979). Keep in mind that it is possible that someone has changed the springs on your automatic advance, or even otherwise modified it...in fact, you might have any of the ATU models in your 1970-1978 bike.
In late 1972 BMW made changes to the automatic advance (centrifugal advance unit, automatic timing unit, ATU). This was done in TWO stages:
To the R50/5 after chassis 2901787 with the second change as of chassis 2905857; to the R60/5 after chassis 2932689 with the second change as of chassis 2946096; to the R75/5 after chassis 2973307 with the second change as of chassis 2996220. The first Service Information bulletin came from Munich, dated Nov. 1972, and the bulletin was numbered as 4/72 (041 M)e.
The second bulletin was also from Munich, numbered 1/73 (056 M)e.
Both referred to SI 284e, Group 13, page 2., which is unimportant HERE.
Don't be overwhelmed...I'll get deeper, but will explain it all.
[Parts shown below described as 'fully suppressed' mean the Authorities (police) models]
From November Bulletin:
Earliest ATU mechanism: Bosch 023 202 005 BMW 12-11-1-351-571 15°30' timing. Note that this early mechanism had a 110° dwell. The updated mechanism: Bosch 023 202 007 BMW 12-11-1-353-639 12°30' timing. NOTE that some were 110° dwell, some were 78° dwell, same part number. The 78° ones were also more abrupt on the cam lobe shape of the ATU. Updated mechanism, but with the breaker plate fully electrically noise suppressed, complete assembly, Bosch 023 202 008; BMW 12-11-1-354-404. 12°30' timing.
Here is a .gif of one of BMW's Euro bulletins (yes, Snowbum has Euro info too!). Pay a bit of attention to this chart. Note that the End of Advance is NOT the typically discussed 2,000 rpm and 3000 rpm, but is more precisely said here as 2500, 2200, and 3200. This chart information is NOT likely found ANYplace else but right here on this website. Note also the advance range and dwell details. If you are hopping-up an Airhead, you may well 'see' that faster advance, and more advance range may well be used, together with a longer dwell angle, for quite high RPM, such as for racing. I've added the 012 unit under the chart.
What is DWELL ANGLE? Dwell can be described in two ways. One is the number of degrees the points touch each other electrically...in other words, it is the number of degrees of rotation during which electric current can flow for charging the ignition coil(s) magnetically during engine operation. It is that fraction of a rotation that the points are closed versus open. But, dwell is REALLY the time in degrees that the ignition coil is being charged; or, better said, the time that current is flowing in the primary circuit of the coil. Dwell thus can be applied to mechanical points, as well as mechanical or electronics means of coil charging or coil current flow. To explain this a bit more, you can easily see that with a cam lobe (two lobes on the Airheads) the points are, due to lobe shape, open and closed for certain degrees of rotation. The points need to be closed for a MUCH MUCH longer time than they need to be open, to ensure coil charging. The cam lobe shape also affects the speed at which the points suddenly open (and close...there are racing points cams, used a long time ago, that had different shapes on the rise, than the fall, of the lobe time). But, consider the two basic types of FULL electronics ignition. These do NOT use points. One type of electronic ignition uses timing built-into the module. BMW uses that type AND shape of the butterfly vane. Thus, the width and shape of the butterfly vane in the 1981 and later Airhead ignition has that type of dwell control. Using a combination method allows BMW to eliminate false ignition triggering/sparks, and BMW modified the late eighties modules to eliminate false triggering possibilities. I get deeply into that subject in the ignition article: http://bmwmotorcycletech.info/ignition.htm.
The discussion in that article is actually needed, as coils and modules were changed numerous times by BMW and some combinations will not work well, or, can be UNreliable. It is relatively easy to measure mechanically as well as electrically. The longer the dwell time, the more time for current to flow and charge the ignition coil. BMW changed the dwell initially because the Authorities (Police) motorcycles, which had metal shields over the coils (and retained more heat), let the coils overheat....and later BMW found that the 78° dwell was more than enough for coil saturation at any reasonable RPM, so all units were changed to 78 dwell (until the canister models). There is NO good reason for you to change an ATU for dwell reasons, although if I was racing an airhead on the racetrack at very high rpm, I would want a longer dwell ATU.
More on timing, dwell, etc:
The BMW engine combustion chamber characteristics and sound radiation from the fins is good at telling you, by noticeable audible pinging (pinking), of an over-advanced or poor gasoline octane situation. Hence, it could be 'acceptable' to advance the timing until pinging at low-mid-rpm is heard under high throttle, then back off a bit. I don't recommend it.
Using a dwell meter you probably do not have a 2 cylinder dwell meter switch position. If you have a 78° dwell timing cam, and set the dwell meter to the 4 cylinder 4 stroke position, the dwell meter should read 39°; that is, 1/2 of true dwell. In the same manner, the 110° timing cam will read 55°. I think timing by dwell angle to be almost foolish on a stock BMW. The main purpose of setting points by DWELL readings is to ensure adequate coil charging. Coil charging is NOT ANY PROBLEM on ANY BMW Airhead motorcycle, until you approach red-line rpm, and even then if the points and points spring is in good condition, there will likely be NO difference, for a modest change in dwell. Some old-timers don't use point gap, but dwell angle, to set the points gap. This can result, with some point sets, or worn points, in excessively narrow points gaps, which can cause points wear from electrical arcing outside the design range. Please do not set points by dwell angle, unless you are very familiar with various effects. If you are just curious, go ahead and measure.
You have already learned that the reason for the change from 110° to 78° dwell was that the Police models, with their heat retaining metal shielding around the ignition coils, were having coil failures. 78° is PLENTY of dwell for coil saturation, at Airhead engine speeds. Racers using exceptionally high rpm might get a wee bit more ignition performance by installing the early 110° cam, but the points don't like super-high rpm, although the stiffer spring points spring mentioned well above (and a Porsche spring for the canisters) do help.... and racers are likely to be using other ignition means, such as electronics, perhaps even crankshaft triggered ignition.
CONFUSING THE ISSUE FURTHER (??), IS THE FOLLOWING:
Only a bit later than BMW's phasing-in of the noted changes, BMW was finding a lot of pinging (pinking), in countries where the gasoline had a relatively low octane. BMW again changed the automatic advance, so that the prior maximum advance which was obtained at about 2200 rpm, was now 2800 (some literature in a few places, erroneously), 3000 or 3200 rpm (also depending on which literature you are reading). The previous 'new' mechanism, 12-11-1-353-639 retained that same BMW part number.BUT the Bosch number on the automatic advance changed, and the ONLY ACTUAL change was the two springs. There is even more on this later in this article.
0232 002 007 Bosch, BMW 12-11-1-353-639, was now Bosch 0232 002 010
The fully suppressed assembly was 0232 002 008, was now 0232 002 011
The early springs were BMW 12-11-1-356-142, the newer, very slightly stiffer springs, were 12-11-1-356-546. So, by simply changing those springs, the ATU got a new number from Bosch, but not from BMW, and the maximum advance point went up roughly 1000 rpm higher. That is, the maximum advance point was no longer ~2000 rpm, but ~3000 or ~3200 rpm. (most are actually ~3000).
NOTE that Performance bikes, with higher octane gasoline, will perform SLIGHTLY better with the early springs giving the faster advance and one might consider using the 110° dwell unit, which helps SLIGHTLY at very high rpm (only).
An even later automatic advance unit was made, carrying the Bosch number ending in -012, and was used on the late /6 and /7 to 1978, still with 78° Dwell. Units ending in Bosch numbers -007-012 had advance limited to 34° BTDC.Only the 005 unit had 39° BTDC. The early advance looks the same but the holes in the cross plate allow the advance to swing a bit more open. Those hotting-up their bikes need to know about this.
In 1979 BMW went to the canister system. 1979 and 1980 models had 120° dwell, using POINTS. With these later coils, and the canister points ignition, the ignition was considerably better than before. One particular improvement was especially nice: The engine valve-opening camshaft no longer had the sometimes troublesome (breaking or bent) extended nose. In addition, the new camshaft had a flat forward end having an OFFSET slot, and a matching offset tang on the canister. Problems with slightly wobbly cam tips were gone; the points were no longer exposed to dirt, etc. that was possible with the old style method of 'sealing' the points area, there was lessened chance of oil contamination, and the entire ignition system had higher output and more precise timing, especially comparing one cylinder to the other.
In 1981 BMW kept the canister but went to full electronic ignition (except for the ATU which remained mechanical). The dwell was now 104°, and the ignition output increased, and became even more dangerous for electrical shock hazard. Along the way, BMW changed coil characteristics and specifications. Over the years to the end of production of the Airheads in 1995-1996, there were three changes to coils and numerous changes to the ignition modules. This is a complex subject, and is FULLY treated here: http://bmwmotorcycletech.info/Ignition.htm
The important timing point for all single spark plug BMW's, is NOT the "S" static (idle) marking, but the MAXIMUM advance point. The maximum advance marking on the flywheel of early models is a F-marked dot depression. Later models use a LINE next to a Z marking.
There is also an OT marking on the flywheel for top dead center (exactly and fully pistons outwards), used mostly by owners for setting valves and putting the flywheel or clutch carrier back onto the crankshaft in the correct position with regards to the bolts.
Some bikes have some markings with two lines, they are 6° separated, the official limits for double images using a strobe lamp, and have other uses, not important to this article (they show maximum allowable differences between cylinders using a stroboscopic lamp).
I am well aware that there may be conflicts here with SOME publications regarding setting the timing at the S mark. Many years ago, the idle (often called STATIC, since it could be done at engine off) timing mark (S) was used on points-containing engines, by setting the (engine off) points opening position, with an ohmmeter, or voltmeter, or test light, or even some thin paper like cigarette rolling paper. Strobes were not plentiful nor overly cheap, and the above methods sufficed. It was always a compromise, as the range of advance and the maximum advance point are far more important. PROPER METHOD is to set timing so the mark, F or Z, occurs at and above the advancing RPM limit (raise RPM to find that point)..... and let the S mark be wherever it does, within reason, unspecified.
All BMW airheads use a similar mechanical ignition advance assembly. The timing weights area on the timing units tends to wear VERY slowly, and the wear tends to INcrease the amount of TOTAL timing advance available...which is usually just fine (although sometimes the timing is less stable).....but may be marginal on some older BMW's, such as the R60 models, which tend to need super premium fuel, especially with considerable carbon deposits in the combustion chamber. This is somewhat moreso if one has 39° advance unit. In fact, many of the old models are fitted with compression lowering plates, to allow use of lower octane fuels. The very mild valve timing of the R60 tends to make it rather a lot more sensitive than other models to pinging (pinking). That is because quite mild valve timing ACTS as if the cylinder pressure is higher. The best cure for pinging, after all else has been tried, including de-coking, is conversion to dual plugging, which offers many advantages, and FEW disadvantages...cost being just about the only one of importance. Without going to dual-plugging, you should try using the more restricted advance unit or just add sleeves to the pins on the wider range ATU, and add the slightly stiffer springs if not there...AND, be sure to reducing the weights size, to raise the maximum advance to about 3500 rpm. This rather often works well enough, and is extremely cheap to do.
Octane in this article is R+M divided by 2, that is, the American calculation. In some instances, an engine may exhibit pinging (pinking), especially if a high compression model, unless you have as high as 98+- octane gasoline. Generally speaking, early higher compression engines are OK with 91 or 93 octane fuel, with the R50 and R60 often being marginal. A machine that can use the faster advance of the early ATU's (the ones with weaker springs, and ~2200 RPM for the maximum, as opposed to ~3200 RPM), will accelerate very slightly faster from low rpm with large throttle amounts. Having the wider advance plate can help sometimes with lower rpm power output.
BMW airhead engines USUALLY give plenty of warning about pinging, and in many instances a small, perhaps 3° more advance (sometimes a degree more), is advantageous...this is especially so at higher elevations, where pinging may not show up even on 87 or even 85 octane grade gasoline, especially on later Airheads with lower compression ratio, as especially common on U.S. shipped bikes. Move the Z or F markings into to the upper half of the window, and see. Pinging, if it occurs, will usually show up first on poorer gasoline's, at near sea level, on a hot day, under moderate to heavy throttle, typically well under 4000 rpm.
If a timing increase is to be tested: If, at the rpm of maximum advance, the Z LINE mark (F or F dot on early motorcycles) is not centered, then make a small adjustment to the timing...by adjusting the points plate position on early models...or rotating the canister on canister models. Points MUST be in good condition and the gap set FIRST, before attempting to do anything about timing. If you have a points-in-canister model, such as a 1979-1980, be aware that the outer bearing holder MUST be in position when checking the points gap. You should lubricate the CAM (and the FELT on pre 1979 models) with the proper greases, VERY sparingly, and the inside of the automatic advance also has a grease type assigned to it. I like a very TINY droplet of good oil on the weights pivots.
With the maximum advance set as above, now check the S, idle timing. It should be approximately correct. Note that early ATU's require an idle rpm below 1000 to get a true S mark reading....as the earliest ATU's BEGIN advancing at ~800 RPM. At this point you would know you have approximately the correct RANGE of advance, and if you want to, move the timing such that the Z line or F or F dot is a SMALL amount above the center of the little timing window, at high rpm (something above the maximum advance rpm, so you are sure the automatic advance unit has quit advancing). Going beyond 3/4 up the window is likely to result in pinging,,,,especially with regular low octane gas and at sea level on a hot day, less critical on the lower compression models of the eighties and later. Those having high compression motors using premium gas should be cautious about advancing timing. HOWEVER, the BMW airhead combustion chamber is such that it tends NOT to hide pinging from your ears. Sometimes, depending on the motor model, year, modifications, gasoline, timing unit, ETC., one can move the mark fairly close to the very top of the window. Be cautious. You may find a bit more performance, and fuel mileage....particularly if you ride all the time at high altitudes. NOTE: While there is no guarantee, a bit of extra advance, if no pinging at, say, 3500 or bit more, tends to NOT be ANY problem at higher RPM.
OT, S, F, Z marks: when the OT mark is centered in the timing window , assuming the flywheel is properly installed, both pistons are fully outwards. The typical use for the OT mark is for setting valves. One must rotate the engine 360° from one cylinder ON COMPRESSION STROKE, to the other cylinder on compression stroke, when setting valves. 1981+: the LINE next to OT is TDC. It is easiest to start with the LEFT cylinder, since you can watch the valves and know what the compression stroke is.
S: Static timing. Lines, if present, are 3° from S, for maximum limits for split images.
F: Full advance, at rpm for that or above. For 1970-1980, the actual F place is a machined dot-depression.
Z: 1981 and later, full advance. If a line is present, Z is that line.
2. 2 mm (.08") on the flywheel is ONE degree (1.5 mm, .059", on R65 and R45 which have smaller diameter flywheels). The 1980's and later flywheels are called clutch carriers, and are not a continuous metal part showing in the window, but the information is similar.3. Later models may have two lines, one above, one below a timing character letter, these are spaced at 3 degrees from the timing mark, and are there for reference to factory spec limits. These lines are 6 mm separated (.236 inch).
4. If your strobe light shows double timing images, that can be due to one or a combination of the following: worn timing chain, bent cam tip (on NON-canister models), worn automatic advance unit, unequal timing bumps of the two bumps on the cam of the automatic advance unit, worn chain sprockets, worn chain guides.
5. 1979 and later models have either points (1979-1980) or a Hall Element (1981+) (a type of transistor that is magnetically sensitive) in a canister, driven by a new style nose of the camshaft: flat, with OFFSET keyway. CONTRARY TO SOME "INFORMATION" in some aftermarket books like Clymers and Haynes, you can NOT put the canister into the engine with the keyway wrongly engaged (due to the keyway being offset). This type of drive is more stable, and cam tip irregularities are eliminated. The automatic advance units in these canisters is known to get gummy or otherwise stick in an advanced position, usually this happens after FULL engine warm-up, and the engine will idle way too fast. You may be able to remove the small oval plate on the side and spray in some oil (NEVER WD40), and fix the problem for awhile. A full disassembly and cleaning is the proper answer, often with some minor metal burnishing. The effect is provable by slowing the engine using a stop like a brick wall in front of the front wheel (or use the brake), and easing the clutch to slow the engine to normal rpm....if the strobe shows advanced timing, you found your problem. An article on the canister and electronics will be found here at: http://bmwmotorcycletech.info/ignition.htm
6. Ignition cutout problems after some miles down the road on 1981 and later models is quite often due to a failure to clean off, and replace, the transistor-type heat sink grease, located beneath the ignition module, which is under the fuel tank. Every two years is probably OK. Allowing repeated cutting-out from excessive heating will result in module failure. Radio Shack has heat sink grease. Use of common autoparts stores Dielectric Grease is acceptable. The white-colored Dow Corning DC-340 is the BEST coating. Riveted later modules are exempt.
Be sure to read article #30: http://bmwmotorcycletech.info/ignition.htm
(a) The airheads before 1979 had an extended nose camshaft, points in their own 'cavity' (which needs to have its rubber surround intact!). Those bikes used TWO EACH 6 volt coils, 1.5 ohm primaries, connected in series, for a total of 3 ohms. If you use coils with lesser resistance the points will be burned & not last long. For two stock coils of 1.5 ohm primary each, or one twin tower coil of 3 ohms, or, in some instances even lower primary resistance coils, you can use a points amplifier, and the points will then last a VERY long time if you regularly (~5,000 miles) faintly lubricate the advance cam.
(b) In 1979 and 1980, points in a canister was used.
(c) In 1981 to 1984, all models (EXCEPT the G/S and ST), used 2 each 6 volt coils, but they are higher performance coils, drawing more current. These do NOT work quite as reliably with points amplifiers unless the amplifiers are capable of handling the extra current.
(d) From 1985 (and a few earlier bikes like the ST and G/S) a single coil was used, originally gray in body color (which cracked and were NOT reliable). The Bosch number on those bad coils was 0 221 500 200. They can be upgraded with an Oilhead coil, or, BMW has a more expensive replacement for the Airhead, 12-13-1-244-426, it looks a bit different, but fits OK. The Bosch number on that coil is 0.221.500.203. Lots of ignition problems from old failed gray coils, including the acting up of the electronic tachometer. I prefer using two of the 6 volt coils from 1981-1984, and you CAN remove one gray dual tower coil and install two of these earlier coils, which are VERY reliable. BE SURE to read article #30! Around 1990, BMW went to Bosch number 0 221 500 203. These, and the so-called Red Coils, have a low primary resistance, ~0.5 ohm, and the module was changed to handle the increased current AND with some internal changes to shorten what is called the time-out period and there was another change in the module to improve stability of the spark. This is complicated, and the information is in my ignition article. One should NOT use the old modules with this coil, but has been done.
(e) The very last two changes to the modules incorporated a INTEGRAL RIVETED module onto a heat sink. There was a third version, just different mounting, but it was the first of the three changes on the way to the final module. The intermediate modules...and ALL earlier versions too...are NLA. Thus, there is ONE only FINAL module version, for ALL motorcycles with BMW's Bosch electronic ignition. If you have an earlier mounting (on the horizontal pad under the fuel tank), you will need minor mount changes, parts are available, see the on-line fiche). The last of the modules that are RIVETED supposedly never need heat sink paste renewal; and, the very last version (with turquoise lettering) works with ANY of the BMW coils, including the very last low primary ohms version, of 0.5Ω. The ignition article points out the differences between ALL modules and coils, etc.
This SOMEWHAT NERDY section requires some 'critical thinking'.
BMW made some changes as the cleaner emissions models were phased-in. One change was that BMW changed the CAMSHAFT sprocket by THREE degrees; this is same as SIX degrees measured at the crankshaft, as far as valve timing is concerned. BMW changed ONE flywheel marking, by repositioning it, when the three degree change in camshaft timing was done. The NEW timing point was placed 6° BEFORE the original marking, as seen in the direction of the moving flywheel. That was 12.5 mm on the flywheel, in the direction OF THE TDC (OT) MARK. NOTE that the STATIC ignition timing (also called idle timing, or the S mark) was NOT changed, it remained at 9° BTDC. Obviously, OT, top dead center marking, was not changed. Think about the differences of no-change at S, and a 6° change at the maximum advance point. ((Yes, if you have a wrong flywheel, or just want to know, you can measure the S mark and see which flywheel you have)).When BMW made those changes to the camshaft sprocket timing, BMW suggested those "upgrading" the ATU should "repaint" the timing marks. This was because if one put a 007 unit into a 005 bike, the F mark would not come up far enough if trying to use the original F mark. If one timed at the S, it was OK...but you would have to ignore the F mark....but, S is not the REAL place to time a bike, the F mark is THE important one, leaving S to fall in the window someplace.
It is possible for you to purchase a bike with wrong components, done by the previous owner (or?). It is also possible for you to install the wrong components, particularly the ATU and camshaft sprocket when doing a timing chain job, so be cautious!
You may be interested in reading this article: http://bmwmotorcycletech.info/timingchain.htm
Those articles contain more insights the camshaft and ATU; and, see other recommended reading, at the very beginning of this article).
Below are two ways to access an old advisory bulletin from Butler and Smith (BMW importers-distributors, from long ago). Here is the flywheel marking bulletin, and some notes on it from me:
If you prefer to access from the Internet: http://bmwmotorcycletech.info/flywheel-markings-B&S-bulletin.jpg
Information on Butler and Smith is located here:
bmwmotorcycletech.info/roundel.htm The information is in a large red-outlined box.
A reference article. See a fuller listing of links at the very top of the page you are reading this at. http://bmwmotorcycletech.info/F,OT,S,Z.htm
Coils, Spark Plug Caps; Module; Heat sink paste renewal; Miscl:
1. Be sure to use the proper coil; they are different for points versus electronic module models, and some modules will possibly fail if used with the low ohm coil.
2. The old black Bosch coils, with the lightning bolt on the side, can be used with ANY of the ignition modules. These old black Bosch coils, with the lightning-bolt on the side, which are BMW 12-13-1-244-142 (these coils are NOT marked 6 volt), are the best old coils to use with the electronic module ignition. In all cases of using two separate single tower coils, terminal 15 on one coil goes to terminal 1 on the other coil. That leaves one terminal 15, which is for the GREEN lead; and the remaining terminal 1 lead goes to the BLACK lead.
If you are replacing a two-tower stock coil with two separate lightning-bolt coils, you will have to fashion mounting...one can go where the original coil was, the other to the rear....under the relay bracketry. Have the rear coil fire the right plug. A common reason to install the lightning-bolt coils is that a stock grey-bodied early twin tower coil has cracked, and started to fail, usually with moisture conditions. If you can dry out the coil, you can epoxy or otherwise seal the crack....but this is a very temporary fix....and sometimes or often will not work. Even if the coil has OPENED its secondary winding, this still may work to get you home.
3. Coil secondary's are approximately 4000 ohms for the two separate coil models, and approximately 8000 for the twin tower single coil model. For any coil, clean the top now and then, maintain a good rubber boot fit and condition, and inspect the metal contact at the bottom of the tower after removing the boot (inspect wire clip too)...clean out any internal tower corrosion, use a bit of silicon grease (on the RUBBER insides).4. Early spark plug caps were ~ 1000 ohms, and are fine for any POINTS ignition bike. ELECTRONIC module/Hall element bikes MUST use 5000 ohm caps. NGK 5000 ohm caps are OK. Caps that measure 4000-8000 ohms are OK. NEVER use resistor plugs!!!!
5. It is perfectly acceptable, on ANY airhead, to short-circuit a spark plug wire CAP at its inside metal fitment, to a cylinder fin, to ground out the ignition. Do it securely; and simply install the cap to the plug, lay the plug metal onto the cylinder/head, and SECURE it there. NEVER, on ANY airhead, allow the spark plug cap to be off the spark plug or be ungrounded, with the ignition ON....you can ruin a coil and other items and the resulting damage may not appear for months or years later. One of the better methods of securing a spark plug metal body to a cylinder head is to use a long spring sold at hardware stores as a Sash Rod spring. A screen door long spring will also work.
6. There is a detailed article on the canister electronic ignitions elsewhere's: http://bmwmotorcycletech.info/ignition.htm
...but here are a few hints on troubleshooting.
© Copyright, 2013, R. Fleischer
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Last check/edit: Saturday, November 26, 2016