Snowbum's BMW Motorcycle Repair & Information Website

If problems are just beginning, I suggest you drain the oil and remove and replace the oil filter.  Install fresh FULL SYNTHETIC oil. The oil that has the best reputation for no or least sprag problems is 15W50 Mobil 1.

If things are more than just beginning, you need to be more aggressive :
A possible fix, temporary or?:    Put some Rislone or CD2 Engine Oil Detergent into the oil.  You need NOT change to fresh oil for this process.  In fact, it is my belief that this works best on dino oils, but certainly works on part synthetics and synthetics too.   These are solvent-detergent cleaners.   Rock the bike, in gear, backwards if you have to, to enable engine starting, or push start it.  After putting quite some miles on the bike, perhaps a couple of tanks of fuel, drain the oil, replace the oil filter and install fresh oil, full-synthetic.  15W50 Mobil 1 is recommended by me.  If the Rislone or CD2 treatment does not work, you will have to do a lot of work to remove the starter sprag clutch and replace it (DO NOT try to just clean it).   Because of the labor involved, many will think about waiting until they need other major work, such as a clutch overhaul.

HINT:  Push starting is part science, part art. Use 2nd or 3rd gear, and do not use the clutch as an instantaneous release of the lever, but not too slow either.  You want the engine to rotate, but not way too jerkily intense, and at the same time you don't want the rear tire skidding (which will usually happen if you use first gear).

WHAT fails with the starter clutch?:
They get some sort of hard-to-see deposits, sort of clear coating ....and there may be sprag wear.   These things are not conducive to the sprags locking into the starter functioning position.  The sprags themselves will also wear out by changing shape. The shape is CRITICAL.  The springs may be gunked-up too, see next paragraph.  If you have the early fewer sprags version, REPLACE IT with an updated parts assembly.

At the time of the Aug. 2016 update to this article, I had seen TWO of these sprag clutches (of the later types...not the earliest K bike ones before the factory updated ones) that have failed, withOUT any apparent excessove wear on any part. I had, still do, ONE of these in my shop for a deep study, eventually. Photos of it are above in this article.  I was so curious about it, that I paid the owner to send it to me. I have compared it to a brand-new one, with no good conclusions, beyond the fact that dimensions have not changed much. My suspicions are still that it failed due to unseen-by-the-eyeball microscopic  surface glaze, or other type of surface change... on the inner friction 'drum' part (and likely also the sprag parts that contact it). There IS wear, but it 'seems' small.  It MIGHT, and probably is, enough to cause problems.  The owner told me he had tried both Seafoam and CD-? additives.  The owner did not try Rislone. I am not sure of exactly how the owner used the additives. I have cleaned the starter sprag clutch, it seems to manually work fine, but I have not installed it into a K bike for full testing.  I may never, as I have closed my BMW repair shop.

No one that uses Mobil 1 (a true full synthetic), has had a problem that has been reported to me. It is entirely possible that OIL is the  major problems factor .....or not.  I do not want to imply that wear is not a concern. I have seen worn starter sprag clutches; and the springs, if gunked-up enough, could cause reduced tension.  Synthetic oils do not tend to leave gunk.  My suspicion is that it was also the oil that enabled excessive wear, when that is seen, but I cannot be 100% sure about it. The only way I can get a handle on that, is maybe to actually be present at some teardowns, and MEASURE side-play,  and so on. I am a bit hesitant to state that it could be a problem, because there is no indication of side-play wear on the needle bearings in the starter sprag clutch here ...that I can see ...nor on the ones I have looked at in that particular regard (two of them).  Keep in mind that a starter sprag clutch works by friction, and it is not supposed to be friction of metal to metal, but friction using an oil film between the parts. Most do not understand how oil films work. There is an in-depth article on this website for anyone who wants details.

I think there is sometimes, or often, more than just one thing at work with the starter clutch problems. I suspect basic failure modes; oil film failure where the result is mechanical damage; or, failure due to a glaze or gunked springs.   I am beginning to think that all the other failures, which manifest themselves as wear failures, are due to a combination of oil, perhaps side forces, and maybe there is some sort of production tolerance ...and maybe contaminant/protectant problems also at work.

I am aware that many have never have had a problem with the later starter sprag clutches, and many have used fairly conventional oils. My mind jumps to the same situation with camshaft and followers, in the Airheads. Many got HUGE mileages without problems; others, even running the same oils, had problems. I have personally witnessed these engines apart. What was the answer? Seemed to be heat treatment variations. For the Classic K bikes, I think we have a combination of effects, and anecdotal evidence tends to point towards recommending the use of a REAL synthetic oil (Castrol Syntec is NOT!), like the Mobil 1 that I recommend. It may well be that the higher amounts of ZDDP in Mobil 1 is helping with the WEAR failure modes.  In addition, in what APPEARS to be the case, Mobil 1's formula does not leave the surfaces such that the sprags do not properly grab, nor leave deposits in the springs.  For those who think that ZDDP may be involved, Mobil does have a V-Twin oil which is their full synthetic with additional ZDDP. See paragraph below.

Until someone does a serious laboratory analysis, right down to the surface's molecular analysis, I doubt we will know much more.  Because of the huge amount of labor involved ...not to mention the $$$ parts cost ...I personally use ONLY Mobil 1 in my K bike.  The majority of information I have points towards it being much less involved with problems.  I have nothing against someone using Mobil 1 20W50 oil, called V-twin oil, that has even higher amounts of certain additives.  The 20W50 is a bit better if someone rides mostly in the hot Southwest, etc., where temperatures can exceed 100°F relatively often; and, more particularly if large amounts of rpm and throttle are often used, perhaps a big load, such as a sidecar, etc.

I leave the Mobil 1 15W50 in my K1100LT for the entire year; unless I have put more than 8,000 miles on the bike. A FULL synthetic, like this oil, really does not need to be changed often, particularly if one does NOT do a lot of very short commutes particularly in quite cold weather.  My K11 works hard; so is hard on oil ...it hauls a big sidecar. I do change the filter at the same time, sometimes every other oil change.   Note that it is the oil's ADDITIVES that wear out (they evaporate, get used-up, etc.), and that is the prime reason to change the oil.  An in-depth analysis by a laboratory showed that the additives in Mobil 1 hold up quite well over time and mileage. You get, sometimes, what you pay for.  I also use only BMW filters. I have no problem with folks using aftermarket filters, but I find them highly variable internally in quality and performance, so I don't use them.

K bikes seem to use a fair amount less oil if the oil is of premium quality and use less oil if the oil level is not maintained all the way to the top of the sight glass. I like the middle. I use higher levels, just almost reaching the top of the sight glass only if on a long tour.

There are other synthetic oils that likely do a good job; particularly the Golden 4 type from Spectro Oils .....and Spectro does make a full-synthetic version.   If the Mobil 1 becomes hard-to-find, I may switch to the Spectro.

My ideas about a very intense cleaning of the springs, and sprags, ETC., and a hand or increased forces test for one way action, possibly working well, and thereby reusing a previously slipping sprag clutch, are not just my own ideas.   Well after I had been promoting the cleaning (with caveats!), I began getting a report now and then of folks having good luck with the process.  One report in particular, by a trusted source ....told me that a number of times after removing a starter sprag clutch that has failed or gotten intermittent, but exhibited no signs of bad mechanical wear, it could be repaired by a very thorough cleaning in strong solvents. I have, myself, done this to TWO, and I also used ~2000 grit paper in 'de-glazing/polishing' the contacting surfaces; with no further problems from the starter operation; so it was nice to hear that someone had used just solvents to effect cures.  I would recommend the polishing by super-fine grit paper, as I did, plus very thorough solvent cleaning (and scrubbing by turkish toweling with the solvents).  I used a 50-50 mixture of MEK and Acetone.  There is so much labor involved, that replacement may be your choice, since the above might not work.  It probably would tend to work, well, more likely, if the chemical detergent additive worked reasonably well first.   I'd not use too gritty a paper.  I probably will try 1200 or 1500 grit next time, but won't try anything coarser.