A description of starting problems, starter sprag clutch, various fixes, photos, etc., are in an article entitled "Starter Clutches In K Series Bikes", published in BMW Owners News, authored by Paul Glaves, in the December 2004 issue. I recommend you obtain a copy of that article. Paul's article, as well as mine, below, lists some possible easy fixes, such as oil additives, etc. MY article, below, does not cover a few things in Paul's article.
You really REALLY do not want the starter sprag clutch to fail. The LABOR just to get to it is VERY HIGH. The entire rear end of the bike must come off, including wheel, driveshaft, rear drive (and Paralever if you have that), alternator, clutch, secondary gear case, etc.
The one-direction sprag clutch is driven by an intermediate gear associated with the starter motor. See the SKETCH below the photos. The sprag clutch, intermediate gear, and more is located in a secondary gear case, rear of the engine crankshaft.
The 3 starter sprag clutch photos below are from my "analyze things for the Club" projects. This particular starter sprag clutch is of the later updated type having additional sprags. The earliest K100 bikes had fewer sprags...the updated assembly turned out to be SOME of the answer to problems, but not entirely.
In the photo below, the red arrow points to a 'sprag'. The sprags are all individually sprung, and they can rotate slightly. Further explanation is below.
In the photo below, the red arrow points to the surface the sprags are associated with. The sprags are always very lightly in contact with that surface on the gear part, below, when the engine is at rest, or, engine is running. When the engine is running, the direction of the part, below, is not that which will cause the sprags to greatly increase their friction to the boss. The sprags are freewheeling; ..or, think of it as just not transmitting power; that is, only a very light pressure, via an oil film, is had in the freewheeling mode. A fair amount of oil flows over and through the parts.
When the starter motor is engaged, the starter rotates the intermediate gear, which rotates the below part. The DIRECTION of rotation will, normally, cause the part in the above photo, and the part in the below photo, to 'lock-up', which means they will rotate as one entire assembly. From this, the engine is made to rotate during starter motor operation. When the starter motor is NOT being energized, the assembly free-wheels.
Below is an annotated copy of the sketch from BMW's fiche for a K100. To eliminate confusion, I have removed the alternator from the sketch. Here is a different way of describing how it works:
The starter clutch is a constantly running device that is designed to transmit force in one direction, and it is not supposed to have any appreciable grabbing engagement until the starter rotates, which is supposed to cause the sprags to move the other direction, and that movement should LOCK-UP that clutch, so the starter rotates the engine. When the starter is NOT powered, the starter sprag clutch is free-wheeling, with no appreciable friction (one hopes) in its innards. The starter sprag clutch is a ONE-WAY power transmitting device. One direction, it locks up, other direction it rotates freely, with no coupling between the two main parts other than quite minor friction of sprags against the oil film contact involved.
What are the SYMPTOMS and simplest 'fix' for a starter sprag clutch acting up? What is one of the first things to do at signs of any acting-up?
IF the starter rotates (very audible) but the engine does not turn over, or poorly:
You might have screeching noise. Intermittent, perhaps. Common to NOT have screeching noises however. OFTEN you can start the engine by push-starting, pushing the starter button AS one lets out the clutch, to start the fuel pump. May vary with temperature conditions. Sometimes one can temporarily 'fix' the problem by having the bike in gear and trying to move the engine counter to normal rotation direction by moving the bike backwards a small amount. It is possible for a combined problem, with the starter motor having a slightly dead spot it stopped at the last time you used it.
If problems are just beginning, I suggest you drain the oil and remove and replace the oil filter. Install fresh FULL SYNTHETIC oil and a new filter. 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 a bit 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 solvents-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.
04/29/2010: Update slightly; +clean up excessive verbiage, etc.
10/08/2012: Add QR code; add language button; update Google Ad-Sense code; very minor other editing.
04/21/2013: Revise article for clarity. Add photos. Add more commentary.
03/06/2016: Update meta-codes, layout.
© Copyright 2013, R. Fleischer
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Last check/edit: Friday, August 19, 2016