AntonLargiader
Well-known member
High-mile K bikes can get a slipping starter clutch. I experienced it over twenty years ago occasionally, but I currently have a K11 in the shop with a severe slipping problem, so I dug into it.
As background, the starter does not have a solenoid/bendix/pinion like the boxer starters (and most cars) do. The starter drives a gear which has a one-way clutch connecting it to the engine shafts. If the starter is trying to turn faster than the engine, it locks up. If the engine is turning faster, it slips. The starter can turn the engine but not vice versa. However, sometimes you can press the starter button and all you hear is the whirr of the starter motor and the engine itself doesn't spin. That's a pain.
Conventional wisdom has been that the slipping is due to oil varnish that gets into the sprag, and you can direct some carb cleaner at the right angle to maybe wash some of it off. I think the owner had done this. Since I was already down to the rear main seal, I went ahead and pulled the bellhousing off and inspected the sprag. I found severe wear on the individual sprag cams:
Left: the sprag cage from this K11. The flat spots are wear. They're huge.
Center: from a used assembly I had. Less wear, and you can see oil varnish
Right: a new sprag assembly from BMW
Here's a better look at one of the K11 sprags.
Here's an oblique view of the less used cage.
And here is an oblique view of the new sprag:
Bottom line: if you have a chronically slipping starter, you may actually need to replace a worn part. Cleaning this will not restore its condition. That said, the one I removed was very, very dirty. There was old oil sludge in the cover that had clearly not been flushed out during use.
The $130 sprag cage goes inside a $100 steel ring which in this case was also worn and is being replaced. A gear driven by the starter fits inside the sprag; that one is $700+ and thankfully is not worn. That gear has different part numbers for K75 vs. all of the 4-cylinder K bikes, but I could not tell any difference at all on visual inspection.
Accessing this takes a minimum of a few hours, and more likely a day. You remove everything behind the transmission, the starter, transmission, clutch, clutch carrier, alternator, starter, coils, and alternator drive. You will probably want to drop the back of the engine slightly to give room for removing the bellhousing. You will need to support the bike as the center stand is on the transmission that you remove. It's not a trivial task. When you get to the sprag, make sure you note which way it goes in!
As background, the starter does not have a solenoid/bendix/pinion like the boxer starters (and most cars) do. The starter drives a gear which has a one-way clutch connecting it to the engine shafts. If the starter is trying to turn faster than the engine, it locks up. If the engine is turning faster, it slips. The starter can turn the engine but not vice versa. However, sometimes you can press the starter button and all you hear is the whirr of the starter motor and the engine itself doesn't spin. That's a pain.
Conventional wisdom has been that the slipping is due to oil varnish that gets into the sprag, and you can direct some carb cleaner at the right angle to maybe wash some of it off. I think the owner had done this. Since I was already down to the rear main seal, I went ahead and pulled the bellhousing off and inspected the sprag. I found severe wear on the individual sprag cams:
Left: the sprag cage from this K11. The flat spots are wear. They're huge.
Center: from a used assembly I had. Less wear, and you can see oil varnish
Right: a new sprag assembly from BMW
Here's a better look at one of the K11 sprags.
Here's an oblique view of the less used cage.
And here is an oblique view of the new sprag:
Bottom line: if you have a chronically slipping starter, you may actually need to replace a worn part. Cleaning this will not restore its condition. That said, the one I removed was very, very dirty. There was old oil sludge in the cover that had clearly not been flushed out during use.
The $130 sprag cage goes inside a $100 steel ring which in this case was also worn and is being replaced. A gear driven by the starter fits inside the sprag; that one is $700+ and thankfully is not worn. That gear has different part numbers for K75 vs. all of the 4-cylinder K bikes, but I could not tell any difference at all on visual inspection.
Accessing this takes a minimum of a few hours, and more likely a day. You remove everything behind the transmission, the starter, transmission, clutch, clutch carrier, alternator, starter, coils, and alternator drive. You will probably want to drop the back of the engine slightly to give room for removing the bellhousing. You will need to support the bike as the center stand is on the transmission that you remove. It's not a trivial task. When you get to the sprag, make sure you note which way it goes in!
