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Slipping starter clutch

AntonLargiader

Active 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:

View attachment 92862

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.
View attachment 92861

Here's an oblique view of the less used cage.
View attachment 92860

And here is an oblique view of the new sprag:
View attachment 92859

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!
 

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One has to wonder what oil this bike had in it. I removed the starter clutch from my K75 at about 300,000 miles. I cleaned off all the gum and varnish, inspected for wear and put it back in the bike. It continued to work fine for the next 69,000 miles until the bike was totaled in an accident.
 
I did one last year on a K1200LT with only 13000 miles on it. The sprag came out in pieces must of being defective. Talk about eating up time to replace it is another story.
 
My guess is the owner has had this problem for a long time and just kept hitting the starter until it caught (for years). Probably didn’t bother to change the oil either (for years).

Anton, if you don’t mind, I’m going to add this to the Flying Brick DIY/Tech Library. A picture is worth a thousand words.





:dance:dance:dance
 
Sure, Lee.

No idea what oil. I can ask.

Yes, I'm sure the owner did let it zing. I did, too, because waiting it out was the only way to get it to catch in a reasonable amount of time. It truly was in its death throes. Normal use, IMO, shouldn't cause much wear as the sprags should be gliding over an oil film when the engine is running so there is only the slight rub as they engage each time.

The spare that I had, I think that came from a totaled bike that I bought for its fairing many years ago. Or another bike that someone gave me some time back. Regardless, I don't know the behavioral history of that sprag clutch so I can't opine on the origin of the wear that it has.
 
I can't say I would recommend this, but I got through a summer with an intermittently slipping sprague clutch. Engine off, put the trans in 2nd gear and hold in the clutch. Roll the bike backward and pop the clutch. If you hear the starter turn backwards it will start normally. I was waiting for cooler weather to take it apart and fix. Early models had no way for oil trapped in the sprague outer race to escape. Later ones had holes drilled in the outer race to allow oil to escape and flush out any sludge.

Frank
 
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