Hi all.
After the recent problem i had with the clogged fuel tank vent pipe bike now is back on the road. Put about 150km on it and in general it seems ok.
The only problem i face, since i m totally noob in K bikes is its idle behaviour.
Bike needs more than 5 minutes with the choke on and on the very start actually needs the second scale of the choke lever. For 3-4 minutes. After that period engine needs another 4-5 minutes on the first choke scale and  stabilises its rpm to 1000-1100 a noise is heard from the engine - crankcase (?)
Since a video will give you the picture i would like your experience on this.

Thank you.

at 00:41 you can hear exactly the  sound that worries me. It goes away after 10 minutes warm up. 
After that 10 minutes of the engine working idle sets to steady 1100 rpm and that noise is barely audible.

Diesel knock! I think we've all had that at some point - search the forum for lots of possible answers. The most likely causes are:

1. Dirty injectors
2. Air leak on the inlet stubs
3. Unbalanced throttle bodies.

If the RPM stabilises at 1100, I'd be inclined towards the air leak theory initially because idle RPM is 960-980 according to BMW. It's not something that will do any damage as it's (only) a misfire.

Dai wrote:Diesel knock! I think we've all had that at some point - search the forum for lots of possible answers. The most likely causes are:

1. Dirty injectors
2. Air leak on the inlet stubs
3. Unbalanced throttle bodies.

If the RPM stabilises at 1100, I'd be inclined towards the air leak theory initially because idle RPM is 960-980 according to BMW. It's not something that will do any damage as it's (only) a misfire.

Hey Dai! 

I've read almost everything  But reading finally made me terrify! Monkey nuts, Crankshaft pivots, Cam chain, Tensioners, even thermostat problems. 
God, i m more confused than ever !
Thats why i've posted the sound, hopping somebody to respond " Yes!!! Its the X thing, had the same sound! "

Which places are prone to air leak so to search?By inlet stubs, you mean the four rubber hoses that go into the throttle bodies? Have in mind that this sound became worse after patching the leaking crankcase breather hose !

The four inlet stubs under the throttle bodies that bolt to the crankcase. Spray some WD40 (or similar) around them when the engine is running. If the RPM suddenly goes up and the knocking stops, you've found the air leak.

FYI: WD-40 is no longer as flammable as it used to be.  It still says flammable on the can but apparently the safety Nazis got to them so it is no longer as fun as it used to be. (Made great blow torches with dad's when I was a kid.)

Dai wrote:The four inlet stubs under the throttle bodies that bolt to the crankcase. Spray some WD40 (or similar) around them when the engine is running. If the RPM suddenly goes up and the knocking stops, you've found the air leak.

will blow some butane gas with my flame torch ( Dremmel ). Guess will do.

duck wrote:FYI: WD-40 is no longer as flammable as it used to be.  It still says flammable on the can but apparently the safety Nazis got to them so it is no longer as fun as it used to be. (Made great blow torches with dad's when I was a kid.)

thank you! i second that!

What's the condition of the S-shaped hose which you can see near the throttle position sensor, which goes from engine to bottom corner of the airbox? In the brief flash of it in your video it looked like it had been taped up. Any cracks causing leaks will make the idle sound like a box of rocks. One quick way to eliminate the engine breather or vent hose as a source of poor idle/performance is to unscrew the engine oil filler cap on the right hand side whilst idling and note the significant idle drop. If the idle does drop further the hose is likely not a cause.

I use a propane torch with a very, very small stream of gas (not lit, of course), to determine air leaks at the throttle body rubber mounts.

Two Wheels Better wrote: If the idle does drop further the hose is likely not a cause.

Thank you. I have already checked that S hose. It's the crankcase breather hose as described in clymers . Found out It was cracked at the end that goes into the air box and as you correctly noticed I patched it with a fair amount of soft electrical tape till I receive the original .
Do you think that it might still be leaking? I mean is the pressure too high there?

George_Pap wrote:

Two Wheels Better wrote: If the idle does drop further the hose is likely not a cause.

Thank you. I have already checked that S hose. It's the crankcase breather hose as described in clymers . Found out It was cracked at the end that goes into the air box and as you correctly noticed I patched it with a fair amount of soft electrical tape till I receive the original .
Do you think that it might still be leaking? I mean is the pressure too high there?

It's a chance. Pop off the oil filler cap and you'll know.

Read that in a post and did that test already .Engine stalls and almost goes out.

Two Wheels Better wrote:... poor idle/performance is to unscrew the engine oil filler cap on the right hand side whilst idling and note the significant idle drop. If the idle does drop further the hose is likely not a cause.

Can I ask, does this apply to K100 16V? On mine, removing the oil filler cap makes no difference at all to idle speed (or anything else). If everything is as it should be, what would you observe when the filler cap is removed?

stanthomas wrote:

Two Wheels Better wrote:... poor idle/performance is to unscrew the engine oil filler cap on the right hand side whilst idling and note the significant idle drop. If the idle does drop further the hose is likely not a cause.

Can I ask, does this apply to K100 16V? On mine, removing the oil filler cap makes no difference at all to idle speed (or anything else). If everything is as it should be, what would you observe when the filler cap is removed?

If nothing changes then you need to look at the crank case breather pipe and vacuum pipes. It means some or all of them are goosed. This is a very cheap repair but slightly fiddly.

92KK 84WW Olaf wrote:

stanthomas wrote:

Two Wheels Better wrote:... poor idle/performance is to unscrew the engine oil filler cap on the right hand side whilst idling and note the significant idle drop

Can I ask, does this apply to K100 16V? On mine, removing the oil filler cap makes no difference at all to idle speed

If nothing changes then you need to look at the crank case breather pipe and vacuum pipes. It means some or all of them are goosed. This is a very cheap repair but slightly fiddly.

The crankcase breather being the Z-pipe that everyone talks about?
Mine looks fine but I have a question about a motronic 16V. How does a leak in the crankcase breather affect the air flow? The breather vents into the airbox, downstream of the filter but upstream of the throttle bodies. Since there's no air flow sensor and the airbox is, in effect, open to the air, how can an air leak here affect the idle?