BMW K bikes (Bricks)

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NickyNicky

NickyNicky
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I'm working on an 86' K100, and I've been chasing my tail a bit trying to figure out a situation where the bike bogs out with throttle. At various stages it's also had trouble starting and idling smoothly at times.

It's been stripped down over the past year. The last time I ran well (although the clutch slipped a lot, so it was hard to tell) was before I disassembled the whole bike for a clutch job. I also painted the tank at that time. When I was separating the transmission from the engine, it actually tipped over off the stand, which in the back of my mind makes me question if I messed something up, perhaps if some oil got in the cylinders or something.

The bike stands as follows:

- "GP style" muffler
- air intake direct from plenum to AFM to filter
- new fuel filter
- new injectors 
- new fuel lines
- new spark plugs
- new plug wires
- "new" used ignition control unit
- "new" used throttle bodies
- "new" used TPS
- "new" used hall sensor

Things I've tried/checked:

- no air leaks around TBs and plenum/s-tube
- throttle bodies are relatively synced
- TPS adjusted
- ignition timing adjusted (centered mark on hall sensor)
- played with the air bypass screw on the AFM (it went all the way in and the idle was still going up)

In adjusting the air bypass screw, something interesting Ive noticed is that the engine revs up and smooths out when I manually open the AFM a bit further. I'm guessing that this means that the AFM is not adjusted properly, but as mentioned the screw is all the way in. It also seems like I'm getting instances of reverse airflow back up to the AFM which will flutter the valve open and closed and likely mess with the mixture.

I'm leaning towards replacing the pump and regulator, but I don't want to just throw cash at this thing. I'm particularly concerned about the reverse air flow. Has anyone seen this before?

    

RicK G

RicK G
admin
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Everything you say there says to me it's running very lean at idle. There is air getting in somewhere. Have you sealed under the manifold rubbers it's a common cause.


__________________________________________________
"Man sacrifices his health in order to make money.
Then he sacrifices money to recuperate his health.
And then he is so anxious about the future that he does not enjoy the present; the result being that he does not live in the present or the future; he lives as if he is never going to die, and then dies having never really lived."   Dalai Lama


Bikes 2004 K1200GT 1998 K1100 LT, 1993 K75 RT, 1996 K75RT, 1986 K75 GS, 1979 Z1300 Kawasaki
    

NickyNicky

NickyNicky
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I put black RTV between the plastic flanges and the cylinder head. I also replaced the bushings and secured them with new band clamps! As mentioned, the s-hose from the plenum to the block is also new. I'm running a coupler that is pretty well secured between the plenum and the AFM. It all looks good visually, although maybe something isn't sealing the way I expect. I'm not sure how hard this should be!

    

Laitch

Laitch
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@NickyNicky wrote: When I was separating the transmission from the engine, it actually tipped over off the stand
The bike stands as follows:- air intake direct from plenum to AFM to filter
Things I've tried/checked:- played with the air bypass screw on the AFM (it went all the way in and the idle was still going up)
How far was the engine's drop to the floor from the stand?

Intake air should go through the air filter then through the AFM, then into the plenum and then into the throttle bodies. If that is how it's set up, what you seem to describe is backward and impossible, except when backfiring. Smile

Be certain the air flow meter connection is undamaged, clean and tight. Verify three throttle body vacuum connections are capped tightly and that the fuel pressure regulator's vacuum hose to its throttle body stub is connected and not leaking.

The air flow screw on the AFM is adjusted at idle using a CO tester or the lean drop method described in this post.


__________________________________________________
1995 K75 81,000 miles
another K100 running rough - reverse intake flow? Usa-lo10
    

Suzi Q

Suzi Q
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Daft (or not!) question:
Is the AFM in the same orientation as OE, i.e. with the vane axis vertical. Reason I ask is because there are pics of custom builds out there on the interweb that show a bike with the AFM on its side. In my experience this plain doesn't work - it results in the oscillating that you describe.

EDIT: At low tickover you will see some oscillation in the vane. Moving the vane manually will increase revs, but then kill it. Without seeing the thing in action, it does sound as if that all might be fairly normal.

EDIT2: Do you know the history of the AFM? There is stuff on the interweb where folks have turned their Ks into monster horsepower beasts by fiddling with the sensor detents on the AFM. Not sure if I believe this, especially since, when I tried it, it made no discernible difference apart from screwing things up if you went too far (as these 'wonder tuning' tricks usually do) Check the seal around the plastic lid on the AFM - has it had a....visitation?


__________________________________________________
Sometimes I'm not really Suzi Quatro.
    

NickyNicky

NickyNicky
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@Laitch wrote:
@NickyNicky wrote: When I was separating the transmission from the engine, it actually tipped over off the stand
The bike stands as follows:- air intake direct from plenum to AFM to filter
Things I've tried/checked:- played with the air bypass screw on the AFM (it went all the way in and the idle was still going up)
How far was the engine's drop to the floor from the stand?

Intake air should go through the air filter then through the AFM, then into the plenum and then into the throttle bodies. If that is how it's set up, what you seem to describe is backward and impossible, except when backfiring. Smile

Be certain the air flow meter connection is undamaged, clean and tight. Verify three throttle body vacuum connections are capped tightly and that the fuel pressure regulator's vacuum hose to its throttle body stub is connected and not leaking.

The air flow screw on the AFM is adjusted at idle using a CO tester or the lean drop method described in this post.

It fell, or rather kind of rolled, from less than a foot. No real visible external damage, but a lot of oil spilled out from what I believe was the starter hole.

Yes, lol, the air goes from filter to AFM to plenum.

I will verify some of those connections and seals, but I've already looked them over. Although I think they are ok, it is *possible* that I missed something.

Thanks for the link on the AFM adjustment. I'll need to find a CO tester.

    

NickyNicky

NickyNicky
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chris846 wrote:Daft (or not!) question:
Is the AFM in the same orientation as OE, i.e. with the vane axis vertical. Reason I ask is because there are pics of custom builds out there on the interweb that show a bike with the AFM on its side. In my experience this plain doesn't work - it results in the oscillating that you describe.

EDIT: At low tickover you will see some oscillation in the vane. Moving the vane manually will increase revs, but then kill it. Without seeing the thing in action, it does sound as if that all might be fairly normal.

EDIT2: Do you know the history of the AFM? There is stuff on the interweb where folks have turned their Ks into monster horsepower beasts by fiddling with the sensor detents on the AFM. Not sure if I believe this, especially since, when I tried it, it made no discernible difference apart from screwing things up if you went too far (as these 'wonder tuning' tricks usually do) Check the seal around the plastic lid on the AFM - has it had a....visitation?

That's a good point, the AFM is on its side. I've seen this on other bikes so I assumed it was ok. I'll try turning it.

I don't know a ton about the AFM. The bike was in stock form when I got my hands on it. Likely not a history of modifications. And the bike did run better at that time. Although I did notice some wear on the Phillips crews on the inside of the AFM inlet. This made me question if it had been disembled in the past for one reason or another.

    

Suzi Q

Suzi Q
Life time member
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When I finally track down that guy who posted pics of his AFM-on-its-side-looking-way-cool special....
....Im gonna, grrrr  another K100 running rough - reverse intake flow? 167893
Says everything that can be bad about some of these exotic 'specials'.
Good luck.


__________________________________________________
Sometimes I'm not really Suzi Quatro.
    

Gauntlett

Gauntlett
Silver member
Silver member
Hi Chris, on the subject of the orientation of the AFM, currently have mine upside down, wiring conector under neath for weather protection, do you have any thoughts on this, at the moment it runs absolutely fine, cheers

    

NickyNicky

NickyNicky
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@Gauntlett wrote:Hi Chris, on the subject of the orientation of the AFM, currently have mine upside down, wiring conector under neath for weather protection, do you have any thoughts on this, at the moment it runs absolutely fine, cheers

That's also more practical from a wiring standpoint, since the plug wont be wedged towards the front of the engine, correct?

    

robmack

robmack
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This is a picture from the Bosch technical manual for the L-Jetronic showing a cutaway view of the AFM.

another K100 running rough - reverse intake flow? 856752d1406505703-are-there-stock-afm-performance-modifications-scan0002

In the orientation illustrated (which is 90° off from factory orientation), the incoming air which is trying to push the metering vane out of the way (right to left in the top picture) will have to work against  the spring trying to close the vane as well as gravity pulling the metering vane closed.  That will cause engine running problems as mentioned above.  The correct orientation is to have the vane pivot vertical relative to the ground so that the effects of gravity is neutralised and the incoming air only acts against the spring tension.  Rightside up and upside down would both be acceptable orientations of the AFM.


__________________________________________________
Robert
1987 K75 @k75retro.blogspot.ca
http://k75retro.blogspot.ca/
    

Laitch

Laitch
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@NickyNicky wrote:Thanks for the link on the AFM adjustment. I'll need to find a CO tester.
You don't necessarily need a CO test if you follow instruction #4 in the link I posted but using a tester will be the most precise method. First, correctly orient the AFM as has been suggested.


__________________________________________________
1995 K75 81,000 miles
another K100 running rough - reverse intake flow? Usa-lo10
    

Suzi Q

Suzi Q
Life time member
Life time member
"Hi Chris, on the subject of the orientation of the AFM, currently have mine upside down, wiring conector under neath for weather protection, do you have any thoughts on this, at the moment it runs absolutely fine, cheers"


For the reasons above, and also from memory, it'll run upside down just fine. 
Still a bit cross about the pics of that 'special'.


__________________________________________________
Sometimes I'm not really Suzi Quatro.
    

NickyNicky

NickyNicky
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I put the AFM flat. Even opened it and cleaned the carbon build up. The bike starts pretty good, but it doesnt make it too far up the rpm band before sputtering off.

I'm debating if I should keep playing with the throttle body sync and ignition timing or if it's futile. Id like to eliminate as much as I can before springing for a few fuel pump and regulator.

With these "new" throttle bodies the "screws which shall not be touched" have not been touched. But I can't seem to get them to sync just right with the brass screws. Cylinder 1 is all the way out and cylinder 4 is all the way in. I might swap back to the my "old" throttle bodies, where those screws have already been tampered with.

For the ignition timing, do you guys suggest centering the marks perfectly? My timing gun doesn't have a delay function. To line the marks up, I'm rotating the hall sensor quite a bit.

    

robmack

robmack
Life time member
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Can you test the voltage on Pin 7 (White/Grey) of the AFM?  According to the information at this page (https://www.gomog.com/allmorgan/AFMadjust.html ):

The resistance track sometimes wears through, usually around the much used first third of the wiper travel. This will obviously cause erratic fueling and whilst a track from an airflow meter which has failed for other reasons, from a similar car, could be substituted, some trial and error might be needed to set the slider position satisfactorily.

Resistance measurements do not provide an accurate means of assessing the serviceability of an airflow meter - it is a potential divider and the system depends on voltages not resistance. There should be smooth transition of voltage on terminal 7 as the flap is moved. Typical airflow meter terminal voltages are:-

Pin 6 0v
Pin 7 2.25v - flap fully closed
Pin 7 7.5v - flap fully open
Pin 8 7.6v
Pin 9 12v - nominally 2v below battery
Also, if you're inside the AFM again, check the wear on the carbon track on the resistor assembly. If it looks very worn, you might need a new AFM or find the procedure to adjust the resistor array so that the wiper sweeps another area of the carbon track.


__________________________________________________
Robert
1987 K75 @k75retro.blogspot.ca
http://k75retro.blogspot.ca/
    

NickyNicky

NickyNicky
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I'll check the voltage as suggested.
The track had worn somewhat, and some carbon built up. It seemed a lot less than some I'd seen in pictures. It was actually quite immaculate in there. I cleaned it down with some contact cleaner and rubbed the resistor surface with paper to clean it down.

    

Suzi Q

Suzi Q
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Well if it's starting okay but having problems as the revs rise then I wouldn't be worried too much about throttle adjustment. All those twiddly screws have done their job by the time you reach 2-3k. If it's ticking over, and picking up from tickover reasonably okay then look elsewhere.
The AFM could be the culprit, but the symptoms you describe could also be a leaky spark, so check carefully inside the spark plug cover. I had a very-hard-to-find problem where the OE plug cap was arcing at high revs. The bike started and ran fine but wouldn't really go past 6k.
I also had the same symptoms with a faulty FI ECU. The clue when this happened was that, with the throttle opened and the bike struggling at 6k, I found that it would rev a bit better if I manually pushed the AFM further open.
The other thing could be a blocked fuel filter. Regardless of it being recently fitted, any leftover crap in your tank can quickly choke it up. This includes old fuel residue from a long-stood tank that's been loosened when it was put back into service. Happened to me. 
Just a few ideas.


__________________________________________________
Sometimes I'm not really Suzi Quatro.
    

NickyNicky

NickyNicky
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Starting isn't necessarily easy, especially since it's so cold out here right now. I have it in a garage with a heater, but it's by no means warm.

Yesterday I got it started in 2 seconds by blipping open the AFM manually when I was turning it over.

How did you identify the arcing issue on the plugs?

So far I have the following to try:
1) voltage test on AFM
2) fit my spare ECU
3) recheck ignition timing (looking for input)
4) fit spare TBs again

All of that before getting to the pump, regulator, and filter

    

19Back to top Go down   another K100 running rough - reverse intake flow? Empty Running badly Wed Mar 10, 2021 8:13 pm

Kr4mo

Kr4mo
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@NickyNicky wrote:I'm working on an 86' K100, and I've been chasing my tail a bit trying to figure out a situation where the bike bogs out with throttle. At various stages it's also had trouble starting and idling smoothly at times.

It's been stripped down over the past year. The last time I ran well (although the clutch slipped a lot, so it was hard to tell) was before I disassembled the whole bike for a clutch job. I also painted the tank at that time. When I was separating the transmission from the engine, it actually tipped over off the stand, which in the back of my mind makes me question if I messed something up, perhaps if some oil got in the cylinders or something.

The bike stands as follows:

- "GP style" muffler
- air intake direct from plenum to AFM to filter
- new fuel filter
- new injectors 
- new fuel lines
- new spark plugs
- new plug wires
- "new" used ignition control unit
- "new" used throttle bodies
- "new" used TPS
- "new" used hall sensor

Things I've tried/checked:

- no air leaks around TBs and plenum/s-tube
- throttle bodies are relatively synced
- TPS adjusted
- ignition timing adjusted (centered mark on hall sensor)
- played with the air bypass screw on the AFM (it went all the way in and the idle was still going up)

In adjusting the air bypass screw, something interesting Ive noticed is that the engine revs up and smooths out when I manually open the AFM a bit further. I'm guessing that this means that the AFM is not adjusted properly, but as mentioned the screw is all the way in. It also seems like I'm getting instances of reverse airflow back up to the AFM which will flutter the valve open and closed and likely mess with the mixture.

I'm leaning towards replacing the pump and regulator, but I don't want to just throw cash at this thing. I'm particularly concerned about the reverse air flow. Has anyone seen this before?

Don't mean to be a mr obvious or anything, but have you checked the fuel pressure on the fuel rail?

    

NickyNicky

NickyNicky
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Unfortunately I don't have access to a pressure tester. I could buy one, but I figured that it would cost almost as much as a pump or regulator. I actually got a fresh pump and filter, pending install.

    

Laitch

Laitch
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@NickyNicky wrote:Unfortunately I don't have access to a pressure tester. I could buy one, but I figured that it would cost almost as much as a pump or regulator.
Give Canadian Tire a call. They loan tools and might have a pressure tester. All they require is a deposit which will be refunded on return of the tool.


__________________________________________________
1995 K75 81,000 miles
another K100 running rough - reverse intake flow? Usa-lo10
    

robmack

robmack
Life time member
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Parts Source, a wholly owned subsidiary of CTC, have a tools loaning program.  I've never seen this service from a CTC store myself. Who knows though; CTC is a franchise and maybe policies differ store to store.


__________________________________________________
Robert
1987 K75 @k75retro.blogspot.ca
http://k75retro.blogspot.ca/
    

NickyNicky

NickyNicky
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They used to do it when I worked there 15 years ago, but I think they stopped a few years later. Parts source is a good suggestion!

    

NickyNicky

NickyNicky
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Hey all!

Thought I'd throw an update in here. Robmack lives pretty close to me in Toronto and he was nice enough to lend a heavy hand in troubleshooting with me yesterday. We ended up all over the engine looking for air leaks and any logic pointing to what could be going on.

The biggest evidence was again in fiddling with and observing the AFM. The flap seemed to flutter open and closed during what seemed to be misfire events, especially when any throttle was applied. It would completely bog out the engine. Further to that, holding the flap open a bit let the engine rev up and run more smoothly.

After everything, we decided to just throw in a spare FI control unit that I had kicking around, and the engine just immediately sang to us.

Still some ongoing improvements to be made in terms of throttle body tuning and possibly looking back at the AFM bypass screw and spark timing. Will keep everyone updated.

Robmack was also kind enough to help me get the RPM working on the Motoscope I'm running. It seems the BEP I have is not letting out a proper square wave signal. We tapped it from the OEM signal and it works for now!

Thanks everyone, and especially Rob!

    

Point-Seven-five

Point-Seven-five
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Life time member
Yeah, Rob has an amazing knowledge of these beasts and, as you have learned, is incredibly generous in sharing what he knows.


__________________________________________________
Present:
1994 K75RT
1991 K100RS
1988 K100RS SE

Past:
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

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