1990 BMW K75 stalling when rolling on throttle

[GB805]

Hello, I bought a used 1990 BMW K75. 70,000miles.

The bike will idle but not accept throttle or even be ridden. It will immediately shut off.

Things I have done:
Fuel Pump
Fuel filter
New fuel lines
New hose from Airbox to Crankcase
New Spark Plugs
Checked the fuel injectors. They are fine.
Checked fuel pressure. Its Fine. 40psi
I have put Dielectric compound on all connections.
Replaced all fuses.
I am borrowing Bosch Coils and spark plug connectors off of another k bike and that didn't change anything.
I have borrowed another computer and that didn't do anything.
I have borrowed a fuel timing unit(I think thats what it was) its under the gas tank and I installed it and it didn't do anything.
I have checked and played with the TPS(throttle position switch) and borrowed a good one off of another bike.

Please help me! I have too much money into it at this point and can't figure it out.

What do you recommend???? I think some sensor or something is shot. Thanks.

[Snowman]

Mass air flow meter. Make sure the the electrical connectors are connected, all the air duct piping is connected properly to the meter and the flap opens and closes easily. I had these exact symptoms on a '87 K75. bike would start, idle @ 600rpm, any throttle including the choke and the bike would not run.
I had the bike idling, reached in a opened the flap, and she took throttle.

[johnny]

greetings gb805...

welcome to motobrick.com...

this motobricker is betting it aints gotts nothing to do with the exhaust valves...

keep us informed as you figger it out...

j o

[TimTyler]

My money is on Johnny will buy you a new AFM.

[Brad-Man]

sed air flow meters.

Flap = air flow meter, heated wire = mass air flow sensor

[GB805]

I swapped out an air flow meter off of another bike and still the same. The bike will idle at 1,000rpm and if i give it throttle nothing will happen until I let off the throttle it revs up a bit and I get a kicking from the air box almost like its backfiring or is it a problem with the air box.

I have done many things to this bike and I can only think it would be tight exhaust valves or there is a temperature sensor on a pipe just below the radiator.

What do you guys think? Thanks

[Scott_]

Have you checked for vacuum leaks? especially between the TB's and the head.

[GB805]

Mass air flow meter. Make sure the the electrical connectors are connected, all the air duct piping is connected properly to the meter and the flap opens and closes easily. I had these exact symptoms on a '87 K75. bike would start, idle @ 600rpm, any throttle including the choke and the bike would not run.
I had the bike idling, reached in a opened the flap, and she took throttle.

Hello Everyone!!!! So I have replaced the intake manifolds (the rubber pieces that go from engine to air box). I thought this was gona solve the problem as the engine would rev up when i sprayed starter fluid to check for vacuum leaks. Didn't solve it.

So I went back to double check it all when I remembered Snowmans comment of reaching in and opening the flap and she took throttle!!! But the flap is not sticking to anything inside the Mass Air flow meter. The flap opens and closes perfectly. I haven't found any outside leaks around the box that hold the Mass Air flow meter. What is the problem? The bike takes throttle when I open it????

[johnny]

greetings...

knowing that it takes throttle when you open it... i would open it...

j o

[GB805]

greetings...

knowing that it takes throttle when you open it... i would open it...

j o

Funny joke..Seriously what must I do so that it will open when riding. I can't hold the fucking thing open when I'm riding the bike.

[Scott_]

I don't have a 75, but from what I have read so far, it sounds like 1 of 2 options..

1, the air flow is still leaking in somewhere after the AFM so that there isn't enough air to operate the AFM properly.

2, the AFM has some kind of bind/drag/resistance(it may not seem like much when you move it by hand) that is still enough that the air flow can't move it properly.

I don't know how the air is channelled in the air box to the AFM, but it sounds like something isn't installed correctly. Especially if you said you tried another AFM from another bike, with the same results. By chance did you try your AFM on the other bike?

[rbm]

The AFM's barn door is attached internally to a wiper that rides over an imbedded resistor array.  The movable wiper contacts a small fixed plate that transfers the electrical signal to the AFM electronics.  It is possible that the plate is not making good contact with the wiper.  There is a Youtube video taht shows how to modify the connection to be more reliable:

This might explain why things work when you manually open the barn door; you're forcing a better contact with the movable wiper.

[Scott_]

Ok, so after seeing that video, I've got a better understanding of the operation...

Have you done a compression test. I'm wondering if you aren't creating enough 'vacuum' (ie airflow) to move the door.

Then that comes back around to, have you checked the valve clearances.

[johnny]

greetings rbm...

thanks for posting that up... thats whack... i always thought it was a sail switch...

j o

[rbm]

Nope, not a switch.  It's a variable resistor like a volume control.  What's whack is the wire the guy chooses to bridge the connection.  It's like 18GA!  I'd never choose something that large for that purpose -- it's too stiff.  I'd be using a very thin flexible wire like one from a Ethernet network cable.  Those are 30GA and very finely stranded.  Just a suggestion for the OP if he chooses to do that mod.

I've re-read the thread because I was thinking if the OP swapped the ignition control unit (ICU). If that was failing, it would lead to this stalling off-throttle.  But it appears that he has done a swap with a known working unit so it can't be that.  The file I attached to this post is the L-Jetronic manual for the BMW E12 Series cars.  These used the same FI system as the K-bikes so the information in this file is relevant to the problem at hand.  Maybe there's something in it to assist with the problem; at least the OP can learn how the system works.

Also this link has useful information about troubleshooting a L-Jetronic system
https://dl.dropboxusercontent.com/u/3359580/Bosch%20L-jetronic%20Fuel%20Injection%20Troubleshooting%2C%20Maintenance%20%26%20Repair%20p.pdf

[Scott_]

It's like 18GA!  I'd never choose something that large for that purpose -- it's too stiff.

I was thinking the same thing. Even though he said it was from a bmw car, probably an 8cyl, that would be capable of much more air volume to be able to flex that thick wire.
Definitely too stiff to use on a 3 cyl K-bike set-up.

[CanadaTim]

Good Morning,

I'm jumping right in here with the exact same situation. 1986 k100rs. Going to try repost this as a new topic.

thanx tim

[Solly]

Newbie here but have seen a few things in 30 plus years as a technician. 1st if you have checked all 5 basics {compression,spark,timing,vacuum,fuel,} then you can start looking at the more subtle things as computer strategies and electronics. Keep in mind that a good rule of thumb is if the problem isn't adding up to anything then it's probably manmade and or induced. Would be well to make sure that the fuel lines cannot be crossed and something so simple as the exhaust isn't plugged up. Remember no exhaust out then no air in, Plumbing 101. If you can throttle up by opening the airflow meter vane then check your vacuum, I'm sure someone on this site can tell you what a acceptable measurement is at idle. My guess would be around 15 to 20 inches . Timing will directly affect vacuum as will a exhaust issue. Good luck  Sol

[Solly]

Also those bosch airflow meters have a air bypass screw to set your mixture on them under a soft metal plug {not under the electrical cover but on the side of exterior}, if that screw has come out then the meter will not open because the vacuum will take the path of least resistance.

[GB805]

Hello.

No longer an owner of the k75 BMW.

Here's what happened:

The bike had too many problems. We replaced rubber manifolds (connect air box to engine block)....it worked for a 2 mile ride then it started to die again.

All cylinders were getting good compression. The fuel pump had good pressure. Replaced all rubber hoses, gas line, rubber o rings, etc.

Would spray fuel injector on engine when running and got a lot of suction near manifolds then in other parts of the bike and it was just giving me a fucking headache.

So I offered the mechanic the bike for $500 and he gave me $400. I bought it for $500 but put time and money into it too but whatever. $400 made me forget about it.

5 months later I found some parts that were part of the bike and brought them to the mechanic thinking he still might have the bike. And sure enough the fucking thing is still not running.

So good luck to all you with those air injected k75's.