Bad running 1991 BMW K100

[andreitc]

Hi guys!

So I have a 1991 K100 2V model, which stayed on the cough for the past year and it came up with a 'cold'

I will do my best to describe the problem but I'm also including a video which I shot yesterday in which you can better see the problem along with my 'best' comments.

So for the past 10 years or so the motorcycle worked great. Not even a problem I may say... However, it stayed untouched for like an year or so and when I tried to ride it, a problem appeared..
First of all, I think it was a starting problem, since it didn't start very good (it used to start exceptionally even if I hadn't started it in a long time). At first I tought it was a battery problem, so i bought a new one. However, this was not a problem. Even with the new battery it took me like 10-15 mins to start it up. The starter motor works ok, however it doesn't start that good.
For example, if I'm trying to start it today, it takes like 10 mins to start it. After I start it, if I'm shuting it down and wait for a few mins, or if I'm trying to start it multiple times in like 1-2 hours, it starts ok, but if I'm starting it the next day, it again takes like 10 mins to start it.

But this is not the major problem, however i think it might be related to it..

The major problem is explained in the video attached, and along with it, i will also try to resume here what i tried to do to solve it...
- i changed all the spark plugs with new working ones
- i changed the spark plugs wire in between eachother to check if any of them is broken, and I don't think there is any one broken, since they work OK on the working cylinders
- i changed the gas it had from the last time i started it (like an year ago..) by putting new gas in it
- i cleaned the air filter (however it was in an impeccable condition even before the cleaning)
- i tried to clean the gas filter (fuel filter) -- *
- the coils don't seem to have any problem, since the spark wire are giving "sparks" (you can see in the video)

* (fuel filter) - i tried to clean it with the air compressor. And I also tried to blow into it. However, when blowing, i felt a very very high resistance. I don't know if it is supposed to be having this resistance or not, but it was really hard to blow into it. The air passed through, however, tooking high pressure power from me to pass through.

So that's basically it.. The motorcycle was working great last time (a year ago) when i started it. It used to work very smooth and no problems showed in the video were happening... Nothing happened in between. It just didn't want to start and work as it did after this year of 'staying'.

Has anyone else faced this problem? Do you guys know what it could be? I really want this to work, i have big plans for it

Here is the video:
https://www.youtube.com/watch?v=dKW83oXDI_c

Thank you!

[WD Forte]

I've had a couple of 'flying bricks' an RS and an RT
The RT had a loads of green and oxidised contacts that after a thorough cleaning made the bike run much better.
As yours is now 30 years old it wouldn't hurt to give it a good electrical check over.
(This should be done annually anyway IMO)

As well as all connectors on the harness, there are a lot of grounding/commoning terminals are under the tank on the spine of the frame and should not be overlooked
I'd also check under the seat where the ECU and relay block live
The TPS can be squished with contact cleaner too, I had an iffy that responded well to this.
Also check the soldering at the ignition switch, I found a dry joint on one of mine that caused problems

IIRC the K's use the Bosch jetronic EFI system and like any EFI system it relies on good signals from the sensors and
hall effect ignition pulsers to perform well.

Its hard to diagnose anything at a distance but I'd want to know if the fuel pump and pressure regulator are working
well
I also recall a rotten fuel pipe inside the tank causing erratic issues

TLDR:
its 30 years old check everything.
____________________
bikers smell of wee

[Nobby the Bastard]

What type of valves does it have? Shim and bucket have a tendency to get tighter with time and so the first symptom of them getting far too tight is difficult starting when cold but fine when warm (the gaps open up with the heat change.)
____________________
trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050

[A100man]

That #4 spark plug cap is duff/broken. It shouldn't short (arc/spark) onto your pliers like that. When plugged into the socket it's probably arcing to the head and not creating a spark in the cylinder.
____________________
Now: A100, GT250A, XJ598, FZ750

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600

[andreitc]

@A100man,

Hi, thanks for the information!

However, all the spark plug caps are doing the same thing when removed with engine running. They short on the pliers (or bare hand) and doing the exact same thing.

Can be all of them broken? Or maybe related to the coils?

[Robby]

First thing, please don't hold down the starter button with the engine running, you're hurting my mechanical sympathy.

You've done a lot of spark diagnosis work, so it looks like spark isn't the problem. The only thing you haven't done in the video that I would do, is connect a spark plug to number 4, hold it against the engine, and make sure you have a spark at the plug. I'm going to assume that you do have a spark where you need it, at least a good enough spark for it to run fairly well.

I would ignore the issue with it not revving up with the exhaust manifold removed. Every engine runs like shit with the exhaust manifold removed, it's an important part and it has to do with harmonics and complex maths.

You have spark. That leaves compression and fuel to check.

Fuel is where I'm expecting a problem. As far as I know, this is an early fuel injected bike. So the easy test is to unbolt the injector from the head and see if any fuel comes out of it with the engine running, or trying to run. Compare against a good cylinder.

Your test with compressed air on the filter is not something I would trust either. It's meant to slow down anything going through it, that's what a filter does. If you haven't replaced the fuel filter, do it anyway, it's a service part.

If you have a similar amount of fuel coming out of the injector, the last thing is compression. In this situation I would get a compression tester and do a test across all 4 cylinders.

Whilst tight valve clearances can be a problem that affects compression, they would have to be very bad for it to not fire that cylinder at all, regardless of revs or throttle position.

So check the fuel, then check the compression. With any luck it's a clogged injector (if it's a purely mechanical system) or a corroded connection (if the injectors are electronically controlled).

The solution to electrical problems on old bikes is nearly always a small piece of sandpaper. The hard bit is finding where to use the sandpaper.

[A100man]

[andreitc]

Hi, thanks a lot for the information, let me respond to some affirmations:

[Robby]

I don't know how the starter is wired, but I wonder if it's letting the engine earth better than it's engaged.

Without doing anything else, try running a thickish wire from anywhere on the engine to the negative terminal on the battery and see if it makes a difference to anything.

[temeluchus]