My 1984 K100 has become difficult to start. It fires straight away but doesn't catch but after a few attempts it will start on a part open throttle. It sounds when it "clears its throat" that it's been too rich but it runs well above 1500rpm. Below that it won't idle very well, cuts out momentarily and idle speed is up and down. On occasion the fuel pump starts whining. When this happens I switch off, leave it a few minutes and the pump is then usually quiet. I've replaced fuel and air filters, removed all the block connectors I can find under the tank and cleaned them, including the earth connectors and the connectors to the air flow meter and efi unit.
Is there anything else I should be looking at?

Hi Willie,
When you replaced the fuel filter did you use a genuine BMW K filter or a generic unit? I have know people to be sold a fuel filter which is intended for carburettor use hence negligible pressure and they can burst or split under the pressure developed for FI leading to erratic fuel pressure.

I assume when you cleaned and treated all of the electrical connections with DeoxIT that you have checked the condition of the spark plugs?

Next thought you say the fuel pump sound changes, that sounds like the pump is suffering from cavitation are you sure the inlet to the pump is not obstructed by anything and are you sure the new filter is actually passing fuel through? When it is misbehaving have you measured fuel pressure with a gauge?

Check the resistance value of the water sensor against the value quoted in the troubleshooting section.

Last thought air leaks cause problems with starting and running so check all the little rubber caps on the throttle bodies are OK and the vacuum hose to the fuel pressure regulator.
Regards,
K-BIKE

Valid tips above. In addition, although little to do with the fuel pump, I would check the air stubbs and the crankcase breather hose for air leaks.
Normally fluctuating noise from the fuel pump, when the tank is not empty, has to do with changes in the fuel line pressure. Has the new filter been fitted in the right direction? Have you looked at the suction filter of the pump? Certain there is no water in the tank?

Thanks for that , guys. There's a bit of previous with this bike. I bought it 2 years ago and it was running fine then late last year the fuel pump started whining and it started drinking petrol and running rich. Removing the multi pin plug from the efi unit and putting it back on sorted that problem.......for a day or two. After the problem started again I replaced the fuel and air filters with genuine BMW parts, removed and cleaned the connections on the air flow meter and temperature sensor and the bike was then running well and has done for several months. When the nonsense reappeared I again removed all the connectors and sprayed them with electrical cleaner from Halfords. I thought I had found a bad connection in the multi connector on the right frame rail, as after I had replaced the tank the ignition lights didn't come on and fiddling with this connector restored power. This connector has 4 wires, 2 big pins and 2 smaller ones. I have wiggled it pushed and pulled the wires to see if there's a break but I can't get it to repeat the problem.

If you are talking about the round plug under the gas tank, you have to pull the 4 round connectors from the plug and squeeze them with a pair of pliers.
This connector is a known problem on the K100.

No, it's not the one that plugs into the fuel tank but one on the right hand side of the frame on the outside frame rail. Disconnecting it disconnects the ignition and all the other warning lights on the dash.

Latest update. With the pump still whining shortly after start up I removed the fuel return pipe from the tank to check. When the whining started, the return of fuel stopped and shortly thereafter so did the engine (not surprisingly). I removed the pump and ran it outside the tank just by operating the starter. It seems to be operating correctly. The fun started when I tried to replace it in the tank. The gauze filter on the bottom of the pump wont go back into the aperture in the tank except with a serious bit of fiddling and even then I can't get the pump to lock into place. Do these filters swell with age? I don't recall this problem the last time I had the pump out!
During the week I checked the resistance between terminal 10 on the EFI and earth. I got a reading of 2.87Kohms. Today I tried again and got a reading of 3.1Kohms. I was also trying to check if any of the connections in the wiring loom would make a difference to the readings but even disconnecting the one marked 6 in the troubleshooting guide I still got the same readings. I'm confused or maybe senility's setting in!

Willie

For the resistance on terminal 10, if the bike was cold, the reading is correct.
I guess today was a bit cooler than yesterday. 3KOhms is the equivalent of a temperature of 17-18 degrees (Celsius).
About the return of fuel stopping when the pump start whining, I can see a problem with the pressure regulator on the back of the throttle body.
What you did is proving that you always have electrical power to the pump but at on point, it stops delivering fuel or the external pressure regulator doesn't relief the excess of pressure.
The pump as an internal relief valve set a 65psi. The external fuel pressure regulator is set to relieve (return) the fuel to the tank when the pressure in the ramp is equal or greater than 36psi.
If this pressure regulator stops working, first the pump will work harder pushing 65psi in the ramp and the engine should be flooded because of the excess of fuel.
I bet you that if you put a gauge in line, when the pump starts whining you will find excessive pressure. I think Ajays had this problem and solve it by cleaning and blowing compressed air into the pressure regulator.

Thanks for that. I had read Ajay's tip and had removed the pressure regulator and blew air through it. I could hear and feel the ball lifting in it and thought it was OK.
What is the purpose of the vacuum switch and how do you check it? I see it was discarded after 1985 so would appear to be not too vital but I would like to check before I buy a new pressure regulator.

Willie, I would attack the whole system rather than individual parts first. That means that you should check the fuel pressure at the injector rail by using simple pressure gauge and 'T" piece spliced in to the supply at the beginning or the end of the fuel rail. Maybe you did this earlier?

The readings, and just as importantly, the gauge behaviour will tell you almost all you need to diagnose the system.

Eg. If the pressure is not 36psi then you have a problem with the regulator or leaks inside the tank. I can supply the list of symptoms and I am sure people can add to them. CF also has a small section on this in his download.

BTW I have a permanently installed gauge on my bike. It is a great resource. It saved me a lot of work and money through guessing.

Hi, all.
I replaced the fuel pressure regulator and the vacuum sensor switch and, joy of joys, it's now running well.
There were a couple of hiccups along the way, though. The wires broke of the pump. the colours didn't match the wiring diagram that I have and so I managed to reconnect it the wrong way round. Easily found and remedied and now, hopefully in the past for a long time!!!!

Another happy customer!
Why would you shop anywhere else for advises?
Now, to pay us back for our contribution, you should share your expertise with others. You may think that you don't know a lot, but look at what you accomplished!

Great job. Congratulation.

Bert