I drove my '85 K100RT (mfg date 8/84) into the shed for the winter, removed the battery to trickle charge in the house.  Sort of started in May and I was able to ride it a few miles but it was difficult to start; rolling the throttle quickly would kill it.  The engine runs while the starter is depressed and then quits a few revs after the switch is released.
-fuel pressure at 38psi when cranking, drops to 20 and then slowly to 0 when not cranking.
-correct new plugs and wires from euromotoelec. the old plugs had the screw tips and a lot of gray powder in the boot.
-read from CrazyFrog's troubleshooting guide that I should have had 2500 ohms at 25°C at the temp sensor, so I replaced it but the new one has the same value at that temp.
-all four injectors emit a bit of gasoline while cranking
-Throttle position switch clicks at the slightest turn of the throttle, both switches show continuity
-disconnected and sprayed electrical contact cleaner on the contacts of the EFI and Jetronic connectors

Am at a loss but will continue with the troubleshooting guide.  I noted that in [url=https://bmwmotorcycletech.info/K100 troubleshooting Starting.pdf]https://bmwmotorcycletech.info/K100%20troubleshooting%20Starting.pdf[/url]'
the diagram for the ignition module is the mirror image of what's on the bike, so I measured everything based on the color IDs of the wires.


  Local dealer tech cut off services at 15 years (better than 10) but at any rate he's even 6 weeks behind for even simple things like routine service.

celhardt wrote:-all four injectors emit a bit of gasoline while cranking

Which spray pattern does it most look like?

https://www.injectorrx.com/wp-content/uploads/2013/05/fuel-injector-spray-pattern.jpg

-Dai

How old is the fuel filter? It could be clogged. The injectors should emit more than "a bit" when cranking is happening. Are those British bits, or American?

+1 on getting a new fuel filter.  NAPA Gold 3032 is the standard for bricks.  Also check that all the hose clamps in the tank are tight and not spraying fuel that should be going to the injectors all over the inside of the tank.  DO NOT use the hose pieces that come with the filter!  They are not for use in fuel tanks.

You mention that the fuel pressure drops once the engine catches and the start button is released.  There should be a signal from the ignition control or the injector control box to keep the fuel pump running when engine speed is above 700rpm.  It appears to me you are losing that signal and that is where I would concentrate my attention. 

I believe that signal goes to the coil of the fuel injection relay.  You might want to see if you can put a meter on the coil contacts to see what is going on.  As short piece of wire stuck into the pin socket of the relay connector and sticking out after the relay is put back will allow you to connect your meter for the reading.

Big clue is that it cuts off after releasing the start button.

Next time you do it keep the start button pressed and see if it keeps firing. If it keeps firing its telling you all is good but.....unseat and reseat the plug into the fuel injection control unit [FICU] under the seat. You will have disturbed this refitting the battery. Clean the pins on the plug while you are at it. This may well solve your problem! Point 75 is on this too. 

Next thing you need to check is the electrical plug into the front left of the tank. These go wrong. Open out the pins slightly to make sure it has a tighter fit and better electrical contact. You can buy these from www.tills.de. Most of my Ks have these plugs and I bought enough of them to replace them all.

Laitch wrote:How old is the fuel filter? It could be clogged. The injectors should emit more than "a bit" when cranking is happening. Are those British bits, or American?

I thought the fuel filter was clogged, too, but a new one has not improved anything.  Thx for the picture of what the pattern should look like.  Will try tomorrow

92KK 84WW Olaf wrote:Big clue is that it cuts off after releasing the start button.

Next time you do it keep the start button pressed and see if it keeps firing. If it keeps firing its telling you all is good but.....unseat and reseat the plug into the fuel injection control unit [FICU] under the seat. You will have disturbed this refitting the battery. Clean the pins on the plug while you are at it. This may well solve your problem! Point 75 is on this too. 

Next thing you need to check is the electrical plug into the front left of the tank. These go wrong. Open out the pins slightly to make sure it has a tighter fit and better electrical contact. You can buy these from www.tills.de. Most of my Ks have these plugs and I bought enough of them to replace them all.

Done all that, thx