BMW K bikes (Bricks)

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1Back to top Go down   Fuel Pump power loss Empty Fuel Pump power loss Thu Aug 13, 2015 4:19 pm


Silver member
Silver member
Hi Guys

Background: I bought a non-running 85 K100RT with 55,000 mls that the PO lost interest in. I was assured that it was purely a cosmetic tear down - frame paint/replace rubbers & pipes/new tires - and that the brick ran fine before the strip down. He also did the splines & sent the fuel injectors away to get cleaned. A new 50A alternator was included as well. My intention was to cafe the bike as it was already stripped down.

Current state : Cleaned out gas tank, replaced filter, new plugs, new fuel lines, cleaned and tested the fuel pump. Bike is back together as a cafe using an Acewell speedo with the gear neutral light & alternator bulb mods detailed on this site. Bike starts and runs (but not well) . 

Problem: However when I take it for a spin, it'll potter around up to about 20mph. However as soon as I give a bit of gas it starts to cut out . If I do that a couple of times it will cut out completely and won't restart. Ignition is good as lights are on and starter turns the engine over. My first suspect was the connector under the tank so I replaced that but I still had the issue. Checking the fuse panel at fuse 6 there is no power. Jumper that fuse to 12v and the fuel pump runs. So problem is at or before the relay. Relay bench tests good ( and is the correct relay ). So I believe the issue is at the low voltage side of the relay. Here's the funny thing , if I leave the bike for half an hour or so, it starts and the cycle repeats itself. To me that indicates maybe heat maybe is the issue ? However that does not explain why the bike cuts out when I rev it. So maybe I have two separate issues? 

Does this sound like the ICU ? I'd rather understand the issue before throwing money at it ( as I think is the attitude of this forum). 



2Back to top Go down   Fuel Pump power loss Empty Update ! Thu Aug 13, 2015 4:29 pm


Silver member
Silver member
Well what do you know? I spent quite a few hours on this issue and also in composing my email to the forum. So just minutes after I posted I took out the FI relay holder itself and guess what came out as well - the low tension green yellow wire ! It was not inserted properly. Woo hoo. Gonna fix that and take her out now to check the issue is gone.

1981 R100 cafe- 121,000 miles
1985 K100 cafe - 55,000 miles
1987 K75C - 44,000 miles

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