Fuel pump primer stays on

[mstolow]

Morning,

New to the forum. I have a 1985 BMW k100 cafe racer.

When starting the motorcycle, you can hear the fuel pump primer activate but it stays on even after starting the engine. I generally have no issue when riding but the mechanic says this is not normal.

He thinks the pressure gas valve is faulty. I am attaching a video of what I hear when the bike is turned on. Notice the primer stays active.

Appreciate any guidance or advice. Please be aware I have limited mechanical know how.

[Laitch]

Welcome! Thanks for the excellent video.

Your mechanic is correct. This is not normal but a gas pressure valve is not an item associated with a 1985 K100. Somebody has wired your moto incorrectly or intentionally bypassed part of its control unit system. Was this Brick running when you bought it? Did you design this cafe racer? Please tell us how long you have owned it, how much you have ridden it. Post a photo of it.

A 1985 K100 fuel injection control unit does not prime the fuel system. When the ignition is turned to On, the system is energized, the fuel pump doesn't run until the engine is cranked. When the starter button is pressed, the starter is activated, the engine is cranked, and the fuel pump starts simultaneously. If the engine doesn't start when being cranked, the fuel pump can be heard for a couple of seconds when the starter button is released then it stops. The fuel pump should be only be running while the engine is running or being cranked.

Go to this page in Motobrick.com. Scroll down the list to find the LE Jetronic Training Manual, download it and read it to learn how this engine control system works.

[mstolow]

Thanks Laitch. I bought the bike from the previous owner in the spring. Had some known issue with a transmission leak which was fixed. Other than that I have had no issues. I have not ridden it a lot. He did do all the custom work on it, which has made having it repaired complicated. I have not noticed any problems associated with the fuel but the mechanic did point out that this was not normal.

[Laitch]

That's a good-looking cafe version—a fender to protect components within the frame is a thoughtful move that many re-modelers seem to ignore. Does it still have the original air intake system or has that been modified? Please post a photo if it has been changed.

The fuel delivery system was designed to shut off when revs get too high or when the engine stalls in a crash, both useful safety functions. Fuel injection systems tend to have automatic fuel pump shutoff when engines fail so that fuel does not flood from fractured tanks and hoses after a collision if the engine control system remains energized. Even my '96 Ford truck has that feature.

Feel free to post a more extensive photo array and description within the Project Custom Motobricks section. I use the site's photo gallery function to post photos so I can insert photos within text and review the completed post before publishing it.