Bubbling in the fuel tank?

[Folsoml]

I just picked up my project K1100LT.  I've not had a chance to dive into it yet, but I noticed something I thought was odd when the previous owner was showing it to me.  The bike is not running, but he wanted to show me how it turns over with the starter.  As soon as he turned on the ignition (key) switch, I head what sounded like bubbling inside the fuel tank.  I asked what it was and he said it was the fuel pump.  I've never heard anything like this.  Is this normal?

[Inge K.]

That is normal on a Motronic equipped 4 valve model like yours.

[The Mighty Gryphon]

Bubbling in the fuel tank?  I know you should be able to hear the whine of the pump, but bubbling?

I wonder if this could be a clue to the non start.  I would look to see if there are any breaks or leaks in the lines from the fuel pump that could prevent getting enough fuel pressure to the injectors.

[Folsoml]

Bubbling in the fuel tank? ... I wonder if this could be a clue to the non start. 

This was what I was hoping for.  It is clearly not just the whine of the pump motor.  It sounds like there is a small fountain inside the fuel tank.

[Laitch]

That is normal on a Motronic equipped 4 valve model like yours.

The bubbling would be fuel coming into the tank via the return line from the fuel pressure regulator. If I understand Inge correctly, these models have a return line outlet located in a different place than other models, hence the sound of the fuel return being heard. The sound of a fountain is usually soothing; enjoy it. The latest reasons here for bike failure-to-start have been misplaced spark plug lines and electrical connection shorts.

[F14CRAZY]

something disconnected perhaps? I'd take out the pump and see what's up

[Laitch]

something disconnected perhaps? I'd take out the pump and see what's up

That, too, would be instructive.

[Folsoml]

Thanks for the assistance.  All this I will get to in due time.  The battery that came with it was not holding a charge, and was not strong enough to turn the engine over.  I went to get a new one, but they did not have one in stock.  It will take a few days for it to come it. 

This bike is actually next on my list of projects.  I've not officially started on it yet.  I have a CB900C that I am putting what I hope to be the final touches on right now (trying to resolve an electrical issue that only presents when the engine is warm). 

[Martin]

Folsom a heat gun & a can of freeze spray is very handy when trying to track heat related problems. Don't know about the bubbling in the tank. I have no experience with K1100's, but it does sound weird, need someone with an 1100 to clarify.
Regards Martin.

[The Mighty Gryphon]

With regards to the tank bubbling noise, it wouldn't be too hard to remove the flapper and run the pump with the filler open.  Probably a ten minute job at the most.  You don't need that flapper in there; with it out you can get another 20-25 miles per full tank.