'85 K100 Weird sound from gas tank

'85 K100RT. Bike starts perfectly and runs great. BUT after the engine spins up past 2K rpm, it sounds like a bee hive in the gas tank. RPMs drop down to idle and it sounds normal..revs up, noise come back. And it's LOUD!! Easy to hear it over the engine at redline.

3 new fuel pumps tried- both sizes, new fuel filter,all lines replaced, new fuel pressure regulator. Good spray pattern from all 4 injectors.

Already checked to make sure the pump is seated properly so no contact vibration..Took everything apart and blew out all the lines and old regulator. Noise still there so replaced that too.

I'm out of ideas. Anyone know where this beehive is hiding?

The early K100's had a check valve in the return fuel inlet on the tank (the forward most fitting). It will make a hammering noise intermittently. Is that what you are hearing? Most of these were removed early on and maybe yours wasn't. My tech took a punch and drove it into the tank and then retrieved it through the fuel cap a loooong time ago. The downside is that fuel will flow out of the return spigot when the line is removed for tank removal. A set of quick disconnects fixes that.

Frank

That's one of the first things I looked for. Didn't see one but I'll pull the tank again just because

If it's a buzzing from inside the tank, it has to be fuel pump related.

So you've tried three pumps and they all make the same noise? Sure it's not the normal amount of noise?

I do know that the noise level can be fuel level-dependent. Does it sound better with a full tank?

Tank venting is functional so you're not hearing the pump struggling to pull a vacuum in a sealed tank?

A normal fuel pump makes a low volume hmmmm.

This bike starts with the normal pump sound then transitions to the very loud angry bee hive vibration sound. 3 new pumps, same symptoms.

Blew out all the tank lines and run with gas cap open. No change. And I can see the vibration ripples on the gas..smaller with the normal pump.. Slightly taller when the bees come out.

My WAG is that the pump mount rubber is weak, but don't quote me on that.

New and the pump is correctly positioned so nothing touches metal to metal

Blew out all the fuel lines again, checked the fuel pump again, installed new fuel pressure regulator, replaced the filter again, and the #%#% bee hive still comes back as soon as the revs hit 2K. Quiets down to normal when it goes to idle MOST of the time.

I lifted the tank off the frame wiith engine running to isolate the tank and the vibration is coming from the tank...and you can physically feel the vibration in the fuel return line that leads into the pressure regulator. I released the fuel pump from the tank and held it in the gas but touching nothing..bees didn't drown...loud as ever.

I guess tomorrow I rip out everything in the fuel system and hope for the best..unless anyone has a suggestion.Seems to be pressure related but WHERE??

beemerguru said:

...and you can physically feel the vibration in the fuel return line that leads into the pressure regulator.

Click to expand...

You said in your original post that you changed the fuel pressure regulator? I am still thinking that it has to be either the regulator or check valve in the return fitting. It was built into the return port. Does fuel run out of the return fitting when you remove the fuel line (with enough fuel in the tank to cover that port)?

Frank

Greg,

What have you done to insure that it is not the check valve that has been suggested?

Try taking a long piece of fuel hose and connect it to the return side of the regulator and run it into the top of the tank through the filler opening. That will bypass the check valve.

Does it still make the noise? If not, the problem is the check valve. If it still makes the noise, you need to call an exterminator to get rid of the hornets nest.:

I'll do that this morning and see what happens..but I can't find a picture of this check valve anywhere...or it's exact location. I have a big aluminum dome inside the tank welded to the top of the tank next to the gas cap..but it's solid with nothing to remove.

I think the check valve is basically just a ball bearing in the stub pipe where the return line connects.

Is this bike new to you or does it have a new fuel tank? Is this a new problem or something that has just started to bug you? The check valve was as described before, a ball and spring in the return fuel pipe on the tank. I remember my local tech removed it pretty soon in my '85's life- it would buzz or hammer at different times. I just removed my fuel cap and looked inside. The return fitting has a cap sort of thing welded into the floor of the tank. I remember the tech saying he removed it with a punch and hammer- right now I don't see how that is possible.

Frank

Finally fixed.

May I make a suggestion. I've worked and built from frame up a number of early K bikes so feel comfortable with just about everything that makes them tick.

BUT this was my frst exposure to the check valve which everyone referred to,,but no one explained what it was or WHERE it was or HOW to fix it.

I explain things on the airheads and ADV forums with a detailed step by step that is right most of the time.

Something like:

Sounds like the check valve

Remove left side lower faring, storage bin, and side panels

Remove gas tank

Stand gas tank on the back end on a soft surface

Use a 19mm deep socket to remove the front fuel spigot - only one there next to the gas tank fuel pump plug

Hold the check valve in a vise and remove the small internal bracket, spring, and ball bearing

Clean up the threads

Reassemble everything. Replace fuel hoses if necessary


Maybe 7 simple lines directions for the idiots out here who can't find anything in any manual or ETK that explains this faulty valve location...would have saved me about 3 days and a few dollars.

But again thanks everyone for at least pointing me in the sorta right theoretical direction.

51797 said:

The early K100's had a check valve in the return fuel inlet on the tank (the forward most fitting).

Frank

Click to expand...

Greg,


The very first response told you WHERE it was.

Your second post implied that you had checked for that (twice). If you didn't understand where it was, you could have saved yourself the three days by asking for more specifics.

I never said I was smart...it went right over my head for whatever reason.

Apologies.. And don't do emails at 2am...rots you brain