K75S swoosh when fuel filler is opened

[SDCR john]

 My recently purchased 1994  K75 S is gradually getting road ready for the riding season. One oddity that I have noticed, is that when I am opening the fuel filler (gas cap), there is a pronounced swoosh, as if air is either entering, or releasing from the gas tank. Hard to discern which, but I believe it is air, entering the tank.

 My first inclination, is that the tank is not venting properly. I removed the tank, and threaded plastic trimmer line through the two drain stubs, or openings, at the right rear of the tank. Nothing came out, or appeared to be plugged. The ones that are for overfill.

  Is there a vent built into the locking gas cap?

  Or, is this an issue or do all/most K bike tanks make this sound?

[K1300S]

did someone install the filler cap "backwards"?  if so, the flange blocks a vent hole iirc.

it should open from the front.

[volador]

If you look at your gas tank filler from the position of sitting on the bike, on the left side there is a hole for the purpose of draining accumulated rain water around the gas cap. Ideally when the bike is on the side stand leaning left.....
Problems can be encountered if you put the cap gasket on with the wrong orientation and cover the hole. Or if you turn the cap any direction other than having the hinge face the seat.

In the picture the drain hole is nearest the 6 o'clock position.



The arrow pointing to tube at the rear of the opening is the tank vent which gets a hose (not pictured) connects to the pre-filter/strainer basket at the bottom of the fuel pump.

What do you have going on inside the tank?

[volador]

Here is where the vent hose connects to the strainer basket. Whether or not it makes a difference if the vent hose is connected or missing I don't know. I'm going through the same thing myself

[volador]

the two drain stubs, or openings, at the right rear of the tank

the forward tube is the tank vent and the rear tube is the rain drain

[SDCR john]

Ok, guys, the cap is installed correctly, opens from the front.
I see the overflow hole on the left side of the inside of the cap, and that is clear ( spayed carb cleaner, and it empties at the lower right, behind the right side footpeg.)
 When I had the tank off, I did not notice that there was any tubing attached to the pump, (as in the blue tank photo) and never noticed that tank vent pipe on the inside, rear of the filler opening. I will have to empty the tank and explore.

  In your last comment, about the forward, of the two tank stubs under the gas tank, you say the the forward stub is a vent. Right now, that stub has about a 6" hose attached under the tank, running to a small triangle shaped device that is plugged into the end. Attached to the triangle devise, is additional hose that follows the other drain tube, down to the right footpeg. I apologize for the "triangle" description, but I don't know what that is, or wheat its called.

 

the forward tube is the tank vent and the rear tube is the rain drain

[SDCR john]

Popped the filler housing off, and I see the small vent stub. Its located just if front of the filler opening. Nothing is attached to it, ( see pic) so I imagine that I will need the rubber hose for that attaches to this pipe, and runs down to the strainer.

 Do I need to remove the pump, in order to install the hose onto the strainer? I don't see the strainer through the filler opening.

[volador]

small triangle shaped device

Is this grey plastic check valve what you are referring to ?



If so you can remove it.
The fuel tank vent is/was routed to the front of the crankcase connected to a small tube sticking up located around the radiator shroud bottom. This was for EPA standards later removed. There's a service bulletin about that......as you could get fuel into the crankcase if overfilling the tank.
The crankcase tube has a rubber cap to plug it.


I don't think you need the inside vent hose


 

[SDCR john]

Yep, that is it. Not sure why I thought it was triangle shaped...  I see that the previous owner plugged the other end, at the crankcase.

 So, Even though the inside tank vent pipe, is  missing a rubber hose and is not routed to the strainer basket...the hose to the strainer is unnecessary?

[volador]

Disregard the blue fuel tank photo with the fuel pump strainer.

You don't need the inside vent hose. You have the newer 43mm diameter fuel pump which has a sock type strainer.
Just remove the gray check valve

[SDCR john]

Ok, so just run that line straight down to the right of the foot peg, correct?

[volador]

Yes and you should be good to go

[SDCR john]

Great, thanks for your help, and patience with my lack of details.

[Laitch]

The fuel tank vent is/was routed to the front of the crankcase connected to a small tube sticking up located around the radiator shroud bottom. This was for EPA standards later removed. There's a service bulletin about that......as you could get fuel into the crankcase if overfilling the tank.

If you're logged in you'll see a link to the bulletin at the bottom of this post. What volador is recommending is the way to go. The bulletin's instructions enable the hose to be reconnected if necessary to meet California emissions inspection, but only a K owner in California could tell us if that is still a consideration out there during inspection.

[SDCR john]

Laitch,

Interestingly, my bike came from California, around 2004.  The doctor that I bought it from, relocated to the east coast at that time.

  I did see the pipe at the crankcase, near the radiator. At some point, this was plugged. The check valve was just running a few (about 8) inches from where it exited the underside of the tank. I plan on running a direct vent hose, following the drain line to the right side peg area, ASAP. I guess that I wont need high pressure hose, just something that can handle gasoline.

I very much appreciate all of the help and insight that you folks have offered with this issue. It is about the only thing that I have found on the bike, that is not functioning properly.

If you're logged in you'll see a link to the bulletin at the bottom of this post. What volador is recommending is the way to go. The bulletin's instructions enable the hose to be reconnected if necessary to meet California emissions inspection, but only a K owner in California could tell us if that is still a consideration out there during inspection.

[The Mighty Gryphon]

Although you don't have the hose shown connected to the filter basket it should be noted that the hose in the photo is not the vent hose.  Rather, it is the return from the fuel pressure regulator.  It was connected that way to prevent pump cavitation.  Later model bikes had that hose eliminated and the fuel just runs back into the tank.

[SDCR john]

Deleted the check valve, and routed a new vent line back to the right footpeg, and  "Swoosh" is gone.   Cool, big thumbs up.

Thanks.

[Laitch]

Here is where the vent hose connects to the strainer basket. Whether or not it makes a difference if the vent hose is connected or missing I don't know.

Its absence has made no difference to my moto for the 7 years I've owned it, but the pump could be damaged if air bubbles enter it. That might occur in a tank that has a extremely low level of fuel sloshing around in it.

The use of the sock-type strainer on the 43mm pump obviates the use of that port for pump protection.