'90 K75S Fuel Tank venting..baffled

[PDX_Expat]

Hey all, long time lurker and thanks for the info that I've taken from the forums.  I have run up against a wall and am completely stumped, and after 3 months of trying to figure it, I'm going to ask for help.

Problem: K75s venting fuel out of the venting port (bottom right of tank, front port).

Moved my wonderful old girl to New Zealand with me, but had to give up my man, Prokop in Portland. Service down here has been lacking, they can't figure it out and simply tell me to buy a new tank.

So it Runs beautifully! but after running for a bit and letting it sit (particularly on the side stand.  Fuel, lots of it gets into the venting cavity in the tank.  When run again it will gush and sometimes choke itself to death.

I figured a leak in the tank, and under pressure fuel was getting pushed in.  I cleaned out the tank and coated it ith Por...there were some hairline cracks.  After carfeful coating and double checking that all ports are clear and functional.. It still does it.  tried a couple of other things.. went for a nice ride on a full tank, and parked on side stand... came back to it 2 days later and tank was empty

Yesterday I pulled everything out, plugged up the ports, cleaned out the tank and filled with water.  Connected a line tot he bottom port and blew air..I got a couple of the tinest bubbles from a spot just under the cap...but there is no way a nearly full tank could leak out in under 2 days that way

at this point i'm guessing that a vaccum is being created and fuel is being sucked up the fuel pump venting hose.  But i'm not sure.

Could the fuel cap be the issue? Is it supposed to relieve pressure?

I am stumped and completely frustrated.  A big ride in 2 weeks and I am beside myself.

Any insights would be greatly appreciated! Pics of the bike or inside the tank I'm happy to provide.

Cheers,
PDX Expat

[TimTyler]

Are you sure you don't have a leaking fuel line or fuel pressure regulator?

Actually, that doesn't make sense either. I'm confused.

[PDX_Expat]

Okay Timeline:
Feb 2014 on a 2,000 km ride.  Half way through I drop her on the right side.  Cosmetic damage to belly pan, fairing and engine block...speedo a bit flaky but seems fine now.  Get it home and angry park for a couple months.

I then break down all the body parts and have the entire thing  fixed and painted.  I buff out and paint engine block.  During reassembly i notice the old tank coating pealing off.  I spend many meticulous hours cleaning it out.  Recoat it with Por. assemble and while she runs beautifully, turn it off and watch it gush. Once the venting area full of fuel, it will pour out while riding.

Local shop says get a new tank due to hairline fractures.  I get more of the sealant and this time use a very little at a time.  letting it cure well with each layer.  3 layers and it seems all sealed up.  Filled with water, blow through and everything seems okay.

Reassemble and still no difference.

Here is the kicker, if I run it with the fuel cap OPEN it seems to be fine.  I had figured that pressure buildup in the tank was forcing fuel into the venting cavity.  but now I'm thinking that a vacuum is being created and once turned off the fuel is being sucked up the fuel pump venting hose.

Does the fuel cap relieve any pressure?  I have looked at it and pulled it apart.. it seems in good shape and the seals good.

/shrug.

[rbm]

Constantine from Stuntexclub posted this picture of the inside of a K100 tank on the k100-forum:



He also posted a detailed shot of the part just below the gas cap:



These images shows the routing and construction of those vent hoses.  There is no venting in the gas cap.  A possible cause might be that there are cracks or holes in the vent tube in through which gas leaks.

[PDX_Expat]

thanks! This is super helpful in at least satisfying my need to understand the routing.

I have plugged the ports, attached a hose to the under vent and blown through with tank filled. I'm getting a few tiny bubbles from just under the cap area. But hardly anything.

It has to be sucking the fuel up the pump vent hose.

Let me ask this. What problems are created if I plug that port in the tank and run without that pump vent hose?  I think earlier models didn't even have it.

Including a pic of her. Notice the quick tubing I ran out of the vent so I could easily spot any gushing?  It ran great that day no problems. Then put it in garages on side stand and slurp! Out goes all the fuel

[TimTyler]

> Fuel, lots of it gets into the venting cavity in the tank. 

How do you know this?

> When run again it will gush and sometimes choke itself to death.

I don't understand this "gushing".

>came back to it 2 days later and tank was empty

So nearly a full tank of fuel was gone? Was it on the ground where you parked?

> Connected a line tot he bottom port and blew air..I got a couple of the tinest bubbles from a spot just under the cap...

So you blew air through the overflow tube that connects to the filler cap?

[PDX_Expat]

Because When i drain the fuel from the tank, then remove it...no fuel visible but still hear lots of fuel moving around.  turn it upside down and blow on the bottom vent port and fuel comes out the vent port in the tank.

If i then run it without clearing all that out...after a period of time the bike will sputter and stop running.. when you stop and its off, then fuel just comes leaking, badly out of the vent port.

3/4 tank and it was down to less than 1/3.  Garage full of fumes, obvious spot on ground where it leaked (tube that comes to back foot peadal out of the cup under tank) .

plugged the vent port inside the tank.  connected hose to vent port underside and blew.  Tank full of water...looked for bubbles.  a few from just under the lip where fuel cap sits.  So blew air opposite direction you described.

[PDX_Expat]

RE: PICS

Those pics are interesting but my tank is very different. I do not have that little "burrito" under the cap. the tubes just go up into the tank (which I assume, from the gas i've heard sloshing around in it, is a large cavity akin to that burrito).

I appreciate the puzzling out of it.  I"m sure I've screwed something up in the sealing process, just can't narrow it.. all ports and tubes were sealed and are clear.. checked them a dozen times.

Also re: parts down here.  Not too bad, there is a great dealer, Experience motorcycles, in Auckland who I get all my parts through, but Down in Dunedin a good service shop but not one that has a certified BMW tech.

[TimTyler]

Wacky!

Thanks for clearing that up. I'm stumped.

Can only suggest confirming that your in-tank hose routing is proper, but I'm sure you've thought of that :)

[The Mighty Gryphon]

I am somewhat confused by your descriptions, but what you are describing sounds to me like you have a siphon going on with the vent line.

I suspect that there is a very small hole in the vent line located at it's highest point called a siphon break that you have sealed.  Not being familiar with the internal plumbing of these tanks I have no clue as to where it might be.

During operation I think you are getting some fuel in the vent that starts the siphon when you shut the engine off.  Normally, the siphon break will allow some air into the tube and stop the fuel running out from drawing more fuel from the tank.  With the hole plugged the fuel will continue running until the suction end gets above the fuel level.

Somewhere I have seen a detailed drawing of the plumbing inside the fuel tank, but I can't recall where.  I would suggest finding that information should be your first order of business at this point.

[PDX_Expat]

Clarification, when running it I haven't had anything plugged. All plumbed correctly save for one short rid with low fuel and vent hose disconnected as a test. But realized quickly that this is fine for low fuel. Fuel tank um. No.

Up thread someone mentioned that the cap does not vent. If true i want to know what the little spring valve is for. ;). There is a youtube video of somone breaking down and rebuilding the entire fuel cap. All my seals, springs look fine in the cap. Zero corrosion.

The siphon idea is one to look at; oddly enough, I haven't checked the integrity of the fuel pump vent hose. Will do later and report back.

So excessive tank pressure pushing fuel where it should be;
- or  vacuum being created and fuel being drawn in
-or siphon, which sounds very plausible and no idea how to fix.

Other than hose. Going to run a test running bike for a bit with fuel cap not closed (sealed). One previous yes like this seemed to work. Which is what led me to think its a cap venting issue.

LOL sorry I've been puzzling through this for 2 months now.

Thanks so much for the replies and sounding board.

[Inge K.]

Up thread someone mentioned that the cap does not vent. If true i want to know what the little spring valve is for.

Depending on model/market......euro spec is non vented....us spec have a different vent system, and the cap
is vented one way to avoid vakum.

I would also back up the siphon theory....that the small hole in the vent cavity is blocked and when the heat
from the engine makes the fuel expand and a pressure builds up and the fuel being forced up the pump vent
hose and as the small vent hole is blocked, it seeks its way out via the vent pipe.
To verify, disconnect the pump prefilter vent hose at the top end and go for a ride.

[nortonbrian]

In the second photo, there is a rigid pipe, lower right, thats open. My K75 has this, is there a flexible hose that connects to this, or is it a vent?

[Inge K.]

If it's the one inside the red circle you mean, it's for the fuel pump prefilter...if you have a 07/90 or earlier.

[The Mighty Gryphon]

This "fuel pump vent"; is this something only found on some models?  As far as I can tell neither my '94 K75RT or '92 K100RS has one.  Why does the fuel pump need a vent?  As far as I can see on my bikes, the pump pulls the fuel in the bottom and pumps it out the top to the fuel filter and then out to the rail.  From the rail the fuel goes to the pressure regulator and the excess is returned to the tank. 

That downward facing tube in the vent body photos.   Is there a hose connected to it?  Should there be a hose connected to it?  If that is where the tank is vented, I can see how a hose there could create a siphon. 

[Inge K.]

  As far as I can tell neither my '94 K75RT or '92 K100RS has one.  Why does the fuel pump need a vent?

Your models have a different pump and a different prefilter, where this vent isn't used.
The vent was there to avoid cavitation in the pump.

[The Mighty Gryphon]

OK, another dumb question that helps me understand what is going on.  Is that fuel pump vent connected somehow to the fuel return line?  Is it allowing fuel in the tank to bypass the pump's strainer?

[Inge K.]

It's a hose between the prefilter and the spigot you see in the picture, to avoid vacum inside the prefilter.

[The Mighty Gryphon]

Thanks, Inge!

[PDX_Expat]

This makes a whole lot of sense.
Now, how to fix. ...

The pics above are not my exact tank. The tube routing is correct, but I do not have that "burrito" cavity. My tubes route up to the top of the tank and disappear. The cavity is much larger (as evidenced by the nearly 2 cups of fuel that I can get out of there after the main tank is empty as well as the sound of the fuel moving around in that cavity). Still, same concept.

So what. I'm guessing is that in sealing the tank unplugged the equivalent of that tiny hole. (Which is odd to me as I sealed the tank previously 5 years ago with no problems).

I wil try the test. But that leaves me with the question:  I know that there are some who have left that hose disconnected permanently (read of it that is); what are the implications there?  I read it was introduced to reduce fuel pump cavitation?  But then there is the problem of the open tube up top.

Obviously I don't want to run this way, trying to figure a work around so I can go on this rally ride in a weeks time.

[Inge K.]

Zip tie the upper end of the prefilter vent hose to the water drain tube as high up as possible.
And don't fill the tank to the brim, leave some space for air at the top..........

PS. Check if zip ties is fuel resistant, or use some fencing wire..... :riding:

[PDX_Expat]

I found it!  Last night.. finally.

The tank seal didn't adhere perfectly, and was masking the real problem.

Again, the top of my tank is different than that posted above.  If you sliced it open and looked at the top there would be a flat plate placed in the top with hole cut out for filler and fit over.... Better explain in the pic up top.. if you see the fill hold ring inside the tank it, mine is FLUSH with the top of the tank.  The rest above it is a large sealed cavity serving the same purpose as that "burrito" in the pic.

last night I plugged everything up, put a hose on the bottom Vent tube, put in half a gallon of water and moved it around until i got bubbles.  no bubbles, but air escaping. An hour later of carefully peeling off some of the tank seal....  One of the seems of the plate on the top front left side is leaking.  This explains why on the kickstand it was more likely to leak out vent as well.

Once that cavity fills, and the tank cannot vent, pressure, siphon and its all over.

*if* i can reach it, some JB weld may give me a quick fix while i hunt down a replacement tank.  I have a video I made of the issue last night I can post to Vimeo if anyone is interested.