1986 K75c - Fuel Tank Vent Hose Rounting

[MarcoK75c]

After having read old dry germanic service bulletins and searched here I am just slightly confused....

The attached drawing shows a routing for the tank vent hose that goes from a nipple on the fuel pump suction to a "knobby" connector on the roof just behind the gas cap. I understand the routing but I am perplexed. After diving into the tank a few days ago to replace the fuel filter & $30 hose, I found a decrepit old piece of tubing dangling from the top "knob".... Here are my questions:

• Why does this vent need to go to the nipple on the pump suction?
• This set-up puts the tube below fuel level, if my tank get pressurized because of heat, would this not just force fuel out of my drain and start a nice siphon effect?
• Does the knobby connector have a function I do not understand?

Thanks!

[TX brick]

That little hose from the pump bottom to the top of the tank is supposed to do something when the fuel boils. Fuel boiling was a problem on the early Ks. BMW added under tank insulation and air scoops and I have not heard of fuel boiling lately.


I have run mine with and without the hose(without now) with no problems.

[rbm]

Carefully examine the pictures below of a cut-away K100 tank:






The following annotated version of the above picture was kindly provided by Laitch later in this thread, around post 55:




The pipe you refer to is evident on the inverted top section of the cut-away tank and more clearly illustrated in the detail photo.  You can see the fuel return line routing behind the collector unit and leading to a small spigot.  You can also see the two vent lines, one of which is attached to that cylindrical collector, the other of which is attached to the gas cap mounting ring.  The pipe you point out in the first post is one of these vent lines.  The hose to which MarcoK75c refers is normally connected between that spigot you see pointing upwards and the filter basket surrounding the fuel pump.  This hose connection is to prevent cavitation and improve fuel flow.  If it is missing, returning fuel just pours out the spigot into the tank.

Why does this vent need to go to the nipple on the pump suction? To prevent cavitation at the pump inlet.
This set-up puts the tube below fuel level, if my tank get pressurized because of heat, would this not just force fuel out of my drain and start a nice siphon effect?  No.  There is no direct connection between the overflow drain pipe and that return supply hose.
Does the knobby connector have a function I do not understand?  It's where the return fuel exits.

[MarcoK75c]

Carefully examine the pictures below of a cut-away K100 tank:


Does the knobby connector have a function I do not understand?  It's where the return fuel exits.

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My tank is an older (?) Design, the return fuel simply exits in the upper left of the tank (at the other knobby) and then flows down to the pump. I watched it do this with some cellophane in place of the cap while on the center stand.

The other knobby replaces the burrito in the cut away, but only two connectors going into it, one hard tubing to the drain, the other the tubing in question.


Sent from my iPhone using Tapatalk

[BrickMW]

My 1986 K75c has the same tank shown in the photo.


Sounds like in yours the hard pipe may be cut / removed similar to the red lines I've added. :dunno


The line should be a solid pipe up to the top of the tank then turned down and the flexible line (circled - pic is kinda deceiving because the loose end is hidden behind that baffle) would then be attached to the "out" arrow.
The "burrito" should only have one vent pipe actually plumbed to it - overflow. The return line goes beside the burrito but doesn't actually connect internally. The other vent goes to the filler cap ring to drain water. (Why you should never turn the orientation of the filler cap.)

[MarcoK75c]

Well, a more careful reading of the Haynes manual has confirmed my suspicions that the routing along the inside of the tanks was changed in mid 1986:

From page 135:
"12 With the introduction of the k75 S models in mid-1986 the return valve was deleted completely and replaced by a long stack pipe which passes up to the top of the tank"

This confirms I have the early version of the tank, long return pipe and a knobby connector instead of the burrito.

I went out, pulled the pump, accessed the mesh screen with a hose nipple connection, connected the rubber hose in accordance to the dwgs and mounted everything carefully back together. Started it up and wow...most of the cicadas have left my tank and I can hear only one or two at most now!

Thanks for the help!

[MarcoK75c]

This confirms I have the early version of the tank, long return pipe

This should read "NO long return pipe"

[szabgab]

Hi, sorry to resurrect an old thread, however it contains info, that helped me a lot so far but some other details are still murky in my muddled up brain :) So as I have posted yesterday in the 'what did you do...' thread I am trying to find out, why there is this loud hissing noise coming from the tank. I have done two short videos, where it looks like, the petrol is not coming out of the barbed pipe on the left hand side, but an another line at the other end of that burrito (I love that description :) ). Am I correct to think, the returning petrol should be routed inside the burrito collector and come out of the barbed pipe in order to be routed back with an immersible line to the bottom of the fuel pump?

The videos ->



https://www.youtube.com/watch?v=UCG4aEAgHaI&feature=youtu.be

Thanks!

[daveson]

It's much clearer after a close look at rbm's post. I'm thinking the return pipe does not go into the burrito but beside it. I think attaching a hose to divert it will fix the noise problem.  It should be an easy test.

[daveson]

Actually,  maybe someone was very rough and bent the end of the return pipe towards the right side.  That's my new best guess,  I better leave it at that now.

[szabgab]

Actually,  maybe someone was very rough and bent the pipe towards the right side.  That's my new best guess,  I better leave it at that now.

Dave, thanks for the suggestions. The pipe, which is pointing straight down and has got a barb at the end is on the other side, but there is no petrol coming out of it, it visible at the beginning of the first video. The line, petrol is coming out of does not look to be severed or damaged, so probably it was cut. As I do not have a detailed schematics, I am not sure what goes where, Robert's pictures do help a ton, but I still not see, where the return line is connected to, e.g. if it is indeed routed to the collector or not (although that would be logical, otherwise what would be the point of it). If petrol level lowers somewhat, I might try to film the bottom and back side of the burrito, but being a bit more careful this time, as Soggz is right and poking electronic devices in petrol fumes is probably stupid...

[Motorhobo]

While we're on the topic of tanks, I ordered one of these stop valves to prevent fuel from streaming out the hose when I pulled it off the tank, as discussed in this thread:

http://www.motobrick.com/index.php/topic,6200.msg41288.html#msg41288

I got the valve from Bike Bandit and it's supposed to be the replacement for the OEM part shown.

Unfortunately it's not shown in that cutaway, and if you look at the tank from below where the valve's nipple protrudes, it appears to be welded in there. Does anyone have any experience with this? If so, I'll start another thread -- the issue has come up several times in the past few years, and for me it's not a dribble anymore, but a stream -- it only took a couple hours to drain 2.5 gallons of fuel through the outlet, whereas there should actually be no fuel coming out of there at all.

BTW please refrain from discussing workarounds such as clamping off or plugging the hose. I've been doing that -- what I need now is someone who knows how to replace that valve while I have the tank off and drained for maintenance.

Many thanks in advance --

MH

[Laitch]

Am I correct to think, the returning petrol should be routed inside the burrito collector and come out of the barbed pipe in order to be routed back with an immersible line to the bottom of the fuel pump?

That is incorrect. The the short barbed pipe adjacent to the burrito collector is a vent pipe attached to the filter basket via a hose. The hose that attaches the two is not a high pressure fuel hose, it is a submersible vent hose.

[szabgab]

That is incorrect. The the short barb pipe adjacent to the burrito collector is  is a vent pipe attached to the filter basket via a hose. The hose that attaches the two is not a high pressure fuel hose, it is a submersible vent hose.

Oh OK, thanks  Laitch for clarification. Is that hose necessary? I do not have it in the tank, but it is cheap enough to replace if it makes the bike running better

On the other note, what about that pipe, that is streaming fuel? Is that normal or something out of place?

Thanks!

[daveson]

Motorhobo is fuel drain out when the tank is off your bike?  If it's draining when all connected, is it draining out the exhaust? If so I'd guess injector not closing.

[Motorhobo]

It drains when the tank is off the bike.

[daveson]

I spose it's siphoning
I spose it would stop if you put your finger oven the vent outlets
One of my bikes does that,  I just put a rubber plug on it, since it's just temporary while your working on it.

[Laitch]

You have a neglected relic that has undergone 16th century diagnosis and treatment technique during its lifespan. Chances are the return pipe was plugged and cutting it was the solution. It's difficult to determine from here. Some owners have left the fuel basket unconnected to that short, barbed pipe without any ill effect on performance. I'm one of them. If you believe your enjoyment of this moto is being hampered by an ancestral curse, connect it.

The basket-vent hose-vent pipe scheme was obviated when the system was converted to a 43mm diameter pump using a fuel sock. By its construction, the fuel sock prevents air from entering the pump and causing malfunction. The basket-hose system channels air from the fuel stream before it can enter the pump.

Concerning the fuel return streaming from the pipe—adapt to it. It's the fuel return pipe. A submersible fuel hose could be clamped to it then routed downward. The extension could even dump toward the rear left-side of the tank by using hose of sufficient length then securing it to one of the vent pipes with a zip tie. This was a solution offered up by BMW when some owners complained that the tanks were prematurely running low on fuel and the pumps were whining because fuel was being dumped away from the fuel pump.

[Laitch]

Unfortunately it's not shown in that cutaway, and if you look at the tank from below where the valve's nipple protrudes, it appears to be welded in there.

You might have been sent an incorrect part. That should be a screw fitting. I notice that number has been applied to vents that are clamped to hoses on either end, too.

Here's an extract of Haynes' instructions for working with that valve.

[daveson]

Sorry,  shouldn't have suggested plugging it,  was early hours, was running on auto pilot.
I spose your not interested in Inge's idea of cranking it with the fuse removed, before removing the tank?

[szabgab]

You have a neglected relic that has undergone 16th century diagnosis and treatment technique during its lifespan. Chances are the return pipe was plugged and cutting it was the solution.

Concerning the fuel return streaming from the pipe—adapt to it.

Laitch, undestood, that was something, I have considered doing after reading the initial thoughts on this old thread (I hijacked, sorry for that). However Gryphon's sentence of the burrito being there for fuel saving, and me having awful mileage made me think, if the two are anyhow related (as everything else to try is exhausted, temp sensor reads fine, valves are not tight anymore, throttle bodies synced, no air leaks, new plugs, injectors cleaned)

So how I could revert to the old ways, e.g. fuel return line connected to the collector (if that indeed is the place, it is normally routed to)? The reason, I am somewhat baffled is that if you are correct, and the the barbed pipe is only for air exhaust there is no other outlet from the burrito fuel to escape to. There is a tiny hole on the bottom, that could not dump enough fuel and the overflow line obviously, but that would mean raw fuel being puked onto the pavement at 25-30PSI... Or is there another outlet, I am not aware of?

Thanks!

[Motorhobo]

You might have been sent an incorrect part. That should be a screw fitting. I notice that number has been applied to vents that are clamped to hoses on either end, too.

Here's an extract of Haynes' instructions for working with that valve.

Thanks, Laitch -- that part with the screw fitting is exactly the one I have. I'll take a look at it and see if I can make sense of what the Haynes says, and start another thread...didn't want to hijack this one, but looking at it from the bottom, it made no sense to me how it's supposed to go in there.

[daveson]

Look closely at rbm's photo,  the pipe from the burrito exits the tank at the rear,  right underside of the tank,  its for air,  not fuel.

The photo shows the fuel return pipe roughly parallel with the burrito, but your's has possibly been cut.

I took gryph's post as a joke as if you get gas if you eat a burrito.

Maybe give your bike a full service and do the air fuel ratio test,  it takes all of ten seconds.

[daveson]

In the photo the return pipe goes behind the burrito, then parallel with it, then pops up on the left.

That's how it is supposed to be.  And then a hose from there to beside the pump. But yours is not normal.

[szabgab]

Most of your response was incomprehensible to me. Gryph was joking in his comment but the joke apparently was lost in translation.
I can't answer if there is another outlet of which you are unaware because I'm unsure of the state of your awareness. What I can observe is fuel returning to the tank. It seems you don't like the noise it makes. I've explained what you might do to mitigate that. That's all I've got.

Laitch, the joke was not lost on me, but I thought it was just a funny afterthought for a true statement (e.g. fuel saving). You must understand my frustration, as I have done everything in the sodding book to lower my fuel consumption, but it is a no-go, and in the winter I was getting 29-30MPG, which is awful even for our family car, let alone for a bike, that supposed to be frugal on petrol. I am not a boy racer type and never drive the bike doing 100MPH, so I don't get it.

As to the other thing, I understood your solution how to mitigate the noise, and thanks for the reply, however I tried to find out what happens to the fuel if it enters that burrito as it must exit also, but where?