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Thread: 1994 K75S Unblocking a blocked tank vent?

  1. #1
    BMW uber alles! Zagando's Avatar
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    Question 1994 K75S Unblocking a blocked tank vent?

    New problem:

    I believe one of my tank vent lines is blocked or somehow restricted. If so, it's probably what has been causing the high pitched chirping (subject of another current thread),

    I have tried blowing through a hose attached to the vent pipes protruding from underneath my tank. The rear one lets some air in but the forwardmost pipe is blocked.

    Anyone have any ideas on unblocking it?
    I already tried #12 gauge copper wire; it'll only go in about two inches before it must hit a sharp bend or something.

    I'm going to try blowing some air in with my HVLP turbine to see if that makes any difference (probably not but it's the only other idea I have at the moment.

    Help!
    ---Jeff '94 K75S Berlina

    ex: R100GS/PD , K100RS , R75/5 , R60/2

  2. #2
    3 Red Bricks
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    It's probably plugged with polishing compound.


    Use real compressed air (over 50psi). Remove the tank. You're going to have to clean out whatever you blow into the tank.

    The rear one is the rain drain for your cap. Open the cap while blowing through the rear tube. You should have air coming out a hole on the left side of the filler flange.

    The front one is the tank vent. Air should come into the tank roof area.


    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
    3 Marakesh Red K75Ss
    Mine, Hers, Spare

  3. #3
    BMW uber alles! Zagando's Avatar
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    Polishing compound, ha ha. Good one, Lee.

    Btw, the tank is already off the bike while I replace all the hoses and install the new ABS relay, etc. and I haven't put it back on just yet. The winds turned into rain so I rolled the bike onto the porch to have some shelter. Now it is dark so I'll continue working on it tomorrow or the next day (might take a day trip tomorrow so maybe I'll have to wait until Sunday).

    Plenty of air comes out the little hole by the side of the gas cap when I put my turbine's nozzle on it, so no problem there.

    After trying heavy, stiff wire in the other front vent pipe I snaked in some very flexible PVC covered 18 ga. wire and got it all the way in (AFAIK)---a good 11" or so. Then I put the nozzle back up to the little pipe and it seems to have made a difference. The wire looked fairly clean when I pulled it out but maybe there was something in there that blocked it enough to prevent further air pressure ingress.

    Should there be any resistance at all or should the front vent pipe be just as unrestricted as the other already is?


    Either way, I'll take the gas cap off again and have a good look around inside the tank before proceeding further. I have some gas that I can put in the tank so as to slosh it around and see if there's any, uh, polishing compund or other crud floating around that needs to be evacuated.

    PS: Our housemate has an air compressor on hand so will try that if necessary---but maybe it's OK now (depends on the answer to my question above).

    Thanks, Lee!
    ---Jeff '94 K75S Berlina

    ex: R100GS/PD , K100RS , R75/5 , R60/2

  4. #4
    A bozo on the bus deilenberger's Avatar
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    I don't think it's a new problem.. Have you noticed a "whoosh" when you open the cap to fill up? That's the usual sign of a plugged up vent. As the gas is pumped out, and no air can get in, a partial vacuum is formed.. and the fuel pump has to work harder (causing it to chirp perhaps?)

    To clean out those sort of lines, it's hard to beat weed-wacker line.. it's thin enough to wind through, and thick enough and touch enough to not break. I used it on an '04 R1150RS I bought where the cap rain drain was completely plugged up.. and that drain for some bizarre reason ran a series of loops around part of the cap assembly, soldered in place. Weed-wacker got through, followed by compressed air, and finished up with WD40 (to try to protect it a bit..)
    Don Eilenberger http://www.eilenberger.net
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  5. #5
    3 Red Bricks
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    The plugged vent could very well be the source of your chirping noise. The pressure may have been releasing past the cap gasket. Chirp, chirp,chirp.



    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
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  6. #6
    Dum vivimus vivamus ted's Avatar
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    Maybe it is a cricket that is stuck in the vent
    Ted
    "A good stick is a good reason"
    1994 K75RT
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    BMW uber alles! Zagando's Avatar
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    Thumbs up Detective Superintendant K-Don

    Quote Originally Posted by deilenberger View Post
    I don't think it's a new problem.. Have you noticed a "whoosh" when you open the cap to fill up? That's the usual sign of a plugged up vent. As the gas is pumped out, and no air can get in, a partial vacuum is formed.. and the fuel pump has to work harder (causing it to chirp perhaps?)

    To clean out those sort of lines, it's hard to beat weed-wacker line.......
    Yes, I think you probably nailed it again, Don. Our housemate thought it also might be the source of the chirping problem and told me to open the tank the next time I heard it screeching to see if that made any difference. Believe I mentioned this in my chirping thread a few days ago but anyway I am guessing that your hunch/his hunch/my hunch is correct. I have almost always heard that whoosh every time I stop for gas and open the tank---so often that I thought it was normal!

    I think I still have some red plastic weed whacker line around here somewhere. Although my wife and I will probably be making a day trip tomorrow in another vehicle I may try to see if I can clear the vent before we leave. Then I would almost be assured of perfect performance or at least the bike would be a whole lot closer to normal than it's been for the nearly four years I've had it (Oct. 1st "birthday"). Will let you all know the outcome as soon as I can.

    It will be quite a relief to have the bike running well (poss. better) again and the fall riding season around here is nothing short of fantastic.
    ---Jeff '94 K75S Berlina

    ex: R100GS/PD , K100RS , R75/5 , R60/2

  8. #8
    BMW uber alles! Zagando's Avatar
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    Quote Originally Posted by Ted View Post
    Maybe it is a cricket that is stuck in the vent
    If it's a cricket it must be a really big sucker; one that can chirp at 120dB or more!

    We do have crickets around here but they are outnumbered and outsized by the grasshoppers by far. However, on the off chance iot turns out to be the culprit I'll take it to a taxidermist and have it sent to you, Ted.
    ---Jeff '94 K75S Berlina

    ex: R100GS/PD , K100RS , R75/5 , R60/2

  9. #9
    Curmudgeon At Large Bobmws's Avatar
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    Quote Originally Posted by Zagando View Post
    If it's a cricket it must be a really big sucker; one that can chirp at 120dB or more!

    We do have crickets around here but they are outnumbered and outsized by the grasshoppers by far. However, on the off chance iot turns out to be the culprit I'll take it to a taxidermist and have it sent to you, Ted.
    Cicada, loud little suckers......
    Bob Weis
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    www.earplugco.com

  10. #10
    BMW uber alles! Zagando's Avatar
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    Spent another hour or so trying to get air flowing freely through that front tank vent this morning.

    Got about the same amount of weed whacker line in as the soft wire before; around 11" before coming to a dead stop. Noticed there's a little chamber at the top of the tank (sort of a mini-tank within the tank) that might account for that---the line goes in until it hits a wall or sharp bend.

    Trying from the top side of the tank it only goes in about an inch and a half; again, maybe a sharp bend prevents it from going further.

    Hit it from both ends with compressed air from a monster air compressor. The hoses I managed to attach (halfway or more, not a perfect fit) usually blew off or else leaked enough at the attachemnt point to be largely ineffective. I did notice a slight improvement as I am now able to blow on a hose held up to the bottom vent pipe and not feel like it's a solid seal on the other end.

    This brings me back to my earlier question that no one answered exactly: should there be any resistance through the front vent pipe or should the air pass as freely as the tank overflow vent?

    Maybe there should be some resistance so the gas doesn't freely evaporate or leak out when it's sloshing around inside?

    I now wonder if I'm "good to go" with a little bit of air passing through the vent now; maybe that's all is needed to ease the strain/vacuum on my fuel pump?

    Off on a jaunt to the remote hamlets of San Luis and Cabezon for the rest of the day so will check back later to see what the ever-helpful gurus think.

    Hasta luego, Jeff
    ---Jeff '94 K75S Berlina

    ex: R100GS/PD , K100RS , R75/5 , R60/2

  11. #11
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    Quote Originally Posted by Zagando View Post

    Got about the same amount of weed whacker line in as the soft wire before; around 11" before coming to a dead stop. Noticed there's a little chamber at the top of the tank (sort of a mini-tank within the tank) that might account for that---the line goes in until it hits a wall or sharp bend.



    This brings me back to my earlier question that no one answered exactly: should there be any resistance through the front vent pipe or should the air pass as freely as the tank overflow vent?




    Hasta luego, Jeff
    The way the tank vent works is there is a plate/baffle screwed to the bottom of the filler neck that separates the upper 3/8" of the tank from the rest of the tank. It is not sealed around the edges so air can pass in and out of the area. The. forward vent tube leads to this area. Your wire is running into the top of the tank. That's why you can't push it in any further. There is also a 2" long tube welded into this plate next to where the vent tube is. Welded in. They are not connected. This tube was for the earlier 52mm pumps that had the vent hose leading to the bottom filter cup. You don't use this.



    The front tube IS he tank vent. The rear tube is the cap rain drain.

    Since the small diameter of the tube is the ONLY restriction that should be there, you need to be able to judge if it is restricted more than just by the diameter. The easiest way is to LIGHTLY blow through it by mouth and judge if the resistance is the same/or greater than if you were blowing through a straw of the same diameter.

    On the tank I tested there're was slightly LESS restriction on the tank vent than on the rain drain, but pretty close.


    The tank vent is there to allow air to replace used fuel so you don't create a vacuum AND to releave pressure build up caused by heating up fuel and air in the tank (I think the latter may be the cause of your noise).



    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
    3 Marakesh Red K75Ss
    Mine, Hers, Spare

  12. #12
    BMW uber alles! Zagando's Avatar
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    Thanks for the explanation, Lee.

    It seems apparent that I still have a serious blockage as there is considerable pressure when I try blowing air through the tank vent; almost zero blowing through the rain drain (now I understand the distinction between the two).

    I am without a clue as to what to do now as I was really hoping and expecting to hear that a small amount of air passing through would be sufficient to equalize the pressure but it sounds way the opposite!

    I can't imagine what is blocking the vent so strongly, either. Maybe I should get the proper hose (that will fit snugly with a good hose clamps) and connect one side to the air compressor. Just hope I don't blow up the tank in the process.

    STILL VERY OPEN TO GOOD IDEAS (humorous ideas, uh, not so much at the moment).

    This is holding back all the other work I need to do on the bike before the cold weather begins to set in (it's already on the way here as it is now getting cold at night).

    Off to hit the hay and hope that something positive ensues very soon.

    Thanks again, Jeff
    ---Jeff '94 K75S Berlina

    ex: R100GS/PD , K100RS , R75/5 , R60/2

  13. #13
    Dum vivimus vivamus ted's Avatar
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    Have you tried a solvent like carb cleaner, one with the red pipe extension to concentrate and direct the spray?
    Ted
    "A good stick is a good reason"
    1994 K75RT
    Moto Pages

  14. #14
    BMW uber alles! Zagando's Avatar
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    No, Ted; Hadn't even thought of something like that---I may very well give that a try as I'm running out of ideas here.

    I've got to go to Harbor Freight today to get another siphon so I can get the rest of the old gas out of the tank so I'll pick up a can (blew up my old siphon with the air compressor yesterday).

    Whether it works or not I will probably re-install the tank and get the bike running again; it's driving me crazy not being able to ride. I also need to get it to a good BMW mechanic I know of up in Santa Fe to get the fork and bearing work done, too.

    If the tank vent is still unresolved maybe he'll know what to do; otherwise I'll just have to open the gas cap every so often to relieve the vacuum/pressure if my cricket or cicada starts acting up again.

    Thanks for the suggestion and I'll let you know if it helps.
    ---Jeff '94 K75S Berlina

    ex: R100GS/PD , K100RS , R75/5 , R60/2

  15. #15
    Themason 42906's Avatar
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    I wonder if a gun cleaning brush can be shoved up to the blockage and used to whittle away at it? Just a crazy idea.

    Otherwise, maybe try filling the tube with some kind of solvent like carb cleaner or maybe penetrating oil and allow it to sit over night.
    Experience is what you get when you don't get what you want.

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