K75 Tank Vent question

[jimmyenglish]

Ok this is driving me nuts. Today I'm at the Walmart and I hear, " Would the owner of a black BMW motorcycle please come to your bike as soon as possible?" I go out and there's a huge puddle of gas under the bike. Like .5 gallon! WTF! It's dripping from one of the vent hoses. The bike is on it's ridiculously steep sidestand that I use all the time. I put it on the centerstand and it stops. The tank is about 3/4 a gallon shy of being full. I never overfill and always leave an inch or so from the cap. The bike was in direct sunlight. Upper 70's. I opened the cap and there was no pressure. It and the bike dry up and I drive home. Business as usual.

The coating stuff in the tank was constantly clogging the fuel pump screen so I POR 15'd the tank last spring. The vent in the bottom (I think that's where I heard compressed air coming out) got clogged and there was massive pressure (vacuum?) when I would open the cap. I took the tank off and unclogged the vent. You can blow through every tube. Everything's been fine until today.

So my question is, specifically, what is the routing for the vents on this tank? The Clymer book is scant on detail. Is it just the cap spill and the one in the bottom of the tank? Where are the hoses supposed to go? I bought the bike used and they've always just dangled out the back. Is there another one somewhere? What does the hose to the fuel pump screen do? Sorry for such a long post. I'm trying to be as detailed as possible. Thanks in advance.

John

[cjack]

Ok this is driving me nuts. Today I'm at the Walmart and I hear, " Would the owner of a black BMW motorcycle please come to your bike as soon as possible?" I go out and there's a huge puddle of gas under the bike. Like .5 gallon! WTF! It's dripping from one of the vent hoses. The bike is on it's ridiculously steep sidestand that I use all the time. I put it on the centerstand and it stops. The tank is about 3/4 a gallon shy of being full. I never overfill and always leave an inch or so from the cap. The bike was in direct sunlight. Upper 70's. I opened the cap and there was no pressure. It and the bike dry up and I drive home. Business as usual.

The coating stuff in the tank was constantly clogging the fuel pump screen so I POR 15'd the tank last spring. The vent in the bottom (I think that's where I heard compressed air coming out) got clogged and there was massive pressure (vacuum?) when I would open the cap. I took the tank off and unclogged the vent. You can blow through every tube. Everything's been fine until today.

So my question is, specifically, what is the routing for the vents on this tank? The Clymer book is scant on detail. Is it just the cap spill and the one in the bottom of the tank? Where are the hoses supposed to go? I bought the bike used and they've always just dangled out the back. Is there another one somewhere? What does the hose to the fuel pump screen do? Sorry for such a long post. I'm trying to be as detailed as possible. Thanks in advance.

John

The fact that you have liner paint in the tank is not stock. Those tanks are aluminum. Painting the inside of the tank has the possibility of clogging the vents. etc. Some may have painted the inside to fill small holes in the aluminum which develop from the tank sitting with water in the bottom. But anyway, the hose in the screen goes up to the top and joins a tank vent piece up there which has a metal tube running down the tank and out on the right side. That is the main vent for the tank. It could be that the top of it is plugged, but the hose from the bottom of the tank at the screen was open with the tube running down and outside on the right. Then any vapor pressure building up in the tank would push gas up the rubber tube and down the pipe and out. The filler cap drain also has a metal tube down the inside of the tank and out the right side. Originally, the vent tube outside hose was vented thru a one way valve into the crankcase at the front top of the engine. This was removed later to prevent gas going down into the oil supply. The filler drain and the vent hose then both went to the ground. In Europe, instead of two hoses, there was a cup under the two pipes on the outside right of the tank and a single hose to the ground. Most buy the cup from the dealer and install it to make it easier to remove and replace the tank. Also less hoses.

[jimmyenglish]

Hey Jack. Thanks for writing. So the pump screen hose and the vent below the pump are coupled?

I know of the 2 pipes coming out of the tank, one goes to the cap and the other goes somewhere below the pump.

I will drain the tank tonight and see if I can blow through the pump screen hose.

Thanks again. This is a great forum!

John

[cjack]

Hey Jack. Thanks for writing. So the pump screen hose and the vent below the pump are coupled?

I know of the 2 pipes coming out of the tank, one goes to the cap and the other goes somewhere below the pump.

I will drain the tank tonight and see if I can blow through the pump screen hose.

Thanks again. This is a great forum!

John

The one goes to the cap.
And the other one at the bottom outside goes to the inside top of the tank just inside and in front of the filler cap. The hose from the screen at the bottom of the pump also goes to the inside top of the tank just inside the filler cap. There is some kind of two into one vent junction there inside the top of the tank. It could be, or could have been, plugged up.