Hi, Just wondering if I'm the only one. Removed the flapper valve from a K75 gas tank and am finding I can fill a bit more fuel. But I get a knocking/tapping noise from the tank. Guessing it is the fuel gauge float rising a bit and tapping the tank as I only hear it after I've filled the tank pretty much full. Or is it possible that I've created/discovered another issue? Not sure if I'm going to leave the tank this way but thought I would give it a trial.

best regards
J.Jefferies

The knocking noise is the float as you say when the tank is full.  Nothing to worry about.  

However what is this flapper valve you speak off?  Is it some sort of silly modification?  A photo would be good.

TacKler wrote:The knocking noise is the float as you say when the tank is full.  Nothing to worry about.  

However what is this flapper valve you speak off?  Is it some sort of silly modification?  A photo would be good.

Good to know. By "flapper valve" I'm referring to the metal door/piece you push the fuel nozzle past when filling
an unmodified tank. Modification is to remove that so you can fill the tank more fully.

jjefferies wrote:Hi, Just wondering if I'm the only one. Removed the flapper valve from a K75 gas tank and am finding I can fill a bit more fuel. But I get a knocking/tapping noise from the tank. Guessing it is the fuel gauge float rising a bit and tapping the tank as I only hear it after I've filled the tank pretty much full. Or is it possible that I've created/discovered another issue? Not sure if I'm going to leave the tank this way but thought I would give it a trial.

best regards
J.Jefferies

Is that a CA emissions measure? Photo?

BMW calls this part an Intermediate Plate (part number 16112309404).



The Intermediate Plate originally had nothing to do with sealing or vapors or CA emissions, they were there to prevent you from putting leaded gas into a vehicle that required unleaded.  The unleaded nozzle was thinner and fit into the flapper hole, the leaded nozzle was too thick to fit.  Another benefit on the K bike is it prevents topping the tank, leaving a bit of air space at the top which might allow a full tank to deform in a crash without bursting.  Automobile tanks are also designed with an air space for that reason.

robmack wrote: The Intermediate Plate originally had nothing to do with sealing or vapors or CA emissions, they were there to prevent you from putting leaded gas into a vehicle that required unleaded.  The unleaded nozzle was thinner and fit into the flapper hole, the leaded nozzle was too thick to fit.  Another benefit on the K bike is it prevents topping the tank, leaving a bit of air space at the top which might allow a full tank to deform in a crash without bursting.  Automobile tanks are also designed with an air space for that reason.

oooh! Might be worth replacing. Though I've never filled the tank so high it didn't have some air space.

No photos to post I've canned all of mine a long time ago.

The flapper is an aluminum cup with a small round spring loaded door at the bottom that is mounted under the fuel fill cap.  It's purpose was to prevent the use of leaded fuel which was still available when the bikes were first introduced.  At the time the leaded fuel was dispensed with a larger nozzle than the unleaded.  The hole in the flapper was too small to allow the leaded fuel to be put in the tank.

Because the cup extended down in the tank it reduced fuel capacity by about a liter.  One of the benefits of having it is that the lower fuel level helps prevent leakage from a full tank when the bike is parked on the side stand.  On the other hand, that liter of fuel spreads out my fuel stops another 10 miles.

Also, the space allows for fuel expansion in hot weather without loss. Otherwise, the only place for the fuel to go is through the overflow.

robmack wrote:Also, the space allows for fuel expansion in hot weather without loss.  Otherwise, the only place for the fuel to go is through the overflow.

Good point, fuel expands about 1%/10C.  Since fuel is dispensed at about 15C, if you park in the sun and the fuel temperature rises to 50C the fuel will expand 3.5% or about 35ml/liter.  This means that the fuel in a K bike's tank will expand about 750 to 850ml. 

Fortunately, even with the flapper removed, filling the tank to the opening in the filler should still leave about 500ml of air space in the tank.  I don't know where the tank overflow is located in the tank, but in my experience, I've never seen fuel come out of the overflow down near the master cylinder(maybe it evaporates too quickly), but I have had fuel leak out of the filler and run down the side of the tank when parked on the side stand.

Point-Seven-five wrote:I have had fuel leak out of the filler and run down the side of the tank when parked on the side stand.

Don't know what others prefer, but I always park on the center stand for this reason and because I find it easier to mount the bike when on the center stand and then roll off. Guess I got trained back in the day when parking on the side stand meant oil seeping into the cylinders. And then a cloud of smoke when starting. But I prefer to use the side stand only for dismounting and safety when moving the bike around.