Hi all, long background story here and perhaps a short answer. As reported earlier in another thread, my current tank on the Silver K75 has issues with surging when the fuel level is low. So my thought was to build a replacement using the tank from my parts bike. Got all the necessary parts to fix what was broken, i.e. the fuel pump supporting ring (btw, it was cheaper from the dealer USD $11. than after market suppliers), the fuel strainer, fuel filter and connecting lines. BTW, I used the weird (to me) accordion pleated plastic lines sold by EME for connecting the pump-filter-pressure lines. Don't know how it will work out just yet. But definitely different than the usual "submerged in the tank" fuel lines. So I've got it all together and re-integrated into the tank. Will be after I ride it before I know if it all plays together. But the question I wish to ask  is that there is one pipe (aluminum) that drops down from the top on this tank just in front of the filler cap. Essentially either the one labeled vent line in this diagram: http://www.motobrick.com/index.php?PHPSESSID=d6fc1ed5a9fa2888023d247b081dfe10&action=dlattach;topic=10105.0;attach=20776;image
or one next to it. As the tanks did morph  over the years and this tank is off an unknown (year) bike  I'm not at all sure it's the same "vent line". And it is known that some of the tanks would spray returned fuel forward I'm wondering if I should connect a hose to this tube. It appears to have the bulge at the end which a hose would fit over.  Unless someone has a meaningful comment to the contrary I'll just try it and see if it works, i.e. doesn't explode or anything. But thought I would post just in case someone wants to respond so they can say "I told you so" later.  

I also had fun smoothing out the usual bumps and dents in the side of the tank before spray painting the tank again. On painting tanks I would add that I've decided to use a base coat with a clear coat that has hardener added (the 2000 series of spray cans). It's all but impossible for a DIY person to get decent paint  with hardener in the old style here in California. And Nevada is a long way to go to buy paint.

In order to try and keep the pump cooler in the low fuel situation I fitted a piece of submersible fuel hose to the aluminium pipe that returns the fuel from the FPR. I ran it over to the fuel pump. It may have been a BMW recommended modification.
Regards Martin.

MartinW wrote:In order to try and keep the pump cooler in the low fuel situation I fitted a piece of submersible fuel hose to the aluminium pipe that returns the fuel from the FPR. I ran it over to the fuel pump. It may have been a BMW recommended modification.
Regards Martin.

Martin, where is that return pipe? Think that may be one answer to my question(s).

Martin, wow that is some .... . After I read your post I went back out and searched using applied compressed air to the return pipe. How did you ever get a hose over that exit pipe?? My hand is too big to fit easily in the tank and it took it seemed forever to find the opening. Anyway that's maybe a solution to my original issue of how to fix the surging. But that means all my hard work metal working, painting, putting parts together is all for fun only. Heck I'm still going to try this new tank and see what happens.  

But I'm still trying to figure out what this drain pipe is. Maybe I'll try compressed air up it and see where it comes out. Ok, that I'll try tomorrow.

thanks
J.

There is a drain around the fuel cap that allows water that accumulates around the cap to drain off. There is also a overflow pipe to cope with fuel expansion. Both these exit side by side under the tank. Some bikes had hoses connected to them and some exited into a plastic accumulator  that attached to the frame the hoses normally then ran down to the RH footplate near the near master cylinder. I had to remove the cap and apply a bit of silicone/rubber grease to get the hose on, I believe the pipe is slightly bigger than the other fuel hoses so a bit of effort is required. It took  quite a few goes to get it on. I also had had to use a thumb wheel with a socket to tighten the hose clip. I also have the added advantage of small hands.
Regards Martin.
Accumulator or whatever you want to call it.

jjefferies wrote:I'm still trying to figure out what this drain pipe is.

If the pipe is descending a short distance from the fuel tank's ceiling, it was connected by a hose to a port on the circular fuel filter basket of 52mm fuel pumps in pre-'93 fuel tanks. The system was designed to prevent fuel pump damage by cavitation occurring if air entered the fuel pump during low fuel level in the tank. Air would be evacuated through that hose before it could enter the pump.

That pump style is in my moto but there hasn't been a hose connected to it for 60,000 miles; however, I don't allow the tank to be chronically low on fuel. Photo from Euro Motoelectric.

If the diameter of the pipe is 6mm, it's for the cavitation hose. If it's 8mm, it's probably the fuel return pipe.

Edit: Either way, you don't have to connect anything to it. I turfed the cavitation hose before I got my brick on the road cause it was soft as licorice, probably not made to withstand ethanol blended fuel in those days (but the fuel hoses were so brittle they would crack if I even thought about bending them)

daveyson wrote:If the diameter of the pipe is 6mm, it's for the cavitation hose. If it's 8mm, it's probably the fuel return pipe.

No need to unpack the calipers. A fuel return pipe will have fuel spilling from it when the engine is running.

You can see if it's smaller by looking at it compared to the external 8mm barb. Please don't ask me how to get your eye inside the tank if you can't see it with the filler cap assembly removed. Or a piece of an 8mm hose is a way too loose fit.