Hi guys, new to BMW repair but fairly adept at basic motorcycle and auto mechanics. Recently I was surprised with a gift of a 1986 k100 that my late father sold to a neighbor. The bike had a failed fuel pump and the common leak. I received the bike back with two tanks (I don't know which is original) that had the pump and sending unit, hoses etc removed. The fuel supply and return are pretty easy to figure out, but on one tank the "vent" line has a open port up near the cap assembly (see picture) that I don't know what to connect to. Can anyone tell me where to connect this to?  Also on the other tank, the return side (front) supply has a pipe that end up at 12 o'clock just below the cap flange..The tank I plan on using has a check valve and no hose port so I'm planning on using that tank if I can't find the proper connection. Are these just left open?  Thanks in advance, this is the last step to
Get the bike back on the road. 

Brent

There are two models of fuel pump—52mm and 43mm diameter. The 52mm pump's prefilter basket is attached by a hose to the vertical port described in the text and photo.  The 43mm pump's prefilter is a cylindrical sock attached to the pump. That vertical port at the tank ceiling is unused. Many riders with 52mm pumps don't bother with that port-to-basket attachment and don't have bad outcomes. Its construction might be a case of Teutonic overbuilding.

Put a couple of gallons of fuel in each tank for a few days and determine if the tanks' lower seams have leakage.

There are other differences in early and later fuel tanks besides the fuel pumps. The early tanks have a check valve and return stub on the floor of the tank. The later models have a metal return line as described in the photo's text. There are also two types of fuel level senders.

On the 86 tank that pipe connects to the screen at the bottom of the fuel pump:

Ok, that reallly helps Duck many thanks for the info! I have the sock style pump, so there isn't any connection. Possibly they bought the later pump by mistake. Looks to fit either tank, but I would definitely prefer to use the original older tank. The newer one has been coated with something and over the years that coating was coming off from expansion/contraction and not having fuel in it.

So I should be good to go with the older tank, newer pump leaving that port open?

BRogers wrote: So I should be good to go with the older tank, newer pump leaving that port open?

Yes.

There has been part swapping among many of these motos. I bought a '95 K75 with the later model tank and it had a 52mm pump with a filter basket instead of the 43mm pump with the fuel sock. The pump still operates well and hasn't been connected to its vent port.

Awesome, thanks guys!

You should be fine with the later  skinny pump with the sock.  I don't know if it's true or not but I've heard that that hose had something to do with reducing cavitation at the base of the fuel pump.

Looks like I've got a old and leaky O ring on the sending/float unit. I'm thinking I should be able to find something locally.

One more question guys, does the in tank hose to pump need to be fuel injection hose specifically? I'm not sure if the hose I have in there isn't just bull from the auto parts store. Thanks again!

The hose from the pump to the filter and the short one from the filter to the outpipe both need to be SUBMERSIBLE fuel injected line.

Thanks Duck! That's exactly what I thought. The hose says NBR that is on there. Just priced 1 foot of hose, $30.00!!!!!!

Hi can anyone point me in the right direction for 43mm fuel pump. I had a 52mm pump that broke and i bought a new rubber vibration thing but its for a 43mm pump.
Thanks in advance

Ok, got the bike back together and it won't seem to start. It will crank, fuel pump makes noise, have fuel in the rail but haven't pulled injectors yet. Pulled plugs and getting fire, but prob a little weak. I used a new battery out of another cycle that is a bit smaller than the stock battery. Would a weaker than stock battery cause the bike not to have enough spark to kick? Thanks again for all the help.

Hi Brent first i would put a small amount of oil into bores via sparkplug holes,to improve compression,the Ks can be a pig to start after being stood for a long time.Also use  SLAVE BATTERY 
to give the bike max crank speed.Good luck.

Ok, tried the oil in cylinders, still no go. By slave you mean like jump starting?

Yes,I use a spare car battery with jump leads,try WD40 or a little oil again.After trying to start,are the plugs wet or dry just a thought.

BRogers wrote:Thanks Duck!  That's exactly what I thought. The hose says NBR that is on there. Just priced 1 foot of hose, $30.00!!!!!!

The BMW submersible hose price is per meter. 16121180040

I was totally shocked(Shocked!!! I tell you. ) to find that BMW actually had the lowest price on submersible fuel line that I could find anywhere.  30 something bucks for a whole meter of the stuff!!!  Even AutoZone was more expensive!!!

Point-Seven-five wrote:I was totally shocked(Shocked!!! I tell you. ) to find that BMW actually had the lowest price on submersible fuel line that I could find anywhere.  30 something bucks for a whole meter of the stuff!!!  Even AutoZone was more expensive!!!

They make up for it by charging $487 for a fuel pump.