Return fuel comes from the fuel pressure regulator at a low pressure velocity and is dumped into the tank. Any blockage of return flow would show up as excessive fuel pressure, rich running and hard starting. Return flow in my '95 K75 is emptied from a tube on top of the burrito and splashes at low pressure into the fuel supply beneath.
The return flow from your Brick's fuel pressure regulator should come from a standpipe in the floor or drop freely from the ceiling plate. Which is it? You should be able to identify the location with the fuel cap assembly removed and the Brick idling.
Two ports exit from the floor of the tank—one dumps water from around the rim of the tank cap to prevent it from entering the tank. The other dumps fuel vapor—or excess fuel—if the tank has too much pressure or too much fuel within it. Both are often routed behind the right foot peg plate. Usually hoses are attached to those ports. Sometimes, they dump directly from their ports into a small plastic connector attached to the frame beneath the tank. This device is lyrically called an air accumulator in BMW's parts fiche.
Vapor needs to escape from the tank. It's up to the port in the burrito (maybe in the ceiling plate if your Brick has one) and the vent in the tank cap to keep tank pressure stable. You could remove the hose from the port in the tank's floor and use a wire to gently probe for blockage. To conform to CA's rules, BMW routed the vapor hose from the tank to a port in the crankcase. Attached somewhere in the run of the hose was a pressure valve that opened when tank pressure exceeded a certain value. The vapor would enter the crankcase then be transferred via the z-hose into the combustion air stream via the air box. It caused performance problems in states with more variation in temperature than California's and was rerouted as described in most other states.
If the fuel cap is opened when you stop after riding, can you hear air escaping with a whoosh?
Laitch, very helpful, thanks. I’ve attached a sketch of the tank innards on this beast. Instead of a ‘burrito’, the top 2cm of the tank (A) appears to be the ‘collector’. I can’t tell for sure, but it the edges of this collector are not smoothly joined to the top of the tank, and there may have been a gap between this and the tank top wall. The gap, if one existed, is full of tank sealer all around the edge. There is a black rubber hose from the pump to the filter, and a short hose from filter to a rigid pipe that exits the tank forward on the left side. (F) That exit goes via rubber pressure hose to the forward end of the fuel rail. The aft end of the fuel rail has a rubber hose that goes to a doodad behind the throttle bodies (the pressure regulator, presumably) and from there to another tank nipple farther forward on the left side. This left forward nipple enters the tank as a rigid pipe that goes to the top of the tank (bottom of the collector, actually) where it is welded to the bottom of the collector just forward of the filler, then bends down and forward. (B) From there, there is another flexible hose going to a nipple on the collector behind and left of the filler (C). Finally, there is a rigid line that goes from the collector just behind the filler, centered, that exits the tank at the back right. THIS IS WHERE THE GAS IS GOING. That nipple is attached to a line zip-tied down the frame to the right footpeg, where it puts out gas at a rate of roughly 4 liters/hour (average) while the engine is running. There is also a rigid line from the filler neck area through the tank and exiting behind this gas vent line, which is not part of this problem.
My best understanding of the problem is that as currently plumbed, the fuel returning from the rail goes to the collector chamber. Due to the enthusiastic application of sealant in this tank, there is no longer any exit from the collector chamber other than the vent line, so my fuel pump is pumping more gas out the vent line than into the combustion chambers.
My question is this: If I detach the line labeled “flexible hose 2” at point B and ziptie it onto the top of the return rigid line so it’s above fuel level, will that cause other problems? If I do this, then only vapor will reach the collector chamber, fuel from the regulator will dump at low pressure into the tank, and my ride won’t leave a smelly and expensive trail of fuel behind it everywhere it goes. I don’t see what problem it would cause, but SOMEBODY in Berlin once thought this was a good idea and I’d like to know their reasoning before I bypass what they did. What does the collector chamber do, anyway?
Daveson describes holes in the burrito, top and bottom, to let fuel that gets in get out. I think the collector in this tank had a gap around the top edge that served the same purpose, but that gap is filled now so gas pumped in from the regulator/return line can only exit via the vent hose.
