Cleaning/flushing out a gas tank?

[Feo]

I've acquired a K75RT that needs some fuel system work, I've been researching for a few days, and it's going to get the whole schmeer except for the pressure regulator (I'm pretty sure I can flush that).

The rubber vibration damper has disintegrated in the tank. Worse than any I've seen pictured online. The last of the gas sucked out was black, and there are bits of slimy, spongy rubber-like crap all through the tank. The strainer was off the bottom of the pump, the fuel filter was crammed full of the rubber particles, and it still ran. Wouldn't pull over 4k rpm under a load, but in the interest of not holing a piston, I didn't really give it the old college try.  :laugh

I going back with a ford type pump and strainer, new hoses, and rebuilt injectors. New crankcase breather hose as well, since that is cracking.

The rest of the bike seems in good shape, I think the previous owner was intimidated by the idea of dealing with the whole fuel system, and decided his '68 Mustang looked a whole lot more fun.

So, I got a couple questions to start off with-

What can I use to clean out the residue in the tank that won't hurt paint or screw up the plastic float on the sending unit?

Do I have to disassemble the fairing to get the tank off the bike? (And what's involved with that?)

Any advice will be appreciated.

[Motorandy123]

Here's a thread from another site about the same problem;

http://www.bmwmoa.org/forum/showthread.php?t=47304

Looks like Kerosene or Goof off or paint thinner. The alcohol in the fuel
is suspected of melting this in the first place so rubbing alcohol might work...

[Feo]

Thanks, Motorandy, I hadn't thought of Goof Off.

Well, I just looked at some of the bigger pieces of the rubber (or should that be the substance formerly known as rubber?), and the ones left in the sun to dry have shrunk and hardened up a bit.

Maybe if I could dry out the tank, a shop-vac might get most of the crud.

I'm gonna scratch my head a bit today.

[frankenduck]

No, you don't need to remove the fairing to get the tank off.  You'll probably want to remove the left inner knee panel to get to the hose clamps for the two fuel lines at the front left of the tank.

Then remove the tank.

At the rear corners of the tank are two posts that have large E-cips on them to hold the rear of the tank to the mounting bosses on the frame. Remove those with needlenose piers and then lift the rear of the tank.  Sometimes a small piece of 1x4 lumber is helpful to pry up the rear of the tank without damaging the paint.

Then undo the connector for the fuel level sender. Lift the rear of the tank and pull it back.

I'd recommend just removing the fuel level sender from the tank so it doesn't get in the way while you're cleaning - four large Phiips machine screws on the underside.

[frankenduck]

Related topic: Air Accumulator (a.k.a. "The Cup")

[Motorandy123]

I'm interested in this too as my new k75 with 75K on it probably will need some
gas tank attention soon. I haven't opened the tank for fear of what I might find
in there. Probably should at least change the filter out...

Hey I looked in there today and everything looks great! The filter is 2" from the
pump and looks to be tilted up away from it. Not sure if there is a hose on the
other end. Does anyone know if that is the right orientation?

[Feo]

Okay, I think I've got the gas tank cleaned out.

Nice thing about 98 degree weather with lots of sun, you can leave a tank outside and the gas residue will evaporate. Which is prolly a lot safer than the first thing I was gonna try to dry it out. And no, I ain't telling, there might be lawyers present.  :lol:

I'm waiting on some BMW parts to arrive (including the o-ring for the sending unit), but in the meantime, I'm asking more dumb stuff.

The original return line had a hose inside the tank that went down to the nipple on the fuel pump strainer, right? (wasn't there when I opened this one up.)

Do I need to run a hose off the return line to any place in particular, or just leave it open to dump the gas?

 :dunno2:   <------ (This will be my most used smilie around here)

[Rick G]

That small line to the prefilter is not the return line. Apparently BMW put it there to prevent cavitation at the fuel pump by giving the prefilter a breather.
Don't ask how because it doesn't make sense to me.
The return line end is near the gas cap as well.  I have never replaced that line but block the end at the filter so nothing stray gets in.