Seems to be a recurring problem-A slow leak always down the left side of the tank.  Is this something just a rebuild with new gaskets could fix or is there something more ominous lurking in there?

What do you mean by slow leak down the left side of the tank?

When gas comes out of the tank in the ground at a gas station it can be cooler than the ambient air temperature above ground which will cause the gas to expand. Not to mention that K bikes generate a lot of heat under the tank. If you leave it on the side stand right after a fillup and the tank is warm then . . .

That's a Mk.1 fuel cap (pushbutton type) - there's a problem right there. A new gasket might fix it but you're better off going for the later type fuel cap.

Dai wrote:That's a Mk.1 fuel cap (pushbutton type) - there's a problem right there. A new gasket might fix it but you're better off going for the later type fuel cap.

It's not just the early gas cap that does this. I've had the same thing happen with 86+ gas caps on warm days.

I had the fuel leak from the cap down the left side of the tank while on the side stand.  It was right after I filled the tank on a hot day.  Big stain on a white tank that had just been painted.  Bike was a 1992 K100RS with original fuel filler gaskets.

I never park on the side stand with a full tank anymore.

Point-Seven-five wrote:I never park on the side stand with a full tank anymore.

If I stop for food and gas while out riding I always eat first and then fill up with gas when leaving.

So the lesson here is, always use the center stand, period.

I generally do most of the time due to the oil startup quirk, but I'm sure I've lapsed sometimes-probably right after a fillup, knowing me.

Is there a benefit to the Mk.II cap in anyways? I'm assuming it's a bolt-on swap. It looks easier to repaint, in any case.

No problem with parking on the sidestand, just don't do it on a full tank. 

I'm guessing that either cap will work to keep the rain out and most of the fuel in.  Neither will hold a full tank on a hot day on the sidestand.

There's no problem with using the side stand. However, since you have an 85 with the early piston rings (built before 2/88) it may have smoky exhaust for a while after you start it when you use the side stand.  (Not a big deal.) This is because the cylinders are pointed downward when the bike leans left and the oil behind the pistons seeps past the rings into the combustion chambers. This can be mitigated if you turn the bike off and then tilt it to the right for a few seconds before putting it on the side stand. This lets the oil behind the pistons drain into the sump so there's less oil to seep past the rings. In 1988 they changed the piston rings so that much less oil seeps past them but even the later bikes will smoke on startup if you leave them on the side stand long enough.

The newer gas cap needs the key to be used each time you open it.  Yes, it's a direct fit on 85 and earlier gas tanks.

You shouldn't have a problem parking on the side stand with a full tank, if you have good cap seals and you have a hose attached to the under tank catch reservoir. The only problem will occur if someone throws a cigarette butt into the puddle of fuel that gathers under the drain hose. 
Regards Martin.