I need some help from someone who knows about the 1986 k100 rt. I recently purchased one and I am putting it back together. I took off the air filter box and found a rubber hose with a grey plastic fitting lying uner the box. Someone had taken an old rubber hose attached it to the grey fitting and put a screw in the end of it. I can only assume this is a vaccum line of some sort. I have called the dealership in the area and they are willing to tell me what it does and where it should be connected for the shop rate they charge.

The hose comes out of the center, top of the engine block at the front of the motor. When I purchased the bike the previous owner had repaired some fire damage that the original owner had caused. The fire was limited to the top of the engine and all of the plastic and wiring have been replaced. The bike would not idle when I bought it. You could get it started and keep it running if you used the throttle to keep the revs up. Can anyone tell me what this hose does and where it needs to be connected?

Hi,
If it has to do with vacuum then it is probably connected to the fuel pressure regulator. The regulator is a round object about 5cm in diameter bolted to the throttle bodies and is only accessible by removing the air box where the filter is.

The other end should connect to the throttle body number 4, the rear one.
Having said this, you will not miss it if it is not connected. Its main function is to regulate the fuel pressure (a few psi) depending on the load.

As yours is a 86 model I don't think that any other vacuum devices are present.

is there no way you could take a picture?

I have stripped mine down around the same area so will be able to tell you more.

Also if its attached to the engine casing it is a sump breather and it attaches to the throttle bodies, if you look at the throttle bodies you will see where it attaches to.

If it is the breather it attaches to the rear of the airbox above the throttle bodies, but that's on the aft end of the engine. A picture please?