Hey guys

I'm new to the K bikes and actually have never owned anything injected before. I'm on an 85 K100RS and I have an intermittent loss of power that feels like I'm running out of fuel. I notice it mostly in higher gears. At first I thought it could be a clogged injector problem but I've run Techron injector cleaner through. After that I noticed that when I shut the bike off, I can hear air getting sucked into the tank via the gas cap. When I open the gas tank, I can hear a rush of air to equalize the pressure. Is this normal? Is my cap not venting properly? It seems to me that my pump is fighting this vacuum pressure and could be causing my issue. If that isn't it, I'm assuming I need to pull the filter and maybe the pump and go through it. The tank is incredibly clean, but maybe there is some clogging in the lines. Any thoughts?

Desperado, when you open the cap to the fuel tank, you can see a small hole at the 9:00 position that vents the tank. Be sure your cap does not obstruct that vent hole.  The cap can be installed incorrectly.  By that I mean 180 degrees wrong.  The cap's hinge should be near the rider, not near the controls of the bike.

brickrider wrote:Desperado, when you open the cap to the fuel tank, you can see a small hole at the 9:00 position that vents the tank. Be sure your cap does not obstruct that vent hole.  The cap can be installed incorrectly.  By that I mean 180 degrees wrong.  The cap's hinge should be near the rider, not near the controls of the bike.

thanks for the response, brickrider. the cap is installed correctly. do you think i should remove the cap and see if it's clogged. any tips for that?

so I fired up my air compressor to see if the venting line was clogged. pretty sure it was. after putting some pressure through it, i was able to hear air coming out of the underside of the tank on the right side. could this have cleared my issue up? i'll ride over the next few days and let you know of any updates i have.

As far as I know the vent comes out on the right side rear of the tank..........so you should be sorted.

There should be a collection cup under that outlet as well with a drain tube to the rear of the foot peg.

brickrider wrote:Desperado, when you open the cap to the fuel tank, you can see a small hole at the 9:00 position that vents the tank. Be sure your cap does not obstruct that vent hole.  The cap can be installed incorrectly.  By that I mean 180 degrees wrong.  The cap's hinge should be near the rider, not near the controls of the bike.

Brickrider, That hole you describe is not for venting the tank. It's purpose is to drain rain water that may collect around the cap, so that the water does not contaminate the fuel should the fuel cap be opened after a shower.

The vent for the tank is just under the cap, inside the tank. You can feel the tube inside the tank at the 12:00 position. Both the vent tube and drain tube terminate at the lower right underside of the tank.

Okay, I stand corrected.  Thanks. My apologies for misleading you, Desperado.  I hope all is well with your new-to-you K-bike now that you've verified the vent is open (in the lower right underside of the tank!).

robmack wrote:
Brickrider,  That hole you describe is not for venting the tank.  It's purpose is to drain rain water that may collect around the cap, so that the water does not contaminate the fuel should the fuel cap be opened after a shower.

The vent for the tank is just under the cap, inside the tank.  You can feel the tube inside the tank at the 12:00 position.  Both the vent tube and drain tube terminate at the lower right underside of the tank.

thanks, robmack.

there seems to be some confusion on that drain hole. i've read it both ways across a few different sources. i'm not finding a tube at 12:00, but there is one at 6:00. is that the one? any tips for cleaning that out? it looks like i might need to take the tank off. do one of the overflow hoses connect to it?

Pop the tank off. Two hoses, one power plug, lift the back, slide it back a few inches and off it comes.
Access to the overflow pipes will be obvious on the bottom.Blow a bit of air into them. I would probably do that quite gently, just in case.

There are excellent photos of the inside of the tank somewhere on here. One member cut his in half and posted the shots.
Pretty sure Robmack is right on the location of the overflow inside the tank

This is the vent pipe.

Paul.

Inside the tank

So the two pipes at the bottom are the overflow and drain from the cap.

Sounds like when fuel is being pumped from the tank into the engine, there is no equalising of pressure in the fuel tank.  Therefore the air you hear rushing into the tank is the tank trying to equalise the pressure to ambient.  That leads me to think it is a blocked vent. 

No problems with insects?  I have a problem with b*stard wasps building their mud food sacs everywhere and underneath motorcycles is a common target. 

robmack wrote:

brickrider wrote:Desperado, when you open the cap to the fuel tank, you can see a small hole at the 9:00 position that vents the tank. Be sure your cap does not obstruct that vent hole.  The cap can be installed incorrectly.  By that I mean 180 degrees wrong.  The cap's hinge should be near the rider, not near the controls of the bike.

Brickrider,  That hole you describe is not for venting the tank.  It's purpose is to drain rain water that may collect around the cap, so that the water does not contaminate the fuel should the fuel cap be opened after a shower.

The vent for the tank is just under the cap, inside the tank.  You can feel the tube inside the tank at the 12:00 position.  Both the vent tube and drain tube terminate at the lower right underside of the tank.

on some models (well on mine anyhow )  this vent is attached to the filter basket around the bottom of the fuel pump by a piece of rubber hose ..( which usually deteriorates ..sitting in the fuel )

Hi there, there are four pipe niples on the tank, two front left are fuel supply and return. The other two more or less rear right are one for draining water around the cap an the other to collect fuel vapor and vent it, on california models to the crankcase, and on the others to atmosphere.
As far as I know there is a builtin check valve in the fuel cap, this allows  air to enter the tank, and avoids fuel vapor leaks up to a pressure of 2.9psi.
Hope this helps
Best regards
JAP