MOTOBRICK.COM

TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: jjwithers on February 25, 2026, 07:26:50 PM

Title: K100 Gas on swingarm and airbox after fill up
Post by: jjwithers on February 25, 2026, 07:26:50 PM

I've got an '85 K100 with some K1100 wheels, swingarm, etc. 
but that's not the point.

I filled up the other day and there was quite a bit of gas making it's way down to the swingarm.  Also a bit on the airbox cover.  This might have been happening often as I usually fill up, and then rip down the highway for a 58 mile commute and didn't notice.  This time, i filled up and ran errands around down.  Gas cap is not leaking.  I do not use the fuel gauge as it's broken in the tank. it seemed that after 5-8miles, it wasn't leaking as much.

Curious where to look and what to fix... or don't fill up as much in the future? Gas is too expensive to lose on the road!

-JJ

Title: Re: K100 Gas on swingarm and airbox after fill up
Post by: Edward on February 25, 2026, 07:54:24 PM

Pressure regulator is shot. On the early K its on top of the engine next to the airbox. Should be the same on your k100 I would think. I've had the entire top of my engine holding a pool of gas several times.

https://www.euromotoelectrics.com/product-p/fpreg-451e.htm?srsltid=AfmBOopec_GO6gIjkmp1Ur6vq-VPaCP0CykWNcyVx6Fnpp4OfEUNHBP6

Title: Re: K100 Gas on swingarm and airbox after fill up
Post by: Laitch on February 26, 2026, 10:14:43 AM

There are two metal pipes exiting the floor of the fuel tank right-side end. The forward of the two pipes drains water from around the fuel tank cap so it doesn't enter the tank when the cap is opened. The rear pipe allows fuel vapor and excess fuel to leave the tank. If the flapper valve assembly has been removed from the tank and the tank has been filled to its ceiling, fuel can sometimes enter the vapor chamber and exit from that rear port until the fuel level lowers sufficiently.

If the fuel return line attached to the fuel regulator has degraded, it could be leaking fuel also. That would likely be a continual rather than an intermittent leak. Inspect the hose and its attachment. If your Brick has a thermistor fuel level sender coupled to the left side floor of the tank, fuel can leak from that. Fuel could be leaking from the return line coupling on the left side of the tank. Leaks from these positions also would likely be continual rather than intermittent.

The lower seams of the fuel tank can be prone to leaking if fuel has been allowed to age in the tank during long periods of idleness. Inspect them.

Title: Re: K100 Gas on swingarm and airbox after fill up
Post by: Kaos on March 14, 2026, 12:24:11 PM

Also the rubber seal from the base of the float/electric connector might be worth giving a check.