Fuel tank pressure

[SinNH]

Ok, second K1100, had this '93 for 3000 miles, last couple times when opening the fuel filler cap, "POOOOF" outward blowage of air/fumes. Have not had the tank off since last fall, most of the last 20/30 fuel ups no issue, just last couple "whoosh". Thinking that the hose that vents into the crankcase is plugged.
Any ideas?

Thanks     

[Laitch]

If you open the fuel tank cap and look at the slope of its rear side flapper wall, you'll see a hole. Remove the filler cap assembly then drill that hole out to 8mm (5/16") and that will eliminate the whoosh. The remedy was recommend in this service bulletin, I did it and that ended the condition. It's also possible that the tank vent hose is plugged, too, especially if it has a one-way valve in it. I removed the valve then verified that the hose passage was clear before I went ahead and modified the cap according to the service bulletin instructions. I was able to reuse the assembly gasket. Your late model might just be venting into an accumulator cup hung on the frame.

[Laitch]

Many of us have capped off that port into the engine and rerouted the hose to behind the right foot peg plate or installed the accumulator cup. The one way valve in the line can fail to open and cause tank pressure to rise. It was a California emissions modification that was problematic.