Is it normal for the gas cap to whistle? If I tap it it'll stop for a while. Only happens for a short period of time after riding. I know that the gas tank develops pressure but shouldn't this vent throught the vent hose and not the gas cap? Any suggestions?
There is a pressure/vacuum relief valve in the gas cap that vents to the atmosphere ONLY when the pressure in the tank exceeds it's design parameter. If you look at the cap, itself, you will see a small "nozzle" that fits inside a rubber ring on the frame of the cap at about the 5 o'clock position. This is the external vent passage. The system, because of emission laws, must be sealed under normal operating conditions. When the fuel is used, as you ride, the valve opens under vacuum to allow air inside to prevent vacuum locking of the fuel supply (not as big an issue with an onboard pump, but still an issue). When you park the bike, engine heat will cause the fuel in the tank to expand, (called hot soak) and when the air space is pressurized enough, it will vent back out to prevent the tank from being damaged. This is the same kind of system that OBD 1 cars have. The more fuel you have in the tank, the more the pressure can rise due to internal vaporization. I had California emissions equipment on my 2000 W-650, and when I removed the ugly canister and other assorted plumbing, I had to remove the internals of the valve completely, so there was an atmosphere vent, or the tank would vacuum lock. The wonders of modern emissions......
Found a picture of what I was trying to explain. :o
Yes its normal... If you open the cap it will relieve the pressure. I can't hear mine when I'm riding, so it doesn't bother me much. Some folks have modified the cap to deal with this, but it can cause another problem and isn't worth the effort IMHO.