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Join Date: Nov 2007
Location: Chandler, Arizona
Whistling from the gas cap, also known as a hissing sound, means lack of proper ventilation. First try riding it with the cap unlocked. If it works ok, then it was a venting problem. If you have a CA model with 2 hoses connected to the tank, right in front of the seat, the solution is simple. Remove both hoses, get more hoses of the same size, and run both of them from the gas tank down below the bike. Leave the ends open.
If it is a non CA bike, with only one hose on the tank, first take the gas cap apart and remove anything you find in there. There is a valve in most of them that won't let them vent properly. Now, at the very back of the filler opening is a rubber grommet. This is part of the venting system, and it is very common for it to swell shut due to use of alcohol gas, closing off the vent. Just remove it. It pulls right out. Make sure the vent fitting on the cap that goes into that rubber grommet is not plugged. You can replace the grommet, but if you are using alcohol gas, it will get destroyed again fairly quickly.
While some disagree with this, IMO, there should NEVER, EVER be either positive pressure of a vacuum in a gas tank. It should always be atmospheric pressure, and the vent should be sufficient to release any pressure or vacuum instantly. This is just common sense, same as there should always be at least a slight vacuum in the crankcase when the engine is running. Oddly enough, one of the first emissions control devices they put on cars, back in the early '60s, was a PCV system (positive crankcase ventilation). This actually used engine vacuum to suck air and whatever might be in it out of the crankcase. It worked better at keeping the crankcase clean than it did as an emissions device. For some reason, motorcycles have not yet adopted this method of venting the crankcase. Probably for the same reasons they still have tube type tires and don't have a real charging system ('88 and up Goldwings excepted)
I am a motorcyclist, NOT a biker.
1997 Vulcan 750, purchased about a week ago
2006 Sportster 1200 Low
2013 Royal Enfield Bullet 500, converted to carb
2001 Yamaha XT225, heavily modified
2004 Honda Rebel 250
1979 Vespa P200E
2002 Vulcan 750 parts bike
1994 Yamaha XT225 parts bike