vented gas caps question

[rico53]

The Harley Service manual states that in 1992 Harley Davidson went to a continuous venting system for its fuel tanks and therefore non vented gas caps are utilized. All of the gas caps for sale have a vented right side cap. I'm alittle confused. Should both caps be non vented on a 1992 softtail?

The continuous venting means that when the engine is running, it is always drawing in some air from the charcoal canister. The canister is plumbed to the gas tank. The canister's job is to absorb gas fumes when the bike is parked. When the engine is running it draws in air from the canister to purge the fumes from it so it is ready for the next parking. The canister draws in a small amount of it's air from the fuel tank as this all is hapening.
If you do not have the right cap vented,(it only lets air in, not out) you would draw a vacuume in your gas tank, starving the flow of fuel to the carburetor or fuel injectors.
Venting is good.

Note: the actual plumbing is a bit more complicated but the above description illustrates the need for a vented cap well enough.

[rico53]

Thanks for the reply. Sounds like you're describing California emissions but I think the same principal would apply whether it's a canister or just the vapor valve mounted to the frame. I just wasn't sure how it all worked. Whether the vapor valve vented out or in, whether the gas cap vented out or in, etc. Thanks