they are not serviceable.
The fuel vapor canister has a blue cap on one of the large vents. this is not used.
The second largest vent Goes into the Air cleaner Base via a paper/foil tube, this provides the negative pressure in the vapor canister that pulls all the vapors out.
Then you have 2 smaller vents:
The smallest vent goes to the Fuel bowl vent on the original 4100, 4300 carburetor. this in conjunction with the larger vent pulls all the evaporating fuel out of the carburetor, it helps keep the carburetor dry from moisture condensation and reduces varnish build up. Similar to how a PCV works in the valve cover.
the slightly larger vent, goes to the Fuel tank via the Vapor Return line. this in conjunction with the larger vent pulls all the evaporating fuel and moisture out of the gas tank, and keep it dry, again like the PCV on the valve cover.
if you smell gas more then likely you have left one of the vents on the vapor canister disconnected. If you do not have the main tube going from the Vapor canister to the air intake base, then there is no negative pressure in the system when the the engine is running. in that case the vapor canister is just relieving pressure build up in the fuel tank as the fuel evaporates. or when the engine is running using fuel from the tank then it stops vacuum from forming inside the fuel tank. the vacuum that is pulled through the canister is very small. just slightly negative.
if the canister is not being used you need to leave at least one of the plugs open so the fuel tank does not implode from vacuum while the engine is running.
most people change the air cleaner base and the carburetor so they make the vapor canister less effective and since air is not pulled through it the fuel tends to varnish inside and clog the fuel vent line up. ( i had this issue my canister was disconnected and the fuel vapor return line was 100% clogged with fuel varnish and had to be replaced.)
if you shake the can and hear what sounds like BBs inside your fine, if you can blow through the 2 smaller hose attachments and there is no pressure build up then the canister is still good. if you pull off the blue cap on the larger tube and turn it over some of the carbon inside may spill out so don't do that.
that vapor canister is quite useful when its hooked up and working correctly. at the best the vapor line should be hooked up and the larger tube should go up to the intake before the Air Filter, to help pull out moisture from the fuel tank.
if not using the carburetor vent line, then you can safely plug that off.
