As I am finishing up new fuel lines, with clean, empty tank, it seems like the tank holds a slight pressure, when blowing thru the return line. I'll be using the bosch 044 fuel pump, which moves a lot of fuel, and want to make sure there is no pressure fighting the flow back to the tank. Does the stock '75 cap have any self venting property's, or does all the tank venting go thru the TINY vent port, that heads to the charcoal canister? If so, have any of you added a bigger vent, as a fuel cell would use? Thanks, Tom

The fuel tank vents to atmosphere through the charcoal canister, and only the charcoal canister. The cap is not vented.

Thanks. That is a super tiny hole it has to vent thru!

..........so how does fuel tank vent if there is no canister?

It vents into the cockpit so you can smell it.

Or down, and thru the lower pan, to the road. Still can't believe that pin hole sized vent can keep the internal pre. the same as ext. I guess it's worked this long, eh?

Now that right there is funny...and true.

Wow, soooo true!

If there is no canister plumbed, than you have to add a vent. If you do not add a vent you won’t smell gas fumes, your engine will stop running at very inopportune times and places. And don’t expect a couple of feet length of hose in a loop to work as a vent either…

Actually there is an expansion tank in the pathway...not just the charcoal filter.

.........where is that "expansion" tank?

Sits on top of the gas tank

Click to view attachment

Need one? I've got one left over from my conversion.

The gas tank vent is designed to prevent a vacuum from building up in the fuel system, and as a smog device to control gas vapors. If your tank is pressurized you have got a different problem to solve. Carbs are a low pressure fuel system; you should not have any pressure in the tank…

The gas tank vent is designed to prevent a vacuum from building up in the fuel system, and as a smog device to control gas vapors. If your tank is pressurized you have got a different problem to solve. Carbs are a low pressure fuel system; you should not have any pressure in the tank…

This was based on my blowing thru the return line, into an empty tank, thru a -6 line. I think it builds a little back pressure because the vent tube is only about 1/16th in. diameter inside the tube. I will be running high pre. injection, not carbs. I suppose the pump is pulling more volume out of the tank, than the return line is putting in when running. Comparing it to a fuel cell, with a -6 (3/8 in.) ID vent line, where there is no resistance,it seemed odd. Will see how it runs.