I replaced the seal under the fuel cap a few years ago, and I recently realized that the fuel tank is forming an airtight vacuum as it drains. In fact I recently burned out and replaced a fuel pump, and in hindsight, I realized it probably burned out due to pulling against vacuum pressure in the tank.

When I remove the gas cap after a drive, there is a very noticeable suction.

My question is: What's the proper routing for charcoal canister and fuel tank ventilation? I'm running carbs on a 2056cc engine. Photo here of the canister... the other end of the hoses seem to be loose/uncapped in the engine bay but could be kinked/blocked somewhere.

Click to view attachment

The expansion tank should allow the pressure to equalize. Is something capped off? Or more than likely restricted / pinched. If it had no venting you would eventually just get to the point the pump would have cavitation.

The other end of the hoses in the engine compartment have to be hooked up. The one that connects to the side of the charcoal canister that has the small tank connection should hook to the fan shroud. the other one needs to hook to one of your air filters.

The idea is that the fan pushes air through the canister, and that sucks up the gas fumes from the tank, and carries them back to the intake so they can be burned in the engine.

Could be it.
You are running a return line back to the tank, right?

Thanks, super helpful. I think the entire tank + expansion tank is acting as a closed system, does that make sense?

Here's where the hoses lay in the engine bay. One is doubled-over and (maybe) kinked, the other is against the firewall.
Click to view attachment

Is this the attachment point for the one that goes to the fan shroud?
Click to view attachment

Is there an easy way to tell which is which from the firewall?

Thanks.

I think I'm not... would that be normal with Carbs? (It was converted to carb by a PO)

Definitely hook up the vapor lines like Clay says but I think your problem is more likely here. That tube runs from the filler neck just below the fuel cap to the expansion tank. If that is clogged or kinked, the tank won't be able to equilibrate pressure with the expansion tank and atmosphere.

Click to view attachment

[quote name='flipb' date='May 15 2020, 01:59 PM' post='2815512']

[/quote]

Thanks, super helpful. I think the entire tank + expansion tank is acting as a closed system, does that make sense?

Here's where the hoses lay in the engine bay. One is doubled-over and (maybe) kinked, the other is against the firewall.
Click to view attachment

Is this the attachment point for the one that goes to the fan shroud?
Click to view attachment

Is there an easy way to tell which is which from the firewall?

Thanks.
[/quote]


That is the fan output. Disconnect the hose on the end of the canister with the small connection. Put a small bit of rag in the hoses in the engine compartment. blow through the line you disconnected on the canister end. In the engine compartment, find the line with the rag blown out and hook it to the fan outlet.

Hook the other to the left air cleaner. so it vents inside the rain shield. You don't want to vent it directly inside the filter, the air from the charcoal canister can contain bits of charcoal in it.

Clay