Fuel tank venting

[Marsh]

Does anyone know how the fuel tank venting is supposed to work?
I know about the small separation tank and the pipe to the charcoal cannister. The problem I'm having is pressure is building in the fuel tank and seems to be forcing fuel thru the gas lines into the Carbs and they are leaking from the float chambers when the car is parked. When I open the gas cap, the pressure releases with a noisy whoosh. I tried blowing air thru the separation tank line to the charcoal cannister but it appears clogged. Does this line need to be open to allow tank venting? This is on a 72 TR-6. Thanks for any information.

 

[poolboy]

Yes, that line needs to be open. When mine was blocked with gas due to overfilling the fuel tank, I opened the gas cap, disconnected the line at the Carbon Canister and using compressed air blew the gas back into the tank. Just remember to open the gas cap; you'll eventually hear air whistling into the tank once the blockage is cleared.
But I don't think it was pressure building up, rather a vacuum caused by the lowering of the fuel level, as the engine ran, and no air coming in thru the vent to replace the gasoline.
Pretty sure the whoosh you heard was air rushing in when you opened the cap.

If your float chambers are leaking it is usually either the O-ring around the bottom plug needing replacement...or the float chamber needle valve has allowed too much gas into the float chamber.

 

[TR3driver]

With the vent line totally blocked, you could get enough pressure buildup to blow the float valves open. Make sure the line is clear and the carbon canister isn't clogged, I'll bet the problem goes away.

As a quick test, you could leave the gas cap loose.

 

[trrdster2000]

If it turns out you also need a new "O", for a quick fix go to you local plumbing department (LOWES) and get a couple or pack of #13 "O" rings and you are good to go. Mine have lasted over 5 years.

Wayne