Strange running issue, Fuel tank vent? Options

[sdoolin]

73 2.0L, built to 2056 running dual Dellortos. I have just over 3k miles on the engine since the rebuild, and it has been running strong, and I have been loving it. Went to pick up my wife from the airport last week and about 30 miles into the trip, the car started running poorly. Really poorly. Had been all freeway miles, and I was running at 80+ MPH. Car had been running fAntAstic!

Felt like it was running on 3 cylinders, and running out of gas. No strange/unusual mechanical noises before poor running, or during poor running.

Completed the next 5(ish) miles to the airport and strangely, once I slowed (exited the freeway) the engine picked back up and began to run normally again. Made the 40(ish) mile trip home, running poorly the entire way. Pulled into the garage, shut the car off and heard the STRANGEST noise from in front of me. Sounded like the gas was boiling in the tank.

Quickly opened the frunk and removed the gas cap. Large sucking sound on gas cap removal. So, I am thinking that my tank was not venting properly, not allowing proper fuel to get to the carbs? I have put another 300 miles on the car since this event - no more issues.

I am running a 3 PSI pump, with a return line from the carbs.

Anyone else ever experience this (or similar)?

[sdoolin]

This happened again. Lost power after 20 mins running at sorta high speed (65 MPH steady state). Slowed the car down, power returned. Back up to 65 MPH, loss of power. Got the car home, running OK, but same boiling noise from the tank. Remove gas cap, noise goes away.

So, I was thinking faulty gas cap, not venting. Today I ran 45 miles up on the HWY at 80 MPH without the gas cap. All good (car runs GREAT (and I just jinxed myself)). Stopped put gas cap on, made the return trip at 80 MPH - no issue.

I have not blocked off any of the vent lines in or out of the tank (to my knowledge). But I am going to give them a very close inspection.

I am stumped?

Both times I've had the issue have been just after a fill up. Maybe I am overfilling the tank, and that is affecting the gas cap vent? I don't fill the tank until it overflows, but I do get nearly 13 gallons in it when it is empty and on reserve. Both times I have had the power loss it was quite hot out. Today it is cool and overcast.

Makes no sense...

[wndsrfr]

This happened again. Lost power after 20 mins running at sorta high speed (65 MPH steady state). Slowed the car down, power returned. Back up to 65 MPH, loss of power. Got the car home, running OK, but same boiling noise from the tank. Remove gas cap, noise goes away.

So, I was thinking faulty gas cap, not venting. Today I ran 45 miles up on the HWY at 80 MPH without the gas cap. All good (car runs GREAT (and I just jinxed myself)). Stopped put gas cap on, made the return trip at 80 MPH - no issue.

I have not blocked off any of the vent lines in or out of the tank (to my knowledge). But I am going to give them a very close inspection.

I am stumped?

Both times I've had the issue have been just after a fill up. Maybe I am overfilling the tank, and that is affecting the gas cap vent? I don't fill the tank until it overflows, but I do get nearly 13 gallons in it when it is empty and on reserve. Both times I have had the power loss it was quite hot out. Today it is cool and overcast.

Makes no sense...

Blow through all the vent lines...insects can plug them solid...