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)?

[bdstone914]

Did you not use the expansion tank and charcoal canister and block off the openings ?
What year gas tank is in the car ?

[sdoolin]

Gas tank is (as far as I know) original to the car. Expansion tank is in place, charcoal canister is in place (but not connected in any way to the carbs). Crankcase is vented (more or less to atmosphere), heads are not.

Fuel pump, tank sock, filter and all lines new. Stainless lines in the tunnel.

The "boiling" sound was creepy...

[jvmarino]

Sounds like exact thing that happened to me a long time ago. After a long run on the highway, there was a large vacuum on the gas tank because I had no vacuum relief on the tank. I had assumed the gas cap had this feature, but I think it only has pressure relief. Since I am running carbs, the vent and return lines were plugged off.

After my experience, I added a vent check valve on the vent line at the firewall under the engine. Have not had the problem since.

I think local driving doesn't cause the problem and maybe even a higher fuel level in the tank helps the issue, but once you open the vent line back up, there should be no problem.

Jim

[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...

[Mark Henry]

I had similar symptoms, without the vacuum issue, from a kinked fuel line under the tank.

I agree insects could also be an issue, I have a hell of a time with sand dabbers here, they will plug any small hole.
Pull the 3mm vent hose off the expansion tank and see what happens.

[Mike Bellis]

I actually crushed a tank once without a vent.

Make sure the charcoal canister does not have a plug or cap on the outlet lines. Most likely it's plugged at the tiny vent line right under the cap/expansion tank.

Like said before, blow through the lines toward the tank and see if air comes out.