After a hot, 100 degree, one hour drive yesterday, as I parked, I heard a sound at the rear of my stock '74 that sounded the same as a hot radiator, boiling.  I found that the sound came from little bubbles coming up through the gas directly below the cap.  This continued for several minutes.  The gas temp was 120 degrees.  I'm thinking that for whatever reason this was vaporized gas backing up into the tank from either the fuel line or the return fuel line.

This car has consistently cutout for a few seconds at highway speeds and has stalled at times at slow speeds.  About everything has been replaced or rebuilt, even sealing the tank.  I thought vapor locks were a thing of the past.  Maybe not.  Any suggestions?  

Is this a big block, or small block car? The big blocks run way hotter, and the high underhood temps could cause something like this. Even small blocks can get that hot underhood. How is the exhaust, as far as, how close to the fuel lines in the rear, and at the front, on the frame?
If the bubbles are coming up directly under the cap, then the fuel is boiling up from the fuel line, either the "out" line(3/8" feed to the pump), or possibly the return line from the pump(which, actually, goes into the top of the tank, on the side).
Might need to get a better grade of fuel? This time of year, the fuel co.s are "reformulating" the gas, from the "winter" mix, to the "summer" mix. Just an idea.
I don't think I've ever heard of this problem, that I remember...Adams' Apple38824.8919791667

The only thing I can think of is exhaust heating the return line, and warming up the gas.  Either that or pressure from a malfunctioning cannister purge system, but that's unlikely.

I agree with Joel, try different fuel.  RVP or Reid Vapor Pressure is adjusted up to 16 times a year.  It is the measuer of volitility  of the fuel.  In winter fuel must be more volitile to light properly at lower temps.  In summer it must be lower RVP, or it evaporates too fast and caused ping and vapor lock.  It's a tricky balance. 

Octane booster will raise octane, but it will not change RVP.  So go drive that thing, and fill it up with newer fuel, and see what happens.

kstyer38825.4765509259

Thanks Ken and Adam for your thoughts.  The gas (from the pump!) is only about a month old.  I should mention that I installed, properly, a switch-operated Delco electric fuel pump last month for emergency use only.  The bubbles evidently are coming from the main line because the return line enters the tank on the upper right side.  Vapor line is not attached to the canister.  Carb change due this weekend. Current one is an old '68 Buick Quadajet for 400 cu. in.  Maybe a new fuel pump next.

HH  

I wonder if the vapor separator valve(on the DS/top of the tank) is clogged, creating a vaccuum on the tank while you're driving. Then, after you stop the engine, the vac. in the tank is pulling fuel back thru the main fuel line? Just another weird idea.

Adam—

 

Again, thanks for the reply.  I think I’ve found the answer.  In the ’74 Chevrolet Chassis Service Manual, under the section Engine Fuel, 6M-(page)73 is this little note: “Many pumps have valves with a bleed hole that allows vapors to bleed back to the gasoline tank.”  So that would be back through the supply line, not the return line.  I suspect this condition was exacerbated by the fact that I have a large—about half the size of a Coke can Purolator gas filter in place.  My experience with this type of filer—and even the little plastic type on lawnmowers--is that they never, as we might expect, fill completely full of gas, but retain an trapped air (vapor) pocket.  My filter being over the intake got mighty warm and I assume the trapped vapor, actually under minimal pressure, however, took a while to bleed back into the tank.

 

And I learned another thing: I mentioned in my last reply that I had installed a Delco (external) electric fuel pump at the tank.  I find that it will not pump through the fuel pump with the engine stopped.  Evidently, it cannot overcome the valves in the mechanical fuel pump.  I’m a fair mechanic, but this car has taught me sooo many lessons!!!

 

HH 

Well, then.....that's good news, eh?!

I think you will find using just one fuel pump would work better.  There is no need to use an electric, then the mechanical.  Try one or the other, and it may help overall.

I think I've solved the bubbling gas tank business I have experienced.  On installing a new fuel pump I discovered that the gasoline was routed within an inch of the end of the exhaust manifold rather than being routed around the front of the cylinder head.  It was a non-stock gas line and the bulk of the A/C compressor and brackets kept it mostly hidden.  Being that close to that high heat, I figure that caused the gas--especially when there is no flow--to boil thus the bubbling in the tank.  That is also probably the reason for my frequent stalling (vapor lock) that I've experienced.

Thanks for the comments, folks.  This is a great site!

Let's hope that solves the problem!! Good job, Harv!!