The Stovebolt Site Forums The Shop Area General Truck Talk what were they thinking!

If I'm not mistaken, the gas cap on 1930-70s chevy trucks is a one way vent.
Allows air in but not out.

So if the system is properly sealed then hot gas will build vapor pressure in a parked truck.

With enough pressure this could force gas past the float bowl valve and cause flooding on startup.
Does anyone notice this problem?
Or is there a positive vent somewhere? (there doesn't appear to be any on my '59)

Also, gas tanks aren't designed to withstand much positive pressure... seems like a giant design flaw unless I'm missing something?

That sounds similar to a modern cap that can burp above ~3 psi... I'll have to see if I can make my cap do that too.

The earlier earlier Shop Manuals do state about venting, but the 1948-1952 Shop Manual shows a filler-neck vent-tube. This releases air from the tank when filling with gas (preventing "blow back").

My 1954 trucks never had/have venting problems in the summer.

I do not remember if my 1948 Panel Truck built-up air pressure in the hot summer?

Not sure about all those years you included but, in general, tanks are vented both ways in our old trucks. In and out. By the cap or other means. There are vented caps and there are vented caps. Some are one way. Some are both ways. You can get fooled by the words vented cap. Find how a particular tank is vented, both ways, and then get the proper cap depending on year. Some 70's cars have charcoal filters and one way caps........... and Government mandated emission control from there on. The older trucks vent to the atmosphere. My 2011 F150 4X4 has no gas cap.

Good info guys, thanks.
I was indeed fooled by the words on the cap... it says "not vented" and I saw the same gas cap listed for that year range in the stant catalog

It seems when they say "not vented" they're distinguishing between an open vent and a low pressure relief valve.

I hooked the cap up to the air compressor just now and verified that it does vent, it just takes a few PSI before it opens.

Thanks for doing the testing, and, for your report on the results.

The original cap on my '36 GMC is a vented style of cap. Air flow in and out.

Very good advice all the way. When you get vapor lock an old fashion fix is to put spring type clothes pin's on the fuel line ( pinch end up ), this will relieve a vapor lock until you can install a shield. Hey folks I thought I was the only one with this problem.

Hey Ott3r can't wait for the results.

Has anyone here experienced vapor lock on a Stovebolt engine with normal fuel line routing?

Driving highway 550 last summer ~90 degree day I couldn't run her wide open without repeated engine dying/surging... an anxious hour later I stopped at a gas station and saw little gas bubbles simmering in the filter bowl by the carb... I imagine it was worse in the carb. Driving up hills was really bad... limping along the interstate shoulder at 30mph playing with the choke to keep her from dying.
Once I got to the mountains and drove up into cooler air ~70degrees the problem disappeared.

That was with the stock fuel system on my 59 burb.

I'm adding a return line to see if it helps.

I have had vapor lock in my stock '51 Chevy half ton one time. I was driving to Kansas City and stopped in Eastern AZ in Springerville for lunch. After lunch, the engine stumbled and stalled a bit, so I pulled over and raised the hood. The glass bowl of the fuel pump was boiling like a tea kettle. The temperature was just above 90 degrees and just as important I was at 7000 feet elevation. Once I got going down the highway again I had no further problems. Within an hour or so I crossed the continental divide at 7800 feet with no problem, but I didn't stop.

I have driven the truck in temps as warm as 116 without a problem but that is at my home in Mesa, elevation 1100 feet.

Kent

Thanks for the report, Kent

How close were/are the fuel lines to the engine/manifolds/exhaust system?

Tim,
Everything is in the stock position. The fuel pump bowl, of course, is on the opposite side of the manifolds and exhaust. I think the under hood temp got too hot after I stopped for 15 minutes and the fuel in the glass bowl was boiling away. I had no way of knowing what was happening in the line to the carb or carb itself. Once I got moving again everything was fine.

Kent

My aftermarket vented cap has two springs. One to let air in, one to vent


...click...

Oh, after years of working on Model "B" carbs, the metering jets do make a difference in higher elevation not necessarily for extreme outside temperature variation (s). Depending on the amount of vacuum your 216 thru 261 draws can at time affect vapor lock. For example a #58 is the only one used by GM for both A/T and standard shift 1954's. However 53, 235's and 216's down 53,52,51 etc. are all over the place. 51,52, 56,57 are common metering jets pending location of original manufacture. Lots more if any of you are having wacky carb problems. Also always remember if you have rebuilt a model "B" and tried to clean that weird white stuff off with carb dip or cleaner spray it will not take it off. Get a lemon and squeeze on juice, it will very soon be gone.

Did the fuel have ethanol in it?

Nice video from DADS50! At the end you removed a spring... I take it you modified so that air inlet valve will hang open?

Mick53: ethanol for sure! I think pump gas around here comes with 5-10% ethanol... something not anticipated by the original designers.
I've heard that aluminum tanks actually corrode if ethanol content is greater than 10%.
www.nmma.org/assets/cabinets/Cabinet103/E20_Position_Paper.doc

Here is another trick I was taught by an old mechanic when ethanol first came on the market in the 70's (which by the way was available but with lead in the 20's). Getting back to the story, put one oz. of chain saw mix oil per ten gallons of gas, it helps cushion the valves and raises the octane.

DADS50, I have the original GM optional item that has a lock. I put it up in a clear box on the bookshelf, where are the springs on my new locking cap?

Thats what I did for the issues I was experiencing,
I never had a problem until a few years ago with the new vented caps.On another forum a guy speculated that a few years ago EPA required stronger springs. I dont know if thats true but what I found is that the new vented caps do not vent properly for my truck

Some suggest drilling a hole in the cap. Im not making any recommendations


If you have an original nos cap you should probably use it. I believe the NOS caps work. If I had one I would use it.
The cap In that video is from autozone part CST 6023