Vapor lock

[CWPASADENA]

The Flathead V-8's are especially bad about vapor lock with todays "Corn Gas". We have found adding a little Diesel fuel in the gas usually solves the problem. Real gasoline from the "old days" had a higher vapor pressure than todays blended fuels. Add about a pint for every 10 Gals of gasoline. The diesel will raise the vapor pressure or boiling point of the gasoline.

I know there are some who will say that this is not good for your engine as the Diesel will also lower the octane of the gasoline but we do not have a high enough compression ratio to have a detonation problem.

The Diesel will also add a little lubricant to the fuel which is good for the engine. This amount of Diesel will not fowl the plugs or cause a model A engine to not run right.

My experience and my experience.

Chris W.

[MikeK]

Just reading this thread has given me the vapours! Living in Chicago I see what is called seasonal fuel formulation changes, or what is called winter and summer blend. Mid January the fuel has so much butane content it will boil in your hand. OK for quick sub-zero starts and no problem in injected vehicles with vapor recovery and high pressure fuel feed. Trash in carbureted anything. The 'summer blend' has a much higher vaporization point. Despite both being E10 the summer blend stuff is OK @ 100F, the winter blend stuff makes the A suffer when it hits 75F+.

My Honda lawn mower won't run on the winter blend stuff after the engine gets hot. You can take the gas cap off and watch the boiling vapor bubbles backing up from the hot carb and condensing (disappearing) into the cooler liquid fuel. Not an alcohol problem, a butane problem. Unlike propane, butane is a cat refining product with little commercial market. They try to dump as much as possible into gasoline. What is accumulated from refining during the summer, unfit for that blend, goes into the winter blend. Refining blends vary by region, refinery, and season.

Henry's 1920's - 30's gas had a significantly different vapor pressure, as did the fuel all through the carburetor era. Not too good for 0F starting, though. I'd love to join in and put the blame on ethanol, but that's not the heart of the problem. Evolving and changing fuel blends and vapor pressure specs to suit modern vehicles are the real problem IMHO.

[Tom Wesenberg]

I'm glad Mike posted about the fuel formulation because I've said for years that even when I buy the good gas without corn crap, it still isn't the good gas we had in the 60's and 70's, even if the lead is removed. Today's gas quickly evaporates, and much faster than it did years ago. It also hurts my hands, which I think is the corn crap mixture. Notice how much the plastic gas containers puff up in the heat of summer. They never used to do that.

Before changing out the carb I would add a heat shield like a couple guy have done in the past. I have some white plastic wire wrap with the split seam, and just putting that on the fuel line helps.

In the 80's many cars were made with fans that blew outside air onto the carb or fuel pump for 3 to 5 minutes after turning the engine off. This was to keep the gas from boiling, so we know that the reformulated gas does boil much easier than it did in the good old days of good gas.

[woodypecker]

People who have posted about problems were at altitude after pulling long grades. Even water boils at a lower temperature at high altitude. Ask any cook who has tried to cook in the mountains.

[captndan]

We can science this problem all we want but it comes down to basics. It takes high altitude, more than the car is used to, high ambient temp, low humidity, and fuel with a low vapor point. Add that old cars were designed for low altitude running. No need to design a fuel system that kept gas lines and pumps away from things like the exhaust manifold. So, if you and your car fall into these vapor lock areas do like the old timers did. Tin foil, cloths pins, copper wire, and cool water or ice. Happy Motoring!

[Mitch//pa]

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Here it is in a nut shell
https://en.m.wikipedia.org/wiki/Vapor_lock

[steve s]

I think the Wikipedia link that Mitch gave pretty much settles it.

A few details that some earlier posts had confused:

A liquid's vapor pressure and its boiling point vary in opposite directions. A liquid will boil when its vapor pressure matches atmospheric pressure. So, liquids with lower vapor pressures will have to be heated to higher temperature before they will boil.

Ethanol and gasoline form a highly non-ideal solution: neither the vapor pressure nor boiling point of mixtures can be predicted by simple interpolation between the values for the two pure liquids. In fact, adding small amounts of ethanol makes the lighter fractions (e.g., butane) want to evaporate even more readily, thereby lowering the initial boiling point. This fact was a major technological hurdle that the oil companies had to solve before gasohol could be sold. There was an air pollution tradeoff: ethanol gave a cleaner burn and exhaust, but the increased hydrocarbon load put into the atmosphere (due to evaporation during fueling up, etc.) created more smog.

[BILL WILLIAMSON]

I "wonder" if starting problems, after a few months' storage, "might" be caused by a BIG BELCH of WATER, accumulating in the carb bowl? This is common in small engines.
This could be overcome by pulling the carb drain plug & turn the gas on, until good gas appears.
Bill W.

[Synchro909]

We had a chemist from a fuel company give a talk at a club meeting a while ago on this topic. In brief, what he said was that fuel is a mixture of many different hydrocarbons, each with a different boiling temperature. Vapour lock happens when the fuel or at least part of it boils, producing a vapour. No surprise there! In the old days, enough fuel to meet the demand was able to be produced using a narrow range of HC's with similar boiling temps. They were selected to give satisfactory performance in normal driving conditions. As the number of cars on the road increased and demand followed, refiners had to widen the range of HC's they put in the fuel to keep up with it. Some had higher boiling temps, others, had lower ones. This pattern has continued for many years now and the range of boiling temps is very wide, some of them boiling at very low temperatures. It is these low boiling point HC's that cause the problem.
If you leave modern fuel to evaporate, you will notice that there is a dark coloured fluid left behind that looks like a light oil. Surprise, surprise - that's exactly what is. Oil is only another HC but with a higher boiling temp.
If you now reread the above posts, I'm sure most will now make sense.

[carolinamudwalker]

Well there you go people, Mitch post # 40 has the answer, we just need to all use Aviation fuel. In all seriousness Mitch"s link is very informative. If you want to keep your Zenith carb, why stop at custom heat shields and tin foil draped over the fuel lines? How about a heat exchanger tube over the fuel line,pump water thru it to a small radiator mounted on the front bumper? Of course we will need a 12v pump and fan. Or we can accept that the EPA is hell bent on making the fuel even worse in the near future and bite the bullet and install a modern downdraft carb and be done with it. I am only talking about long range touring hear not your show cars or those that stay within 100 miles from home. Ok I am going back to my closet. Bob

[G.M.]

I live in SW Florida and it's 85 plus a good part of the years. I had many
vapor lock problems with corn gas. On 100 degree days I had vapor lock
on 3 or 400 mile turn pike runs. At 65 or 70 MPH it ran good but if there
was road construction or an accident below 55 MPH VL started. Getting
back up to speed it went away. Also local driving, stopping for lunch for
1/2 the heat build up would cause VL. I did tests for several months under
the same conditions and determined after numerous "fixes" if the pump
was kept cool (fans and heat sinks) the problem went away. The problem
on the interstate proved that as long as a GOOD flow of cooler fuel from
the tank flowed through the pump it cooled it enough to prevent VL. I
installed a small fuel return line with a 'T" in the line right after the fuel
pump back into the filler pipe of the tank. There needs to be enough fuel
flow to the carb for high speed driving so you can't return to much. The
Stromberg carb has a fuel restriction hole in the float valve. It was
determined by Ford a .098 hole was ideal. A few thousandths larger
allowed fuel to push by the valve and a few thousandth less starved the
engine at high speed. This is described in the Ford Service Bulletins.
I found a restriction hole of about .100 allowed enough fuel return to cool
the pump and supplied enough fuel for at least speeds of 85 MPH. I don't
like electric pumps so this is my method. I also now run noncorn gas
because the alcohol attracts water, rusts the fuel tanks and eats the thin
brass float. This won't work on "A"s but defines the problem. G.M.

[BILL WILLIAMSON]

Some of our guys have run out of gas, with a warm engine, & the added gas WOULDN'T fill the carb. "Apparently" due to "back pressure" in the HOT fuel lines, carb, & sediment bowls. Just pour COLD water on all that stuff & IT RUNS AGAIN.
Bill W.

[mshmodela]

Boiling gas in a carburetor float bowl:

https://www.youtube.com/watch?v=yfm6DrB0eV8

[Chuck Sea/Tac]

Dodge, where are you located? Heat related: guessing your Southern Hemisphere .

[Tom Wesenberg]

Originally Posted by forever4

Why does this thread start with post # 2?
What happened to post # 1?

I see all the numbers OK on this thread, but for the past few months my computer has had a problem with the numbers on many posts. It might show 20 posts, but lists a page 2, which isn't right because page 2 should have post #21. If I click on page 2 it just takes me back to the top of page one with post #1.

I thought it was just my computer that was vapor locked.