Vapor lock

I have had a problem several time with my Model A. I would not call it vapor lock. It happens when I slow down or more often when I stop right after a long run at speeds over 50 mph on a hot day. Here in California we have winter and summer gas. It happened once in December when we had a very hot week, over 80 degrees. I think it was because the winter gas boils out of the carburetor easier than the summer gas. When it happens all I have to do is open up the GAV and everything is fine. Depending on how bad it is I have to open the GAV 1 to 3 turns. After a couple of minutes I can turn down the GAV. One time I was going up a long steep grade. I have to open up the GAV 1 to 1.5 turns to keep the engine running right. Once I got over the hill I stopped for gas and the carburetor was so hot a drop of water would instantly sizzle and boil off of it. I used a gallon of water to cool down the carburetor. My analysis is that the carburetor gets hot enough to boil the gas. This is most likely to happen when you pull off of the highway and come to a stop. The residual heat in the engine super heats the carburetor. Since the gas has vaporized in the carburetor not enough can get through the main jet and the mixture is too lean. By opening up the GAV there is a second jet supplying gas and the mixture is corrected. Once the carburetor has cooled the gas is no longer boiling and the GAV can be turn down.

I do not have engine splash pans. Perhaps they might help the air flow to the carburetor and prevent the boiling gas.

Bob
 
Painting the carb gloss appliance white to reflect the heat rather than the black that absorbs heat may help, but the purists with alternators and 16's would point fingers and have a stroke.

Putting a sheet metal heat reflector around the carb may help too, but the purists with heater holes chopped in their firewalls and aftermarket dizzy heat shields would point fingers and gasp.

Alcohol will help too, but only if you put it in a glass and drink it. By the time you're OK to drive again the engine will be stone cold and run just fine. The purists will wave bibles at you and lament the repeal of the 18th and the Volstead act.
 
Ok, re-reading all the post's, heat seems to be a common thread. And the reason this was not a problem in the 30's is because of the fuel blends have changed! (Mike post #40) We hear adding all sorts of concoctions to the fuel: Diesel, ATF, Marvel mystery oil ect. is the cure. But I like Brent's post #2 " Folks will only believe what seems believable to themselves." going with that I like Bob Johnson's post #41 because it is just like the experience I had but was not smart enough to figure out the solution to get home. I know that I am going to hell for this but if it happens to me again that updraft carb. is history. Ill wait for John's report in the model A times before taking the drastic step of tossing the Zenith. Forgot one other post, "each car is different in the way it has worn, how it is tuned, and how it is driven." This may explain why some drivers have never experienced this problem. Sorry don't remember the post. Bob New Bern N.C.
 
carolinamudwalker said:
Ok, re-reading all the post's, heat seems to be a common thread. And the reason this was not a problem in the 30's is because of the fuel blends have changed! (Mike post #40) We hear adding all sorts of concoctions to the fuel: Diesel, ATF, Marvel mystery oil ect. is the cure. But I like Brent's post #2 " Folks will only believe what seems believable to themselves." going with that I like Bob Johnson's post #41 because it is just like the experience I had but was not smart enough to figure out the solution to get home. I know that I am going to hell for this but if it happens to me again that updraft carb. is history. Ill wait for John's report in the model A times before taking the drastic step of tossing the Zenith. Forgot one other post, "each car is different in the way it has worn, how it is tuned, and how it is driven." This may explain why some drivers have never experienced this problem. Sorry don't remember the post. Bob New Bern N.C.
Click to expand...


Plus a lot of them do not have our gas blends and live in the heat of NC in the summer.
 
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.
 
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.
 
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!
 
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.
 
Last edited:
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.
 
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.
 
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
 
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.
 
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.
 
You must log in or register to reply here.
Back
Top