Vapor lock!

Well back in the day mama had a 56 chevy wagon and on a trip to the Smokey mtns. , she started running ruff .
After a good climb she quit and we coasted down hill to a pulling off spot with picknic tables.
My father took the breather off and climbed on top if the engine and said its vapor locked.
I had no idea what that was but hoped we might have to spend the nite.
While they prepared our lunch dad and I walked down to a store and he got a big bag of ice.
He piled that ice on the intake and my first thought was that will crack something.
After we ate and rested all the ice melted , she started up and ran fine all the way home , as traffic had cleared and you could get up some speed .
Later in life I saw it many times at the drag strip.
 
I have a question. Are the bowls not vented to atmosphere in order for the gravity feed to work? Head pressure is all great until it cannot displace what is in the bowl, such a high pressure vaporized fuel? I cannot see how this would work without a vent to atmosphere, and most carburettors I have seen, have a vent some where in order to push fuel through the various circuits and jets. (Vacuum asissting) Some folks are saying the cracking the line to the carb will relieve this problem.
Is it beacause of a full, or partial oclusion of the vent port?
Same with the tank. If air cannot enter, it will not let the fuel out.
Just asking, as I have never experienced "Vapor lock" and wouldn't know it if I saw it.
 
The one other thing that intrigues me is these folks who pull the choke out to cure the 'vapor locking' problem. In your opinion, what is happening during that little exercise that corrects the problem?


I have studied the operation of a Zenith carburetor and personally I cannot understand how this would help. The vacuum that is being created through the venturi will pull harder on the jet however as soon as the bowl is dry of fuel, the extra burst of vacuum is for naught as it cannot pull a vacuum up into the bowl and then into the float needle & seat, and then into the fuel line to get the flow flowing again. So "what" does the choke do to help?

Again, my thoughts are, if there is fuel in the bowl it is indeed possible it will draw more fuel through the jet and richen up the mixture, HOWEVER if fuel is present and being replenished, then that can't be "vapor lock". If there is vapor in the bowl, pulling the choke will not richen the mixture of vapor. Again, I have stated this over & over here but I believe it is fuel percolation in the bowl that boils the fuel and when it evaporates, the fuel dries leaving a sticky residue that tends to clog passages and create a sticky float valve which is causing the problems. Once fuel is able to flow past those objects, the cleansing agents in the gasoline will remove the deposits and things return to normal. IMO, "vapor lock" and percolation are not the same.
 
I have one car that exibits the problem and one that does not. Try the following solutions to verify the problem. If you do not have an indented firewall cool the gas by keeping the sediment bowl in ice. A can in an insulated holder works short term. Permanent solution is fuel pressure. My problem car has a fuel pump and turning it on makes the spiting and back firing stop almost instantly. Many claim the problem is electrical but if fuel pressure cures the electrical problem so be it.

Prior to the gas shortages in the late 70's this car had no problems at temperatures well over 100 deg. As the volatility of gas has increased to get more yield from a gallon of oil and to compensate for the octane loss without lead the problem increases.
 
I think the two terms are used interchangeably .. I would think that fuel perk is generically called vapor lock.. Not to be a PITA but I guess I've thought that perk caused vapor lock,, but,, I haven't thought about it much..
 
Farrell In Vancouver said:
I have a question. Are the bowls not vented to atmosphere in order for the gravity feed to work? Head pressure is all great until it cannot displace what is in the bowl, such a high pressure vaporized fuel? I cannot see how this would work without a vent to atmosphere, and most carburettors I have seen, have a vent some where in order to push fuel through the various circuits and jets. (Vacuum asissting) Some folks are saying the cracking the line to the carb will relieve this problem.
Is it beacause of a full, or partial oclusion of the vent port?
Same with the tank. If air cannot enter, it will not let the fuel out.
Just asking, as I have never experienced "Vapor lock" and wouldn't know it if I saw it.
Click to expand...

Yes, the bowls are vented to the atmosphere. The handiest picture I have is of a B carb showing where the vent was plugged to adapt it to an air filter.

ModelBcarbs015.jpg


I understand your question and have no answer. I agree that it doesn't make sense that cracking the feed line to a vented chamber could work--but I believe the people who say that it does. Brent's idea that percolation (why not call it vaporization?) in the bowl could so quickly gum up the float valve sounds far fetched to me, but I've got nothing better.

I have seen vapor lock in a Model A with my own eyes. I watched, repeatedly, as clear plastic inline filter filled up with vapor and stalled the motor. I've posted photos (seen in a link provided earlier) of vaporization occuring in the sediment bowl--amazingly, large bubbles of vapor could be seen to pass through the system with barely a hiccup. So, there is absolutely no doubt (in my mind) that fuel vaporization can occur in the A fuel path, and that it may or may not kill the motor.

Steve
 
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I have seen vapor lock in a Model A with my own eyes. I watched, repeatedly, as clear plastic inline filter filled up with vapor and stalled the motor
Click to expand...
.

Why did the vapor stop at the filter? Why didn't the vapor pass through the fuel line and vent through the gas tank?
 
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Steve & Gary, ...using your scenario of vaporization in the fuel line, please explain exactly what is happening in your view? I understand the air pocket in-between two liquids, but what is happening when the lower level of fuel in the fuel line depletes? A Model-A carburetor will still perform with a bowl half-full, and as soon as the float drops pulling the needle down, you really shouldn't have any more backpressure to hold the "air bubble" (vapor) in the line. (Remember the bowl is vented where it cannot hold pressure.) So again, would somone please explain in minute details exactly what is happening from the beginning until the end as to why this is called 'vapor lock'?

As far as not believing that percolated fuel can be sticky, all I can say is try it like I did several years ago. Pour a little gas in a bowl off of a Zenith carburetor and warm the bowl. I used a very small flame but you do whatever you feel comfortable with. Just waving the heat under the bowl will cause the carburetor bowl to become extremely warm all over. I just got it hot enough that the fuel simmered and evaporated. It doesn't take long for the fuel to disappear ,and as it was cooling down I stuck my finger in the bowl and the floor of the bowl was sticky. Again, for those that don't subscribe to my theory, then so be it.
 
I'm with ya Brent. I could easily see this "stickyness" occuring and sticking a float valve shut. I've delt with fuel evaporating and cooking out of carbs on old motorcycles all the time. The fuel being cooked out of the bowl is not the hard part to deal with its what's left behind.
 
And for all the other naysayers, the other way to look at all of this is just to remember the motto by which many Model-A restorers live by;

If it can't be fixed with a hammer, ...then it's an electrical problem!!
LOL.gif


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I found the first sentence from the following excerpt surprising because I really doubted it. Anyway I hope this information is helpful.

From Wikipedia:
Gravity feed fuel systems are not immune to vapor lock. Much of the foregoing applies equally to a gravity feed system; if vapor forms in the fuel line, its lower density reduces the pressure developed by the weight of the fuel. This pressure is what normally moves fuel from the tank to the carburetor, so fuel supply will be disrupted until the vapor is removed, either by the remaining fuel pressure forcing it into the float bowl and out the vent or by allowing the vapor to cool and re-condense.

Vapor lock is also less common in other motor sports, such as Formula One and IndyCar racing, due to the use of fuel injection and alcohol fuels (ethanol or methanol), which have a lower vapor pressure than gasoline.
 
I also have this problem .I put this on the other thread but will also post it here . A fellow model a`r who is a second generation model a`r has a theroy that the Zenith carburator with the cast iron housing acts as a heat sink and when you stop the engine and let the engine sit for a while the heat from the carburator turns the gas to vapor and then when you are ready to go the engine will start but soon dies out . Pulling out the choke will keep the car going ,alibet not to good , until fresh gasoline gets into the carb . He changed to a carb that does not have a cast iron housing .
 
I'm not going to argue the point farther but if anyone on here didn't know anybody can edit or post on Wikipedia.
 
Milton said:
I found the first sentence from the following excerpt surprising because I really doubted it. Anyway I hope this information is helpful.

From Wikipedia:
Gravity feed fuel systems are not immune to vapor lock. Much of the foregoing applies equally to a gravity feed system; if vapor forms in the fuel line, its lower density reduces the pressure developed by the weight of the fuel. This pressure is what normally moves fuel from the tank to the carburetor, so fuel supply will be disrupted until the vapor is removed, either by the remaining fuel pressure forcing it into the float bowl and out the vent or by allowing the vapor to cool and re-condense.

Vapor lock is also less common in other motor sports, such as Formula One and IndyCar racing, due to the use of fuel injection and alcohol fuels (ethanol or methanol), which have a lower vapor pressure than gasoline.
Click to expand...
If this is true then, why does this happen in systems like Ford flathead 8's and late chevy 6's and 8's with fuel pumps? Also why does it not happen in modern cars? Wives tale because the person it's happening to has no better explanation.
 
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There has been an extensive post by G.M. on the EV8board about v/lock along with temp cks etc. Interesting. Possible solution to this problem would be to add an elec in-line f/pump on the A.
Paul in CT
 
James Rogers said:
If this is true then, why does this happen in systems like Ford flathead 8's and late chevy 6's and 8's? Also why does it not happen in modern cars? Wives tale because the person it's happening to has no better explanation.
Click to expand...
In modern injected cars, excess pressure is routed back to the tank, therefore the fuel is always flowing in a loop from & to the tank & runs somewhat cooler gas temperature.
If I ever experience this so called "vapor lock," I think I would put a nipple in the carb screen plug, run a hose into a hole drilled in an old gas cap as a way for air bubbles or vapor pressure to escape back to gas tank. Bill W.
 
I really felt this would occur.

jkeesey,
Another source, Webster's Online Dictionary, unlike Wikipedia Webster's isn't open to public editing:

"Do not assume that a gravity feed fuel system is immune to vapor lock simply because there is no fuel pump to "upset”. Much of the foregoing applies equally to a gravity feed system. Vapor lock has been the cause of many a forced landing in aircraft. That is why aviation fuel (AVGAS) is manufactured to far higher vapor pressure than automotive gasoline (petrol). In addition aircraft are far more susceptible because of their ability to change altitude rapidly. Liquids boil at lower temperatures when in lower pressure environments."

James,
Pardon the delay dinner time on the left coast.
"If this is true then, why does this happen in systems like Ford flathead 8's and late chevy 6's and 8's?"

Negative fuel pressure at the pump inlet. Low pressure pumps. Pump higher than fuel level.

"Also why does it not happen in modern cars?"

Fuel pumps are in the gas tank (cool) and have very high pressures relative to the pre 80's vehicles.
 
Next time I drive my car and I am going to let the car sit for more than 5 minutes I am going to do as one post suggested . Close the fuel valve and run all the gas out of the carburator.
 
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