Vapor lock

Bruce Compton said:
Thanks Bluebell: I believe and understand everything you said about the fuel pump systems and vapor lock, but as there is no fuel pump in a Model A and the fuel line is basically vertical with two maybe 2-6" of almost horizontal runs, I still can't see where a section of boiled gasoline (vapor) in this line can stop the flow. Would the head pressure and weight of the fuel in the tank not push the "vapor" down and into the carb?? People are trying to convince us that the vapor or air in a vertical line can shut off the flow even if there was an open end at the bottom (which is the same thing as an open needle and seat assembly). Bruce
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I feel that a vapor bubble CAN STOP the downward flow of gas to the carb.
Example: Stumbling into my "office" in the A.M, wih an empty stomach & a GIANT mug of coffee with Hazlenut Creamer, I proceed to chug coffee to wake up my feeble brain. (??) Shortly a giant air bubble starts to come up, GOD! it hurts:eek: & seems to get hung up & takes some waiting, groaning, contortions, etc to finally "exhaust" itself in a sickening BUUUURP!:(
Now, if I had gulped MORE coffee while that BIG bubble was stuck, somewhere down here, (Pointing to my right upper chest) does anyone think that last gulp of Joe would have gone DOWN? I don't think so!! I REST MY CASE! Bill W.
 

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Bluebell said:
steve s; Bluebell's pressure increase theory sounds good if you read it fast said:
So Bruce, here we stand, back to back, against the mob, pondering the issues of life.

Steve S and others.
I have never put a spanner to a model A in my life, but I can tell you that there is something wrong with your car if a bubble of gas in your fuel line cannot make its way immediately, up into the tank.
The laws of nature / science dictate so. Try putting some fuel in a clear tube, leaving a bubble. Block both ends with your thumbs, and now tip it to vertical.
Does the bubble rise to the top?
Can you keep the bubble down?
Would you call the reason for the bubble being there "vapor lock"?
Would you trust a spirit level?
Maybe you should read my post slowly.

I have not tried to detail all facets of the operating process of a petrol pump, rather I have sought to clarify the reasoning behind the term "vapor lock" for those who do not know.
All the other airy fairy fuel blockage, blocked cap vent issues are all just patterns in the clouds.
I am here on this forum, firstly (and selfishly) to learn, and secondly (the payback) to help people with some of the stuff I have learnt from 43 years of mechanicing for a living.
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Bluebell,

I am not questioning the fact that "normally" bubbles will rise up through a liquid, or that bubble-levels work, and so on. I am merely pointing out that there are conditions under which bubbles may not rise. For example, the sink blockage case I described earlier, or if the tube is very narrow diameter, or if the tube has kinks or constriction in it, or if there is a large bubble that has to get up through a much smaller diameter tube. I suspect that the latter case is something like what people have reported happening when their carburetor fills with vapor that refuses to bubble up through the line back into the tank--cracking the connection to the fuel line to let the bubble escape solves the problem, for the moment.

I too am on this forum to learn and to help others, and I immensely appreciate the contributions from real mechanics, such as yourself.

On the other hand, I do know a good bit about the laws of nature, and they are more complicated than sometimes assumed. The present discussion is an example of a conflict well known in physical chemistry, where the laws of thermodynamics, which control what may, or "should", happen run up against the laws of kinetics, which control the rates at which things actually happen. Eyes may be glazing over at this point, but examples of the "laws of nature" being disobeyed are all around us: For example, since CO2 and H2O are of lower chemical energy than a mixture of organic matter and oxygen, most of the surface of the earth should burst into flames. Or, since a flat pile of dirt is of lower gravitational energy than a high pile, mountains should level themselves out. But, energy activation barriers get in the way (thankfully!). Similarly, a big bubble may not be able to squeeze itself down sufficiently to get through a constriction, even though some of the laws of nature say that it would be much happier, energy-wise, if it did.

Steve
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Sometimes its hard to believe something you haven't seen. Just isn't logical and can't be explained. But while parked in a gas station after just filling the tank on a very hot day and we had just driven 25 miles the gas was boiling out of the carburetor. There was a puddle under my car as big as the whole front end of the car. I thought any second it was going to go up in flames. I ran inside the gas station and got a big cup of ice and put as much ice on the carb as I could. After I got the carb cooled down we left and headed down the road. About 10 miles later while stopped at a stop sign it quit, I got out and again the gas was actually boiling out of the carburetor. When you see this happening it makes a believer out of you. We got to our destination had dinner and when we drove home later that night it was dark, temp. had dropped about 15 degrees and we had no more problems. So what do you call this boiling of the gas in the carburetor? There was no little bubble trying to go back up the fuel line, it was pouring out all over the place from the carb. I suspect that the carb. was so hot that the fuel "vaporized" as soon as it got there. Not in the fuel line.
 
Jon,

I too had a problem believing in Vapor Lock until it happened to me.

After a long drive I pulled into the gas station on a hot (100 degree) day.
After filling up the gas tank, the car wouldn't drive away without choke on or opened many turns on the GAV control.

After some driving, the car returned to normal driving, but every time I stopped at a traffic light, the car would run poor and need extra gas.

After using Marvel Mystery oil it drove O.K.

That was in Colorado....here in Minnesota it runs fine but then again, I haven't run in 100 degree weather.

Vapor Lock is real and can happen in the Model A. But, until it happens to you, I'm sure that most will dismiss it and claim it "can't happen".

Marc
 
Blueb ;

Vacuum Lock , Vapor Lock ; What ever you want to call it, "IT" will put you on the side of the road . Ignition system checks out ok ,carburator ok, gas getting to the plugs is the question. In my case it was a faulty gas cap that vented and then would not vent depending on how the hole in the gasket lined up with the hole in the cap after you tightened the cap .
 
Bluebell, you're not taking into account, the top of the fuel line(unlike an open straw) is plugged with gas. So it would be like NOT taking your finger off the top of the straw. Or perhaps partially off.
 
Chuck Sea/Tac said:
Bluebell, you're not taking into account, the top of the fuel line(unlike an open straw) is plugged with gas. So it would be like NOT taking your finger off the top of the straw. Or perhaps partially off.
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Not only that.........he's on the bottom of the world, where everything is upside down.:eek::D
 
Jon said:
Sometimes its hard to believe something you haven't seen. Just isn't logical and can't be explained. But while parked in a gas station after just filling the tank on a very hot day and we had just driven 25 miles the gas was boiling out of the carburetor. There was a puddle under my car as big as the whole front end of the car. I thought any second it was going to go up in flames. I ran inside the gas station and got a big cup of ice and put as much ice on the carb as I could. After I got the carb cooled down we left and headed down the road. About 10 miles later while stopped at a stop sign it quit, I got out and again the gas was actually boiling out of the carburetor. When you see this happening it makes a believer out of you. We got to our destination had dinner and when we drove home later that night it was dark, temp. had dropped about 15 degrees and we had no more problems. So what do you call this boiling of the gas in the carburetor? There was no little bubble trying to go back up the fuel line, it was pouring out all over the place from the carb. I suspect that the carb. was so hot that the fuel "vaporized" as soon as it got there. Not in the fuel line.
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Jon,
Was that with your new or old Zenith?
Steve
 
It wasn't the new carb. I think it was the Marvel carb. If you remember you helped me get it started the second time along side the road.
 
Jon said:
Sometimes its hard to believe something you haven't seen. Just isn't logical and can't be explained. But while parked in a gas station after just filling the tank on a very hot day and we had just driven 25 miles the gas was boiling out of the carburetor. There was a puddle under my car as big as the whole front end of the car. I thought any second it was going to go up in flames. I ran inside the gas station and got a big cup of ice and put as much ice on the carb as I could. After I got the carb cooled down we left and headed down the road. About 10 miles later while stopped at a stop sign it quit, I got out and again the gas was actually boiling out of the carburetor. When you see this happening it makes a believer out of you. We got to our destination had dinner and when we drove home later that night it was dark, temp. had dropped about 15 degrees and we had no more problems. So what do you call this boiling of the gas in the carburetor? There was no little bubble trying to go back up the fuel line, it was pouring out all over the place from the carb. I suspect that the carb. was so hot that the fuel "vaporized" as soon as it got there. Not in the fuel line.
Click to expand...

John,
Do you (or does anyone) have a theory of how vaporization can lead to a puddle? I'm struggling with this one. I think it's clear that a puddle the size of one you describe can only result from a stuck float valve, although the Marvel I believe has a largish capacity float bowl. One might think that pressure from vaporization in the float bowl could push liquid up and out of the jets, but the Zenith float bowl is vented to the atmosphere--I assume the Marvel too. I'm trying to visualize how a vapor bubble could hold the valve open, and I'm not seeing it.

Steve

Steve
 
Years ago when working on old Datsuns with a large glass window in the float chamber, I was surprised at how fast the gas boiled when I turned off a warm engine. This was BEFORE the "crap" gas, even. There's probably lots of boiling stuff going on that we can't see or explain.
"Maybe" the MMO helps by raising the boiling point???
Gut feeling tells me that some boiling is in the carb chamber where the long skinny filter resides???
My cars have iron sediment bowls, so I don't know what goes on inside with vapor bubbles,etc.
When looking at Chuck's glass sediment bowl, (which has a small plastic fish in it, "swimming" just below the surface), I pondered, the gas comes in through the center of the hole in the strainer & into the bottom of the bowl. Question is, how does the gas get back up through the strainer & up past the air space & up to the top, forward outlet & on down to the carb???????? Maybe Mike/K. can susplain it.
OH! Chuck's fish is a "bottom feeder" feeder & eats the rust/scale. "Maybe" I should interrogate the fish for an EXPLANATION??? bILL w.
 
Either the people that experience this problem are cursed of those that do not are blessed. You can have two cars driving on the same tour buying the gas at the same stations and some will have the problem some won't. I wish there was a common denominator. I live in South Florida, 90 plus degree days, stop and go traffic and corn gas, I'm blessed with no problems.

I'm interested to know if those that swear that "it" cannot happen in a gravity fed system ever experienced the symptoms stated here and if those that are cursed with the problem in the past ever swore the problem could only happen with a fuel pump?
 
Steve, What does a model A have at the tank end of its fuel line?
Is there a a soldered nipple (weatherhead type fitting)
Is there a tap?
Something must be stopping that bubble from returning up to the tank. It could be a blob of solder inside the fitting, dirt or rust scale.
If there is a tap, is it the original fitting?
Does it stop the bubble climbing, and make it take a down hill path at any stage?
Did Henry originally use steel tube, and has someone replaced it with copper pipe? (makes a big difference!)
As any one who has chased a fuel leak knows, petrol has a very low surface tension. The bubble only runs back up to the tank, because it is displaced by petrol running down to fill it.
If the heat is so great that the fuel cannot approach the carb, (without being vaporized) and the fuel is being boiled in the float chamber, then you have a problem that needs to be addressed from another angle.

In Jons case, Was the needle valve in good condition? Was the heat in the carb enough to swell the sides of the float (filled with air) and make it stick down?

Some of you guys will only experience these problems on rare occasions. Because of this most of you will just wait it out, and not actually fix the cause. (if your happy that's great!)
The thing is, that the fuel IS different from when Henry built his cars, and even he didn't get everything right. He like most of the others that stayed in the game built cars to a price.
If he had a reccuring problem, he arrived at a modification.
He is not around to fix a modern fuel issue in an old car. that is up to the owner.
In the sixtys and seventies a lot of cars had a fibre insulator under the carb to put a heat barrier between the carb and the manifold.
Even a war time Jeep (Nearly half built by Ford) had an insulator (incorporating an atomiser) under the carb.
Maybe you guys who have a regular problem, and don't do shows, might consider trying this. (if you car didn't originally have one)
Here is a quote I ran across years ago, and from a mechanics perspective, I draw on it.
You might find it useful; (Steve, read it slowly:D)
"The greatest cause of non observation, is preconceived opinion".
 
I believe my problem is most severe. Even on a day that is in the upper 70s if i shut my A off after it is thoroughly heated up, it will not start. i can hold the choke out and crank like crazy, not only will it not start, but not a drop comes out the carb. This is a car that cold starts and runs perfectly until it's shut off.

Here's what I've tried all at the same time:

Swapped out the Zenith for a known good tilly
Insulated the gas line, sediment bulb and carb.
Added a double dose of MMO to the tank
Installed an insulator between the carb and manifold
Verified that there is good spark when it won't start

After 30 - 40 minutes, it starts and runs like nothing happened

One other thing I'm thinking of trying is to mill about .060" off the intake manifold where it's bolted the the exhaust manifold to gain heat. We don't need no stink'n extra heat in the summer.
Any additional ideas are welcomed.....
 
I have no real need to comment here other than to thank Tom W. and others for explaining the carb boiling phenomenon, which I was afraid to post for fear of getting flack for it. It gets a little hotter here than it does in Wisconsin, but the elevation all over hilly Misssouri varies (guestimate) from around 500 to maybe 2500 ft. or so, and my favorite pastime is exploring the most hilly and least hospitable areas of it in my A. The fuel boiling in the carb has caused me to rebuild my whole ign. syst. again and take extra care with the timing to make sure the ign is not contributing to high exhaust manifold temp which would aggravate the condition. The boiling carb problem probably behaves a little differently with each car that experiences it. Mine ( '30 CCPU with 7:1 CR, hotter cam, etc and B manifold and carb) behaves very similar to what Tom explained. This hasn't happened to me with ethanol free fuel, BTW. Here are some things that I think about relating to it. I'll stop at a stop light or have to drive slowly through a town on a hot day and then it just won't run above an idle, requiring the GAV to be opened two or more full turns in order to get up to speed. Then once I'm on the open road again, the volume of air going through the carb starts to cool the carb and I can start winding the GAV back down to, in my case, 1/2 turn open. This is OK when the road is level and a road speed of 50 mph can be maintained. But my favorite haunts are hilly and when the throttle butterfly shuts going down hill, the carb can't cool and I have to open the GAV back up to get it to run something like smooth going up. Since the engine needs more volume of FA going up hill, the carb cools and it starts to run almost normally at the top, if it's a long, gentle hill, but then it's time to shut the throttle down again, and besides, the uphill run has now raised the exh manif temp, which will find its way to the carb. Also, the less-than-ideal FA mixture also raises the exh temp. On my way back from camping and kayaking last week end I dealt with the situation for about 250 miles, not having access to unleaded. I got good at guessing ahead of time where to set the mixture, once I had diagnosed the cause of the symptoms. Does MMO really help, and how much per tank?
 
Correction. Missouri's highest elevation is 1772 ft ASL, at a beautiful place called Taum Sauk Mountain, not 2500 ft. It is about 800 ft and fairly level in my area 30 miles east of Kansas City. The southern half of the state, much of it in the Ozarks, is the gorgeous part, perfect for pleasant touring.
 
So last night I went out for take-out, it was about 95 degrees with high humidity. My truck ran fine in the heat, I got to the take-out joint, parked and shut off the engine and left it for about 10 min. When I came out it would not start. I could get a "putt" out of it by pulling out the choke. Simply would not start. I read somewhere that pouring water over the body of the carb. would remedy "vapor lock". I did this, got in and it started right up and I drove home with no more problems. I noticed the Temp. on my gauge was pegged on hot while it wouldn't start, but it stayed in the green when the truck was running.

Gravity system, Zenith2 carb, glass sediment bowl with micro filter full of fuel (didn't see any bubbles), stock set up.

Vapor lock?
 
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