fuel vapor lock issue

labjack

Member
Hello all, I have a 63 4000 4cyl gasser that seems to be vapor locking. From a cold start it will run 30-45 mins. and then stall out(rather abruptly). If I let it set a bit it will run again but for a shorter time due to warmer starting temp I assume. When it stalls if I open the carb bowl very little fuel comes out. If I then open the sed bowl I get good flow from the tank. I assume this breaks the vapor lock? So, would it help if I routed the fuel line around the front of the engine instead of between the valve cover and the manifold? Note: this past winter I changed the fuel shutoff from the lever type to the screw type which uses a slightly smaller gas line. Would that have any bearing on this problem?
Thanks, Mark
 
(quoted from post at 18:48:25 05/31/19) Hello all, I have a 63 4000 4cyl gasser that seems to be vapor locking. From a cold start it will run 30-45 mins. and then stall out(rather abruptly). If I let it set a bit it will run again but for a shorter time due to warmer starting temp I assume. When it stalls if I open the carb bowl very little fuel comes out. If I then open the sed bowl I get good flow from the tank. I assume this breaks the vapor lock? So, would it help if I routed the fuel line around the front of the engine instead of between the valve cover and the manifold? Note: this past winter I changed the fuel shutoff from the lever type to the screw type which uses a slightly smaller gas line. Would that have any bearing on this problem?
Thanks, Mark
would be looking into the carb itself, not the line, based on your description.
 

Forget the idea of vapor lock, gravity flow fuel systems cannot vapor lock. Since you have little at the float bowl but plenty at the filter bowl it sounds like your problem is between. Have you cleaned the screen at the top of the sediment bowl? Also some carbs have an additional screen in the inlet fitting after the sediment bowl. Next time you use it prepare yourself to check for spark immediately after it stops. I ran a 172, the same as yours, for many years under heavy load at full throttle on a 1/4 line, running straight down between the block and the manifold.
 
I used to have a 64 4cyl gas that did the same thing when they started changing the formula for gasoline. I made a new line out o copper tubing and re-routed it from going thru the manifold. As far as no vapor lock on a gravity flow system, when the tractor would die I could take the fuel cap off and watch the bubbles come up thru the gas in the tank from the gas boiling inside the fuel line.
 

I had a problem very similar to what you describe with my 8N. I tried various things over two summers, the problem only occurred in hot weather. I finally traced it to a faulty ignition switch. The switch would go open after a period of time and then operate again after it cooled a while.

If the fuel line is routed as from the factory it should not need to be rerouted. After all it did work for many years. To verify fuel flow: There should be a drain plug in the bottom of the carb fuel bowl. Remove the plug and install a fitting. To the fitting attach approximately 8" of clear plastic tubing. Route the tubing up along side the carb, leave the top end open. Note, the open top of the tubing must be above the carb to prevent fuel from over flowing. Now turn on the fuel and the fuel level in the plastic tubing should rise to the exact level of the fuel inside the carb bowl. When the tractor stalls the tubing will show the level of the fuel present in the carb bowl.

Vapor Lock, is a condition where the carb is located higher than the fuel tank. A pump is then used to lift the fuel from the tank to the carb inlet. The inlet side of the pump must lower the pressure in the fuel line, to cause atmospheric pressure in the fuel tank to push liquid fuel into the pump. At elevated temperatures the fuel is prone to flash to vapor. In this case the pump only receives vapor, instead of liquid fuel. It is said to be "vapor locked".

Gravity Flow Systems, have the carb located lower than the fuel tank and do not use a pump. Since there is no lowering of the pressure it is typically said that vapor lock can not occur in these systems.
 
Well, if there is no vapor lock then I agree it sounds like there must be some sort of blockage in the fuel system. I guess I will have to check everything from sed bowl to the carb and clean the carb. Only thing I don't get is it always seems to run longer when cold than when it has just cooled off a little. That sounds more electrical but on one occasion after the stall I checked for spark and it seemed fine. I've checked the bowl many times after the stall and it is always lacking flow. Might have to check the spark again, too.
Mark
 
So, did the rerouting solve the problem? Can this system effectively vapor lock or not? I'm getting conflicting info.
 
There is no need to reroute the fuel line. OEM routing works fine.

Make sure that the heat shield is in place and avoid gasoline with ethanol if possible.

Verify that the cap is vented to avoid vacuum lock.

Dean
 
Yep, about "most" of the posters use the term vapor lock without a clue as to the meaning. Simple test would be, if it doesn't have a fuel pump then it isn't vapor lock. If it does have a fuel pump, the it "might" be vapor lock. Somebody needs to carve that in stone. :!:
 
Heat shield is in place and cap is definitely vented. It has always ran on gas with ethanol before but I can try it with non ethanol fuel. It's getting hard to find though.
 
(quoted from post at 21:07:25 05/31/19) I used to have a 64 4cyl gas that did the same thing when they started changing the formula for gasoline. I made a new line out o copper tubing and re-routed it from going thru the manifold. As far as no vapor lock on a gravity flow system, when the tractor would die I could take the fuel cap off and watch the bubbles come up thru the gas in the tank from the gas boiling inside the fuel line.

Bubbles from gas boiling in the line is not vapor lock. Vapor lock is an air or gaseous bubble in the line that fuel cannot get past and which keeps a fuel pump from being able to pump fuel. Since this is a gravity fed system with no pump, there can be no vapor lock.

With that being said, if the fuel is boiling in the line and producing enough gasoline vapor bubbles, the bubbles traveling upwards can restrict the amount of fuel than can travel downwards and starve the engine of fuel, but the cause of the starvation is insufficient flow due to the large amount of vapor bubbles traveling upwards, which is not vapor lock.
 
Ok, so it's a matter of semantics then. If that is indeed the problem what is the fix? Original heat shield is in place. Plus I have the fuel line wrapped with a piece of larger radiator type hose. Short of rerouting I don't know what else to do. This has happened in relatively cool weather also with the temp gauge being maybe a little over halfway up to the red zone.
 
(quoted from post at 00:02:32 06/01/19)
(quoted from post at 21:07:25 05/31/19) I used to have a 64 4cyl gas that did the same thing when they started changing the formula for gasoline. I made a new line out o copper tubing and re-routed it from going thru the manifold. As far as no vapor lock on a gravity flow system, when the tractor would die I could take the fuel cap off and watch the bubbles come up thru the gas in the tank from the gas boiling inside the fuel line.

Bubbles from gas boiling in the line is not vapor lock. Vapor lock is an air or gaseous bubble in the line that fuel cannot get past and which keeps a fuel pump from being able to pump fuel. Since this is a gravity fed system with no pump, there can be no vapor lock.

With that being said, if the fuel is boiling in the line and producing enough gasoline vapor bubbles, the bubbles traveling upwards can restrict the amount of fuel than can travel downwards and starve the engine of fuel, but the cause of the starvation is insufficient flow due to the large amount of vapor bubbles traveling upwards, which is not vapor lock.

Ok so not Vapor lock but fuel starvation, re-routing from factory orignal to not thru the manifold did solve my problem.
 
(quoted from post at 21:54:50 05/31/19) Ok, so it's a matter of semantics then. If that is indeed the problem what is the fix? Original heat shield is in place. Plus I have the fuel line wrapped with a piece of larger radiator type hose. Short of rerouting I don't know what else to do. This has happened in relatively cool weather also with the temp gauge being maybe a little over halfway up to the red zone.

Labjack, just go back through and read the posts again. You have been told what you need to do.
 
(quoted from post at 06:55:34 06/01/19)
(quoted from post at 21:54:50 05/31/19) Ok, so it's a matter of semantics then. If that is indeed the problem what is the fix? Original heat shield is in place. Plus I have the fuel line wrapped with a piece of larger radiator type hose. Short of rerouting I don't know what else to do. This has happened in relatively cool weather also with the temp gauge being maybe a little over halfway up to the red zone.

Labjack, just go back through and read the posts again. You have been told what you need to do.
guess we are just in the jaw stage at this point, as he said he was going to clean the carb but hasn't and he said it even has done this in cool weather, but is still stuck on re-routing.
 
And, it is not "semantics"! A fuel pump pumps liquid. It can not suck vapor on its inlet and it can not compress vapor on its outlet, hence, the "vapor locked"[b:2e480c5f9f] fuel pump.[/b:2e480c5f9f]
 
Have you verified ignition timing and proper functioning of the centrifugal advance?

Retarded ignition timing, which can be caused by incorrect static timing or a non functional mechanical advance mechanism, will cause exhaust temperature to be much higher than normal.

Thanks to the EPA, todays gasoline is much more problematic than pre EPA gasoline in vintage farm tractors. Ethanol is not the only reason.

Dean
 
Just re-read your initial post.

Another thing to investigate is the ignition switch.

If your problem is fuel starvation, your tractor should restart almost immediately after shutting down, though it may not run well enough to work.

If your tractor shuts down abruptly, as if someone turned the ignition switch off, and will not restart for 15 minutes or more, change the ignition switch.

Dean
 
Ok, so it's a matter of semantics then.

Not semantics. Two very different things. With true vapor lock on a fuel pump you can let it sit as long as you want and it's not going to fix itself, not even enough to let the engine start and run for a few seconds. With the fuel boiling in the line of a gravity fed system, if you let it sit it will start and run at least until it gets hot enough to happen again.

Is the sediment bowl itself full when when the problem happens? If so, then there's most likely something blocking the flow between the sediment bowl and the carb bowl. It could be the needle valve sticking or the filter at the top of the sediment bowl needs to be cleaned, or the screen at the inlet to the carb if it has one.

Or you could be fooling yourself about how much fuel is flowing from the carb when the problem happens. You have only said "very little fuel flows" when you open the carb drain, but you haven't quantified that. The flow will always be lower at the carb drain than at the input side of the sediment bowl after the bowl first empties because the flow is restricted by the size of the orifice at the needle valve as compared to the full diameter of the fuel line. Next time the problem happens, hold a clean container under the carb and see how long it takes to drain a pint of fuel into the carb.

If it really isn't fuel flow, your symptoms also sound like the problem could be that the coil is going bad. Have you tested for spark when the problem happens?
 
Ok, so not semantics and not true vapor lock. It does sound like the fuel starvation situation that has been described. And, I do agree it shouldn't have to be rerouted if it worked before with oem routing. So, maybe have to clean everything and check timing and carb setting, etc. To lean a mixture is the one that causes excess heat....is that correct?
Lots of good answers and info so far, thanks everyone.
Mark
 
This is going to sound very off the wall considering all that has been said so far, but...........a
friend of mine had a very similar problem with an old Allis. He and I went thru the same thought
processes as you all with various amounts of money spent on parts e.g. coil. What the problem
turned out to be was dead insects in the fuel sediment bowl housing. So, I guess my advice would be
to blow out the sediment bowl and housing.
 
(quoted from post at 10:02:38 06/01/19) This is going to sound very off the wall considering all that has been said so far, but...........a
friend of mine had a very similar problem with an old Allis. He and I went thru the same thought
processes as you all with various amounts of money spent on parts e.g. coil. What the problem
turned out to be was dead insects in the fuel sediment bowl housing. So, I guess my advice would be
to blow out the sediment bowl and housing.

Tim, labjack stated in his original post that he had "good flow through the sediment bowl" which would be difficult if it were plugged with dead insects. I asked if he has checked the screen which of course screens the gas leaving the bowl but he has not answered that. He may still be fixated on vapor lock.
 
I have not cleaned the screen yet, but I am going to go through the entire fuel system from sed bowl to carb to make sure that is not the problem. I'm sorry I used the term vapor lock, apparently not the correct term. I guess fuel starvation due to vaporization would be the way to describe it. I'm not certain that is the problem either but it acts that way. It always runs great for that first 30-45 mins. If the fuel system was blocked it doesn't seem like that would happen. And when engine cools off for 15-20 mins it will run good again but for less time. That's why I think it's the fuel vaporizing issue. I would rather it be blockage, I think that would be easier to fix. Gonna find out soon. Will be seeing the tractor Wednesday (don't live with it :)). Hope to get started on trouble shooting then.
 
(reply to post at 17:48:25 05/31/19)
apor lock is a problem that mostly affects gasoline-fueled internal combustion engines.

It occurs when the liquid fuel changes state from liquid to gas while still in the fuel delivery system. This disrupts the operation of the fuel pump, causing loss of feed pressure to the carburetor or fuel injection system, resulting in transient loss of power or complete stalling. Restarting the engine from this state may be difficult.

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.
 
(quoted from post at 22:19:00 05/31/19) No.

By definition, a gravity flow fuel system CANNOT vapor lock.

Dean

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.

https://en.wikipedia.org/wiki/Vapor_lock
 
Pay attention to my comment below about the ignition switch.

Insufficient information, but your problem could be the difficult to diagnose aftermarket ignition switch gremlin.

Rather than fight this gremlin, I would simply replace the ignition switch with a "same as original" switch from DC to see if the problem goes away. It could turn out to be $25 well spent.

It took me nearly 10 years to finally get my hands around this gremlin, but once understanding it, I've had no more such issues with vintage Ford tractors.

Dean
 
(quoted from post at 08:12:00 06/02/19) Pay attention to my comment below about the ignition switch.

Insufficient information, but your problem could be the difficult to diagnose aftermarket ignition switch gremlin.

Rather than fight this gremlin, I would simply replace the ignition switch with a "same as original" switch from DC to see if the problem goes away. It could turn out to be $25 well spent.

It took me nearly 10 years to finally get my hands around this gremlin, but once understanding it, I've had no more such issues with vintage Ford tractors.
Thanks Dean, will try that too. DC?
Dean
 
(quoted from post at 07:21:48 06/02/19)
(quoted from post at 22:19:00 05/31/19) No.

By definition, a gravity flow fuel system CANNOT vapor lock.

Dean

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.

https://en.wikipedia.org/wiki/Vapor_lock

sotexbill, This article does not cite any sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (July 2007)


Bill, absolutely there can be a reduction of pressure over the column of fuel in the line. However, it would certainly be so small that it would be very difficult to measure, and the gas in the tank is still under more pressure to push its way down the line and to the carburetor, so I will say that the effect is insignificant.
 
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.

That is absolutely false. In a gravity fed system the difference in pressure between the top and bottom of the system is determined by the difference in external atmospheric pressure at the top and bottom, external to the gravity fed system itself, That does not change merely because there are air bubbles in the system.
 
(quoted from post at 14:38:20 06/02/19)
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.

That is absolutely false. In a gravity fed system the difference in pressure between the top and bottom of the system is determined by the difference in external atmospheric pressure at the top and bottom, external to the gravity fed system itself, That does not change merely because there are air bubbles in the system.

Apparently Wikipedia disagrees with you. Boiling gas can create pressure and temporarily stop the flow of gas in a gravity system or so they say????

https://en.wikipedia.org/wiki/Vapor_lock
 
(quoted from post at 14:37:59 06/02/19)
(quoted from post at 14:38:20 06/02/19)
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.

That is absolutely false. In a gravity fed system the difference in pressure between the top and bottom of the system is determined by the difference in external atmospheric pressure at the top and bottom, external to the gravity fed system itself, That does not change merely because there are air bubbles in the system.

Apparently Wikipedia disagrees with you. Boiling gas can create pressure and temporarily stop the flow of gas in a gravity system or so they say????

https://en.wikipedia.org/wiki/Vapor_lock


sotxbill, This article does not cite any sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (July 2007)
 
Bingo.

Wikipedia, though often interesting, and often correct, is considered a reliable authority by no one.

Dean
 
(quoted from post at 17:37:59 06/02/19)
(quoted from post at 14:38:20 06/02/19)
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.

That is absolutely false. In a gravity fed system the difference in pressure between the top and bottom of the system is determined by the difference in external atmospheric pressure at the top and bottom, external to the gravity fed system itself, That does not change merely because there are air bubbles in the system.

Apparently Wikipedia disagrees with you. Boiling gas can create pressure and temporarily stop the flow of gas in a gravity system or so they say????

https://en.wikipedia.org/wiki/Vapor_lock

Absolutely any fool who wants to can go to Wikipedia and write whatever they want. It is not true just because it's on the internet. If you search Google for vapor lock and read any of the results other than Wikipedia, you will see that they all specifically say that it happens when a pump cannot pump the fuel due to the vapor bubbles.

When fuel boils in a gravity fed system the bubbles are lighter than the liquid fuel and so they rise to the top while the fuel continues to try to flow toward the bottom. If the amount of bubbles that are forming and rising in the fuel line is more than a certain amount, which differs based on the diameter of the line and the exact nature of the fuel, then the rising bubbles can take up so much of the cross section of the fuel line that the fuel flowing down cannot keep up with the demands of the engine to keep running. But once the engine dies fuel continues to flow, even if the fuel is still boiling in the line, until the carb fills up enough that the engine will start again, as I said, even if the fuel is still boiling. So the bubbles in the gravity fed system do not stop all fuel from flowing as is the case ina true vapor lock with a fuel pump.
 

The following paragraph is a quote from Wikipedia, Vapor Lock:

"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".

Over all, I agree with the article however, I do believe it could use a review and clean up. For example, I would revise the above paragraph to read as follows:

Gravity feed fuel systems are not immune to vapor forming the line. If vapor forms in a vertical section of the fuel line, its lower density acts to reduce the pressure developed by the weight of the fuel. This pressure is what normally moves fuel from the tank to the carburetor, so the fuel supply can be reduced or disrupted until the vapor is removed. The vapor can be removed, either by the remaining fuel pressure forcing it into the float bowl and out the vent or by cooling causing it to re-condense.


The carburetor inlet on my 8-N is 12" below the bottom of the fuel tank and the fuel tank is 8" deep, approximately. The sediment bowl inlet extents into the tank approximately 2" so we need to subtract that. Therefore, with the tank full we have a column of liquid gasoline; 12" + 8" - 2" = 18". For each foot of gasoline we have 0.312 psi, therefore an 8-N would have 0.312 x 1.5 = 0.468 psi of pressure at the inlet of the carburetor, with a full tank of fuel.

From the above it can be seen that should the fuel line contain vapor it would act to reduce the pressure available to push fuel into the carburetor bowl. Since the pressure is reduced the importance of clean screens, as others have posted, proper operation of the inlet needle (float) valve and venting of the carburetor is apparent. Running with a full tank of fuel and the sediment bowl valve opened to the reserve position would improve the pressure available. In addition to having clean screens I would suggest the screens should not be finer than that used when produced by Ford. In line filters should not be needed if the original screens are in place.
 
I disagree. Same pressure acting on carb inlet, whether acting against liquid or vapor at that inlet.
 
(quoted from post at 08:05:25 06/03/19)
The following paragraph is a quote from Wikipedia, Vapor Lock:

"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".

Over all, I agree with the article however, I do believe it could use a review and clean up. For example, I would revise the above paragraph to read as follows:

Gravity feed fuel systems are not immune to vapor forming the line. If vapor forms in a vertical section of the fuel line, its lower density acts to reduce the pressure developed by the weight of the fuel. This pressure is what normally moves fuel from the tank to the carburetor, so the fuel supply can be reduced or disrupted until the vapor is removed. The vapor can be removed, either by the remaining fuel pressure forcing it into the float bowl and out the vent or by cooling causing it to re-condense.


The carburetor inlet on my 8-N is 12" below the bottom of the fuel tank and the fuel tank is 8" deep, approximately. The sediment bowl inlet extents into the tank approximately 2" so we need to subtract that. Therefore, with the tank full we have a column of liquid gasoline; 12" + 8" - 2" = 18". For each foot of gasoline we have 0.312 psi, therefore an 8-N would have 0.312 x 1.5 = 0.468 psi of pressure at the inlet of the carburetor, with a full tank of fuel.

From the above it can be seen that should the fuel line contain vapor it would act to reduce the pressure available to push fuel into the carburetor bowl. Since the pressure is reduced the importance of clean screens, as others have posted, proper operation of the inlet needle (float) valve and venting of the carburetor is apparent. Running with a full tank of fuel and the sediment bowl valve opened to the reserve position would improve the pressure available. In addition to having clean screens I would suggest the screens should not be finer than that used when produced by Ford. In line filters should not be needed if the original screens are in place.


Ken, All that you described is very nice but it is still the same as what I posted earlier. The main object of our exercise here is to get labjack off from his fixation on "vapor lock", which he is now calling "fuel starvation due to vaporization" and onto something that is actually likely to solve his problem.
 
(quoted from post at 11:05:25 06/03/19)
The following paragraph is a quote from Wikipedia, Vapor Lock:

"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".

Over all, I agree with the article however, I do believe it could use a review and clean up. For example, I would revise the above paragraph to read as follows:

Gravity feed fuel systems are not immune to vapor forming the line. If vapor forms in a vertical section of the fuel line, its lower density acts to reduce the pressure developed by the weight of the fuel. This pressure is what normally moves fuel from the tank to the carburetor, so the fuel supply can be reduced or disrupted until the vapor is removed. The vapor can be removed, either by the remaining fuel pressure forcing it into the float bowl and out the vent or by cooling causing it to re-condense.


The carburetor inlet on my 8-N is 12" below the bottom of the fuel tank and the fuel tank is 8" deep, approximately. The sediment bowl inlet extents into the tank approximately 2" so we need to subtract that. Therefore, with the tank full we have a column of liquid gasoline; 12" + 8" - 2" = 18". For each foot of gasoline we have 0.312 psi, therefore an 8-N would have 0.312 x 1.5 = 0.468 psi of pressure at the inlet of the carburetor, with a full tank of fuel.

From the above it can be seen that should the fuel line contain vapor it would act to reduce the pressure available to push fuel into the carburetor bowl. Since the pressure is reduced the importance of clean screens, as others have posted, proper operation of the inlet needle (float) valve and venting of the carburetor is apparent. Running with a full tank of fuel and the sediment bowl valve opened to the reserve position would improve the pressure available. In addition to having clean screens I would suggest the screens should not be finer than that used when produced by Ford. In line filters should not be needed if the original screens are in place.

The bubbles do not reduce the pressure. This is a vented fuel tank and a vented carb bowl, so the pressure at the top and the pressure at the bottom are dictated by the external atmospheric pressures at those two heights. You cannot change the pressure at the top or the pressure at the bottom without changing the density of the atmosphere itself.

The rising bubbles keep the downward flow of fuel at a lower flow rate than normal due to the bubbles taking up a percentage of the total cross section of the fuel line so the fuel cannot flow down as fast as if there were no bubbles.
 
(quoted from post at 13:42:21 06/03/19)
(quoted from post at 11:05:25 06/03/19)
The following paragraph is a quote from Wikipedia, Vapor Lock:

"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".

Over all, I agree with the article however, I do believe it could use a review and clean up. For example, I would revise the above paragraph to read as follows:

Gravity feed fuel systems are not immune to vapor forming the line. If vapor forms in a vertical section of the fuel line, its lower density acts to reduce the pressure developed by the weight of the fuel. This pressure is what normally moves fuel from the tank to the carburetor, so the fuel supply can be reduced or disrupted until the vapor is removed. The vapor can be removed, either by the remaining fuel pressure forcing it into the float bowl and out the vent or by cooling causing it to re-condense.


The carburetor inlet on my 8-N is 12" below the bottom of the fuel tank and the fuel tank is 8" deep, approximately. The sediment bowl inlet extents into the tank approximately 2" so we need to subtract that. Therefore, with the tank full we have a column of liquid gasoline; 12" + 8" - 2" = 18". For each foot of gasoline we have 0.312 psi, therefore an 8-N would have 0.312 x 1.5 = 0.468 psi of pressure at the inlet of the carburetor, with a full tank of fuel.

From the above it can be seen that should the fuel line contain vapor it would act to reduce the pressure available to push fuel into the carburetor bowl. Since the pressure is reduced the importance of clean screens, as others have posted, proper operation of the inlet needle (float) valve and venting of the carburetor is apparent. Running with a full tank of fuel and the sediment bowl valve opened to the reserve position would improve the pressure available. In addition to having clean screens I would suggest the screens should not be finer than that used when produced by Ford. In line filters should not be needed if the original screens are in place.

The bubbles do not reduce the pressure. This is a vented fuel tank and a vented carb bowl, so the pressure at the top and the pressure at the bottom are dictated by the external atmospheric pressures at those two heights. You cannot change the pressure at the top or the pressure at the bottom without changing the density of the atmosphere itself.

The rising bubbles keep the downward flow of fuel at a lower flow rate than normal due to the bubbles taking up a percentage of the total cross section of the fuel line so the fuel cannot flow down as fast as if there were no bubbles.
y whatever mechanism, whether it be bugs in screens, rust, heat or whatever, flow can be slowed a lot before trouble because an all-is-well flow rate is several times the fuel use/burn rate.
 

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