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06-03-2010, 09:48 AM | #41 |
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Re: For vapor lock nonbelievers
If vapor lock is a problem, add a gallon of diesel fuel to your tank. No more problems.
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06-03-2010, 10:02 AM | #42 | |
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Re: For vapor lock nonbelievers
Quote:
note the added 1/8" bowl vent hole added mid-1930 for improved hot weather performance.....
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06-03-2010, 10:19 AM | #43 |
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Re: For vapor lock nonbelievers
Recently took the Model A for a short drive on a day that was not overly hot. The vehicle has the new modern Zenith carburetor. When I stopped for a vehicle check as this was a new engine, the fuel was perculating in the carburetor. This also happened to us on a high altitude trip on a hot day. Ok! Is perculating and vapor locking the same thing ? An original Zenith carburetor has a vent hole in the top half. Many years ago carbs were made with a perculation valve.
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06-03-2010, 10:33 AM | #44 |
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Re: For vapor lock nonbelievers
If we look at the variables within this process, we can simplify the possible causes.
The net effect seems to be a vaporization or evaporation of the fuel while in the fuel line conduit. As root causes here are a few to consider; Fuel formulation Size and type of fuel filter. Type of metal used to construct fuel line. Diameter of fuel line. Condition of fuel line. I would focus my aim towards the condition of the fuel line and the condition of the surface areas of the vessel holding the fuel. Most research is done in a style known as bench, under ideal conditions or simulated conditions reproduced. The undoing of all engineering is the natural reaction of nature in a non bench condition. As stated" this problem does not occur in the south" In the south they have a hotter summer. We are focused on the hot, We are discounting the cold cycle that they do not have. While the hot cycle would cause boiling and evaporation to the fuel, one factor. The cold cycle causes the fuels water content to freeze and separate within the fuel line line causing reaction within the line. This reaction causes oxidation within the line, oxidation within the line causes a variation from the bench optimal condition. The effects of oxidation within the fuel line can vary, however the effect of oxidation may cause more surface area to be present within the fuel line causing a reactive surface, the oxide may trap and transfer heat to the fuel or the oxide itself may be reacting with the fuel. My first consideration would be to research the fuel line and its metal type, design, and how it has been affected by time. |
06-03-2010, 10:41 AM | #45 |
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Re: For vapor lock nonbelievers
Edit: I was WRONG below. There was initially one vent hole for the bowl in the top, a second was added in May 1930. Read the posts below.
Bill, you're correct. Your notes caused me to do a little research. According to Steve Pargeter's excellent monograph on Model A carburetors an "bowl vent hole" was added in May 1930. Before that there was no obvious vent between the bowl and atmosphere. Perhaps this holds the key to why some Model A's can have vapor lock problems. For a carburetor to work properly the air/vapor above the fuel in the bowl needs to be at atmospheric pressure. In normal operation with the float valve system working properly and the fuel not boiling the fuel level in the bowl doesn't vary much. This results in the volume of air above the fuel in the bowl not varying much. Not much airflow is needed to keep the air at atmospheric pressure so the "vent" doesn't have to be very large. In a Model T Holley carburetor the vent hole is small and under a brass tag. In the earlier Model A carburetors leakage of air past the gasket must usually suffice given the number of Model A carburetors without bowl vent holes which work quite nicely. However, if the fuel is boiling in the bowl and the only vent is the small amount of leakage past the gasket then problems are likely. Fuel may slow or stop flowing into the bowl. If the vapors don't vent quickly enough the engine will eventually stop running as the fuel level in the bowl drops. With the bowl vent hole the vapor will vent to the atmosphere. Perhaps the solution to vapor lock in a Model A is to add the bowl vent hole to the carburetor if it's missing. As note above Paul Moller recommended this. Last edited by David Cockey; 06-03-2010 at 01:47 PM. |
06-03-2010, 11:13 AM | #46 | |
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Re: For vapor lock nonbelievers
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Quote:
The vent is toward the rear of the 3 holes shown in line about the middle of the top of the carb bottom. The vent hole was enlarged during 1929. |
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06-03-2010, 11:13 AM | #47 |
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Re: For vapor lock nonbelievers
Great discussion! If I may be so bold to add my $0.02....All automotive gasolines in this country are manufactured under ASTM D 4814 specifications (www.astm.org) and, although each refinery is different, their product meets the same spec and is fungible (compatible with all the others). Vapor pressure, distillation and vapor lock protection are closely monitored by season and region and are regulated by U.S. EPA - not by oil companies and not by octane grade. Although many of us believe vapor pressure is the culprit in vapor lock problems, it is another value used in the industry called T20 V/L: The temperature at which the vapor to liquid ratio is 20:1 in the distillation curve. Their are 6 classes ranging from 95F to 140F (Arizona in July being Class 1 [140F], and Alaska in January being Class 6 [95F]; the rest of us somewhere in between). Typically in older engines (non-computerized) an exeptionally warm day in spring or an exeptionally cold day in fall may cause driveability problems because of the volatility of the fuel. Otherwise, gasolines are fairly well formulated to work all the time. I'm not any happier than anyone else to have 10% ethanol in our gasolines, but EPA mandated the oxygenate rule to reduce CO and HC emissions and everyone complained about the ethers (MTBE and others), so refineries were forced to add alcohol. I've had several problems with my Model A in the 50 years I've owned it, but none were vapor lock, and most were my own fault. BTW, most gasolines begin boiling around 70F-80F, but must finish at or below 437F, regardless of the components. Diesel fuels (ASTM D 975) start to boil around 380F-400F and will be only 90% distilled at around 570F-670F; they may not completely combust in a Model A engine. (Sorry for being so nerdy).
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06-03-2010, 01:44 PM | #48 |
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Re: For vapor lock nonbelievers
Discussion is good.
I just had a look at a carburetor without the added hole in bowl. There is a vent hole in the carburetor TOP near the float bracket which goes from the bowl to above the vent hole in the carburetor bottom. That area is vented to the atmosphere through the hole in the carburetor bottom. Thanks for the note which prompted me to have another look at the carburetor. I found it hard to believe that leakage past the gasket was sufficient but didn't see the other vent the first time I looked. So Model A carburetors initially had one vent from the bowl to the carburetor, and then a second vent was added around May 1930. |
06-03-2010, 03:29 PM | #49 |
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Re: For vapor lock nonbelievers
Steve, I have the same as the sketchs above - Zenith-2 and no marking on top casting. The 3rd vent hole is 9/32" dia ('28-mid '29)
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06-03-2010, 03:59 PM | #50 |
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Re: For vapor lock nonbelievers
It was questioned in a previous post why doesn't vapor lock happen to everyone. two of my A's have never vapor locked and one will always vapor lock when hot. go figure! premium gas in that one is the easiest remedy for me.
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06-03-2010, 04:09 PM | #51 | |
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Re: For vapor lock nonbelievers
Quote:
Wouldn't it be something if the one that gives you "vapor lock" doesn't have the added bowl vent? |
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06-03-2010, 04:48 PM | #52 |
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Re: For vapor lock nonbelievers
I was having the same problem (bubble forming in the glass bowl) followed by hesitation, cough, and wonderfully powerful backfire (scared the phajeebers out of my kids!)
Anyway, I thought it was vapor lock, but what I think the real problem was the gas tank. I have since put in a dead leg, and drain off the gunk after each fill-up. What I believe was happening is that fuel was drawing out faster than the tank could deliver; the sediment was preventing the fuel from getting to the bowl before the carb drank it all. After the dead leg, I have not witnessed the bubble in the bowl. Just a thought. |
06-03-2010, 04:58 PM | #53 |
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Re: For vapor lock nonbelievers
OK. Explain a dead leg. Maybe a picture?
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06-03-2010, 05:42 PM | #54 |
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Re: For vapor lock nonbelievers
A drip leg, like on your furnace or hot water heater, Correct?
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06-03-2010, 06:58 PM | #55 |
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Re: For vapor lock nonbelievers
Like a sediment trap? You could remove the strainer tube that is in your gas tank filler hole and replace it with a closed-bottom tube that has a hole in the side. The gas comes in, the sediment goes to the bottom, the gas overflows out the side hole into the tank. You pull out the tube to clean it out occasionally.
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06-03-2010, 07:22 PM | #56 |
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Re: For vapor lock nonbelievers
I don't have a picture but it's a T connection comming out of the gas tank. One leg of the T goes straight down and has a drain valve. This "DEAD LEG" catches the junk. The other leg of the T goes to the right and feeds the sediment bulb and carb. The dead leg makes it quick and easy to catch and drain the junk. Once all the junk stops comming, the dead leg can be removed and the shutoff valve can be installed as it was originally installed.
I always like to use a standup brass screen in the shutoff valve, even with my clean tank. It will keep junk from nicking the surface of the valve. |
06-03-2010, 07:37 PM | #57 |
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Re: For vapor lock nonbelievers
I see it now. Looks like a good idea.
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06-04-2010, 06:41 AM | #58 |
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Re: For vapor lock nonbelievers
For vapor lock nonbelievers
That would be me. The catch-all leg is simply a typical gas train set-up for any industrial gas fired burner. Ethanol evaporation is closer to water then to gasoline, so I'm thinking that this makes no difference in terms of vapor lock. Fuel has to flow through the tank, settling bowel [with a clean screen] and down the pipe to the carb where if in proper trim the float valve will stop/start the flow of fuel if the carb is open to the free flow of atmospheric pressure. skip. Last edited by skip; 06-06-2010 at 10:08 PM. |
06-04-2010, 06:47 AM | #59 |
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Re: For vapor lock nonbelievers
This tread / subject is better than "what kind of oil I should use" It should be rated with at least 5 stars.
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06-04-2010, 08:17 AM | #60 |
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Re: For vapor lock nonbelievers (Dead Leg Image)
The lower leg (leg parallel to the emergency brake lever) is always off, and there is a stopper plug in the bottom (just in case). I drain this after each fill-up and before each start up.
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