Vapor Lock ???

Bob Johnson

Bob Johnson

Well-known member
On my return trip from the MAFCA meet in Washington we took the fast route down Interstate 5. Around noon we hit the first of two long uphill grades. Our speed up the grade was about 40 mph. The air temperature was close to 100 degrees. I had no problems going up this grade. My digital motometer hit a maximum of 184 degrees. We stopped for fuel in the valley between the two uphill grades. After filling up I had a hard time starting the engine. It was not getting enough fuel I had to choke it to get it to run. I decided to add some mystery oil to the fuel because I was told it would prevent this problem. We headed up the second grade. Again we were moving at about 40 mph. The outside temperature was over 100 degrees. My coolant temperature was now 189 degrees. My Model A started to act like it had run out of fuel. Since I have had experience with this before I had a solution. I opened up the GAV and the bucking stopped. After a while it stared bucking again. So I opened the GAV more. By the time I had reached the top of the grade I had opened up the GAV 2 FULL TURNS. At the bottom of the hill we stopped for lunch. I checked out the manifolds and carburetor and found that they were REAL HOT. The intake manifold was so hot that when I dripped some water on it the water sizzled just as if it were a hot frying pan. The upper part of the carburetor was also hot enough to boil water. I used about a gallon of water to slowly cool off the carburetor and manifold. Yes the gasoline had Ethanol in it.

My guess is that the carburetor got so hot that some of the chemicals in the fuel were hot enough to boil. Thus the main jet was providing a mixture of liquid and vapor fuel. This condition does not supply enough fuel and the engine acts like it is running out of fuel. By opening the GAV I created a second source of fuel allowing the engine to continue running.

I am not sure what to do to prevent this problem. I could just not drive up hills in hot weather. I could try to isolate the carburetor, thermally, from the manifolds. Or I could figure out a way to provide cooling to the carburetor.

Bob
 
deadhorse.gif
Ohh NO, here we go again!! :p


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You may wish to consider to get a laser thermometer from harbor fright. I take spot check of the temps of parts of my engine... My spark plug bases are ~170-190F, the exhaust manifold near the #4 cylinder has reached 520F the hottest point I've found on the engine. My zenith 1 has reached ~117F...
 
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I have read most of the previous posting on this. However I thought that my experiences might help others. Vapor lock is usually associated with gas vapor in the fuel line. My experiences suggest that it may be fuel vapor in the float bowl, which is different than normal vapor lock and would require a different solution. I am not trying to beat a dead horse just add some information to an old topic.

Bob
 
Bob, let's start by figuring out why the carburetor is so hot. IMHO, 100 amb. degrees outside is not that hot, -especially when you are moving 35 knot winds thru the engine bay. In addition, engine water temps in the 190 degree range is not hot either which means you were not transferring excessive heat from the radiator into the engine compartment.

This leads me to wonder about things such as carburetor type & condition, type of cooling fan, engine pans, etc. You also used the term "we" and "our", does this refer to occupants of the Model-A, --or other Model-As driving with you?
 
Brent,

I was driving a 31 PU with a B engine, B carburetor, B manifold, A distributor, 4 blade steel fan, no engine pans. There was no one else in my truck. I was following a slower Tudor which had a stock A engine.

Bob
 
I wish we could get a handle on this as we read about it here all the time. Two cars set up the same way one has the symptoms one does not. Maybe Tony S. will chine in but I know of no cars in our club that have had that problem even in South Florida heat, although we don't have any real hills except for over passes and land fills.

I'm not saying that it does not occur in Mode A's but thankfully it seems to be far from common.
 
Bob, how did you have your spark advance set? I've heard stories of exhaust manifolds and mufflers getting red hot when A's are pushed hard with the timing retarded, though it's never happened to me. Is it possible that your timing was retarded and the manifold heat was radiated to the carb?
 
Bob Johnson said:
Brent,

I was driving a 31 PU with a B engine, B carburetor, B manifold, A distributor, 4 blade steel fan, no engine pans. There was no one else in my truck. I was following a slower Tudor which had a stock A engine.

Bob
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Mike V. Florida said:
I wish we could get a handle on this as we read about it here all the time. Two cars set up the same way one has the symptoms one does not.
Click to expand...


In my mind, we just did. A four-blade fan will impede airflow, and one of the main reasons why engine pans were installed originally was to direct airflow thru the engine compartment efficiently. Get the air flowing through the engine compartment efficiently and the carburetor will likely drop in temps. Try it Bob and see if it doesn't make a difference.
 
BRENT in 10-uh-C said:
In my mind, we just did. A four-blade fan will impede airflow, and one of the main reasons why engine pans were installed originally was to direct airflow thru the engine compartment efficiently. Get the air flowing through the engine compartment efficiently and the carburetor will likely drop in temps. Try it Bob and see if it doesn't make a difference.
Click to expand...

And along the same thought, check radiator condition, obstructions such as license plate or stone guard in front of radiator. Anything that impedes air flow through engine compartment.
 
May seem a bit stoopid....but, is the fan sucking or blowing?
And yes..... I do have first hand knowledge.:o

Bob
:cool:
 
Bertha said:
May seem a bit stoopid....but, is the fan sucking or blowing?
And yes..... I do have first hand knowledge.:o

Bob
:cool:
Click to expand...

It's blowing. Sucking doesn't exist. Air pressure does the pushing of air through the radiator. There is more air pressure in front of the radiator than behind it because the fan pushed the air back.

You can't suck juice up a straw, but air pressure can push it up.:D
 
Hot fuel in carb?? and line? I'm Curious.. do you have a air maze filter on the carb? I have heard that because the maze is so close to the exhaust pipe that heat can transfer to the carb and raise the temp in the fuel bowl, thereby cooking the gas. I agree about the 4 blade fan. On a moving A as long as the fan belt turns the water pump the fan is secondary, and mostly for idle. Timing, water/air flow restriction, gas cap that expands and blocks the vent hole, brake drag, as was stated by others, are a possibility. The sad part(or not) it will probably never occur again, since the circumstances may never be matched. BTW, did you drive over the Grapevine on I-5.... even my modern cars react to that place.
 
If it happens again try a half a quart of auto trans fluid to a full tank of gas.I have a friend who with a modern car had the same problem,tow truck came out and he poured a quart of trans fluid in the gas tank and the car ran fine after that.
 
No air filter at all. My problem is definitely fuel boiling in the carburetor. It is not the fuel line or the gas tank. As for wooden cloths pins on the gas line, IMHO that is akin to snake oil. Wood is a very poor conductor of heat. Aluminum cloths pins (if they exist) would dissipate heat. It happens to me several times a year mostly when I am running with winter gas in hot weather.

Bob
 
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