At 07:08 AM 8/29/2000 -0400, Bill Schooler wrote:
>Would some erudite individual please explain precisely what is meant by
the term "vapor-lock"? I always thought it had something to do with a
mechanical fuel pump's inability to pump "air"...
Vapor lock is a contition where the fuel boils somewhere in the system and
causes a vapor bubble that interferes with fuel delivery.
For an angine with a mechanical fuel pump, boiling in the line immediately
before the pump could be a problem. I understand this can affect the
Midget 1500 with the TR engine, and the cure there is to reroute the fuel
line farther away from the engine.
More commonly the vapor lock occurs in the carburetor. Sometimes it can
happen in the float chamber where vapor pressure can inhibit fuel from
entering through the float valve This can prevent the engine from running
all together, but this is not the usual case.
More often it happens in the body of the carburetor where hot fuel boils in
the porting between the float chamber and the main fuel jet. In this case
some fuel still gets through the main jet, but it passes as a mixture of
liquid and vapor, so there is less fuel delivered per unit of flow volume,
which makes the fuel/air mixture go very lean. The engine will probably
start and run, but it will cough and sputter badly with very little power.
This commonly happens when you shut off a hot engine and let it bake under
the bonnet for a few minutes, and then try to restart, such as after a
short refueling stop. If you have a manual choke you can choke the
daylights out of it to get more fuel into the venturi to make it run
better. Once you escape the environmental conditions that are causing the
overheated carburetor this condition should go away. Lots of fresh fuel
flow through the carburetor helps to cool the carb body, so heavy choke and
heavy throttle usually speed the recovery.
Incidentally, fuel with 10% alcohol content can be much worse in this
respect. Alcohol has a higher vapor pressure, so it can boil or evaporate
at a lower temperature than gasoline without the alcohol. Alcohol is also
an octane modifier, and is a relatively cheap way to boost the octane
rating of the fuel a couple of points. As such it is common for the lowest
grade of fuel at the pump the be alcohol free, while the mid grade contains
10% alcohol and is a couple of points higher octane rating. Sometimes
these two grades will even sell for the same price; alcohol is more
expensive to produce then gasoline, but the alcohol content is not taxed in
some states. When the price of crude oil goes sky high it is more likely
to find the alcohol included in the fuel. While the alcohol does increase
octane rating, it also increases the prevalence of vapor lock.
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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