Re: Carburetor Icing

Posted by Smitty on July 26, 1999 at 11:11:03 from (208.231.102.146):

In Reply to: Carburetor Icing posted by PTO on July 25, 1999 at 17:45:09:

You are running into the classic conditions for carburetor icing. This depends on relative
humidity, tempurature, dew point, and all of that meteorology stuff.
When fuel is vaporized (turned into a gaseous state from a liquid state)heat
energy is absorbed (Gas molecules are at a higher energy state that liquid molecules). This
is how a refrigerator works.

The addition of alcohol would change the temperature / humidity conditions under
wich icing occurs, because it has different vaporization characteristics than gas.

When the gas / fuel is vaporized, it takes heat away from the surrounding
carburetor parts. Since the throttle blades are right in the place where vaporization occurs
and because they are thin (very little mass) they are where the icing will first form.

The ice will especially affect idle as it plugs up the small throttle opening and
transfer slots. You won't notice it at high speeds as much for this reason. Also, if you
remember your high school science vaporization also is dependent on air pressure. Ths was
the experiment of putting the Coka Cola in a bell jar and drawing out the air, creating
a vacuum. This condition is duplicated in your carb at and near idle, when the
vacuum is high in the manifold. The fuel really vaporizes as it goes thru the small opening
and plugs up the opening.

I have read about some racing carburetorsetups that are very efficient at
vaporizing fuel.
The carb bodies turn frosty!

You will need to figure out a way to get a little heat to your carb or mixture during
times when this is happening. You could:

1. Use a thermostatically controlled air cleaner like old smoggers did in the 70s.
Take some heated air from the exhaust manifold to heat the mixture. If you don't want
to use the vacuum to control it, hook up a bowden cable and do it by hand.

2. Try different brands of gas. Sometimes they have differing vaporization points.
I've found that cheap gas is usually more susceptable on this issue.

3. If you are using a phenolic spacer, try an aluminum one that lets the carb run a
bit hotter.

4. In the early 60s, Ford heated the spacer with the heater line. This was made in
2v and 4v Holley patterns. This will add heat, but there is a funny provision on the
vacuum tap that will bleed vacuum if you use it on a Y manifold. give it a look before You
buy one so you will know if you want to tackle it.

I'm not saying that these ideas will cure your problem. Adding enough heat or
changing other conditions so that you avoid the problem is how to stop this. I do feel that
these ideas will point you in the right direction for doing this, tho.

Since I've lived in Wet areas, this has only happen to me on a few occaisions, and
it was always on humid days.

hope this helps

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