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Old 11-15-2013, 08:27 PM
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Carb icing

OK here is a question I've yet to find an answer to.

Back in the 1980s I had an 81 Honda Prelude. In a crude attempt to mimic the moan by father's 454 Chevy pickup used to make, I inverted the air cleaner lid. Yes it created a moan but nothing like Dad's truck.

One day on the way to school, eventually I was going a maximum of 10 mph under full throttle. Once I flipped the breather lid the correct way, all was fine.

1984 Ford Bronco II. Same deal, except with this vehicle, the air intake system had not been modified.

The conditions: 40s, high humidity (misty, foggy, etc.). Both vehicles were going in excess of 40 mph.

I know carb icing is due to the pressure drop across the venturi and due to high moisture conditions near freezing and is not likely to occur even in subfreezing conditions as long as the air is dry but initially I thought the reason carb icing occured was due to wet, cold air.

And the reason air cleaners have a hot air intake coming off the exhaust manifold was to prevent carb icing. However these systems only pull in warm air during engine warmup. So after awhile they shut off and pull it cold air. So that blows that theory.

The other theory was open air cleaners (like Mr. Gasket chrome air cleaners) increase the air velocity which causes carb icing. But the Bronco's air cleaner was not modified.

So what actually causes carb icing and how to prevent it? What was going on with these two different vehicles?
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Old 11-15-2013, 09:23 PM
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Being a Ford partsman for many years (1979 - 1992) I used to sell a number of

(9E607) air cleaner temperature sensors. These were used to control the vacuum motor

(9D612) out on the end of the air-cleaner's intake "snorkel".

All of the "Big 4" used something similar to this throughout the 70's and 80's on carbureted engines.

When air temps inside the air cleaner assy dropped, the sensor would activate the vacuum motor, swinging the gate closed to normal air intake flow, and divert it so that it drew heated air from the exhaust manifold heat stove.
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Old 11-15-2013, 11:58 PM
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I've had carb icing under both conditions of warm, humid air and very cold air.

In the warm, humid air case, what happens is that the evaporating gas supercools the carb itself and allows the humidity in the air to form ice on the very cold carb surfaces. (Like when alcohol evaporates on your hand and supercools the skin)

I was always taught that you cannot get carb icing in extreme cold because the humidity is in the form of ice crystals that just get sucked through the equally cold carb. HOWEVER - the wonderful feature in the aircleaner snorkel that pre-heats the air also thaws the airborne ice crystals back into moisture that can now freeze onto the throat of the carb itself.

So - I have had carb icing at 65* above with high humidity and at 35 below.

Just pull over and let the underhood engine heat melt the ice and you're good to go again.

If you carry a bottle of methyl hydrate, you can just pour some into the carb and speed up the process of getting going again.
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Old 11-16-2013, 12:19 AM
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Quote:
Originally Posted by Dave57210 View Post
I've had carb icing under both conditions of warm, humid air and very cold air.

In the warm, humid air case, what happens is that the evaporating gas supercools the carb itself and allows the humidity in the air to form ice on the very cold carb surfaces. (Like when alcohol evaporates on your hand and supercools the skin)

I was always taught that you cannot get carb icing in extreme cold because the humidity is in the form of ice crystals that just get sucked through the equally cold carb. HOWEVER - the wonderful feature in the aircleaner snorkel that pre-heats the air also thaws the airborne ice crystals back into moisture that can now freeze onto the throat of the carb itself.

So - I have had carb icing at 65* above with high humidity and at 35 below.

Just pull over and let the underhood engine heat melt the ice and you're good to go again.

If you carry a bottle of methyl hydrate, you can just pour some into the carb and speed up the process of getting going again.
HMMM. Never thought carb icing could happen at 65F! Yeah it doesn't take long for the carb to thaw out.
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Old 11-16-2013, 12:20 AM
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Quote:
Originally Posted by 66GMC View Post
Being a Ford partsman for many years (1979 - 1992) I used to sell a number of

(9E607) air cleaner temperature sensors. These were used to control the vacuum motor

(9D612) out on the end of the air-cleaner's intake "snorkel".

All of the "Big 4" used something similar to this throughout the 70's and 80's on carbureted engines.

When air temps inside the air cleaner assy dropped, the sensor would activate the vacuum motor, swinging the gate closed to normal air intake flow, and divert it so that it drew heated air from the exhaust manifold heat stove.
I've observed the "stove pipe" operation on my 68 Fairlane 302 which uses a wax pellet system. It's always closed toward the exhaust manifold before I start the engine but after the engine warms up, the hot air flapper is diverted to cold air. That's why I thought these systems only sucked in hot air until the engines had reached normal operating temperature.
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Old 11-16-2013, 08:56 AM
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Originally Posted by jseabolt View Post
I've observed the "stove pipe" operation on my 68 Fairlane 302 which uses a wax pellet system. It's always closed toward the exhaust manifold before I start the engine but after the engine warms up, the hot air flapper is diverted to cold air. That's why I thought these systems only sucked in hot air until the engines had reached normal operating temperature.
Yeah ... don't quote me, but I think the thermostatically-controlled version came into play in 1970, IIRC.
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Old 11-16-2013, 09:02 AM
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emission reduction.

I always heard the pre heated air was to reduce emissions, get the engine to ideal operating temperature as fast as possible. an to get the choke off.
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Old 11-16-2013, 10:04 AM
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Nope. It was to keep you from sitting along side the road on a cold frosty morning on the way to work on the turn pike...
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