Ford 390 - Carb Icing
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From: Vancouver Island, BC
Ford 390 - Carb Icing
Wondering if anyone could shed some light on this issue. 390 in my '75 F250 has been running rough and sometimes stalling, and I was told that the carburetor is icing up with water vapour because the air filter snorkel is missing a pre heater hose from a shroud on the exhaust. This shroud seems to be impossible to find or even to find information on. I've heard of other methods of preventing this such as carbs that take coolant, or even have inlets from the exhaust to prevent the jets from icing. I'd imagine anyone with air filters with the open housing must have some other method. Anyone else have this problem or know how to fix it?
Fleet Mechanic
Joined: Nov 2010
Posts: 1,501
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From: Axtell, TX
Like Mike said, many came with a heated carb spacer to keep from icing in cold climates. Like this one Heated carb spacer
If you were in a hot climate and had the heated spacer you could have the opposite happen. You could have problems with fuel vaporizing and a swap to a non heated phenolic spacer could eliminate that.
If you were in a hot climate and had the heated spacer you could have the opposite happen. You could have problems with fuel vaporizing and a swap to a non heated phenolic spacer could eliminate that.
Hotshot
Joined: Apr 2009
Posts: 14,333
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From: Stanley, VA
The shroud that you're refering to is part of the emission control system. This is a cold starting aid that help fuel vaporize while the engine is still cold. Once the engine reaches operating temperature, there is a flapper door in the snorkle that closes the heated air duct and opens the snorkle so cooler air will flow through the carb.
While it's possible that icing could be causing a problem, I kind of doubt it.
The only way to check for sure is the next time it happens, immediatley and I mean right now! pop the hood and look down the throat of the carb. If ice is a problem, you'll be able to see the build-up in the throat of the carb, just above the throttle plates.
The reason I said be quick is because once the engine stalls, the ice will melt VERY QUICKLY and any evidence will be gone.
While it's possible that icing could be causing a problem, I kind of doubt it.
The only way to check for sure is the next time it happens, immediatley and I mean right now! pop the hood and look down the throat of the carb. If ice is a problem, you'll be able to see the build-up in the throat of the carb, just above the throttle plates.
The reason I said be quick is because once the engine stalls, the ice will melt VERY QUICKLY and any evidence will be gone.
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Fleet Mechanic
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Post Fiend
Joined: Mar 2004
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From: Union, Washington
That hose off the exhaust manifold only works when the engine is cold on start up. Once the engine warms up it does nada.
Things to check to determine your problem without guessing.
Compression test
Look at the plugs while doing the compression test, are any obviously bad?
Look at the distributor cap and rotor, do they look dirty or have been arcing?
How old are you plug wires?
Vacuum gauge to look at the intake vacuum, this can tell you a lot.
Things to check to determine your problem without guessing.
Compression test
Look at the plugs while doing the compression test, are any obviously bad?
Look at the distributor cap and rotor, do they look dirty or have been arcing?
How old are you plug wires?
Vacuum gauge to look at the intake vacuum, this can tell you a lot.
Fleet Owner
Joined: Jun 1999
Posts: 24,552
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From: Creedmoor, North Carolina
I live in the Tarheel state, don't have very cold winters, don't drive the truck a lot in the winter, but mine hasn't had any kind of heat injection to the carb. in many years.
John
John
Posting Guru
Joined: Aug 2001
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From: Evanston, WY
I used to have the same problem with my 390 with an open air filter. To solve it, I just went to the junkyard and bought a factory air cleaner with the little outlet on the bottom like is shown in the picture above. I then just jammed the tube (it's kinda like a dryer hose) between the back of the head and the firewall. The tube sucked warm air off the back of the head and stopped the problem. Then when the weather got warm again I just switched back to my open filter.
-Brian
-Brian
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