Carburetor icing will usually occur when the ambient temperature ranges between 30° to 60°F, when the relative humidity is above 65%, and when the wrong grade of fuel is used.
Carb icing on Aisan carbs is real, common, and often misdiagnosed. It has nothing to do with outside temperature being “too cold” and everything to do with physics and missing heat.
The short, blunt explanation
An Aisan carburetor can ice up when:
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Air passes through the venturi
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Fuel atomizes
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Temperature drops 30–60°F instantly
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Moisture in the air freezes on the throttle plate and bore
This can happen at 40–70°F ambient, especially with humidity.
What carb icing feels like
Classic symptoms:
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Starts and idles fine cold
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After a few minutes:
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Loses power
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Won’t idle
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Stumbles or dies at stops
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Shut it off for 5–10 minutes ? runs fine again
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Rinse, repeat
People blame:
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jets
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mixture screws
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ignition
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fuel pump
They’re wrong.
Why Aisan carbs are especially prone
Most Aisan-equipped Toyota engines rely on multiple anti-icing systems working together:
If any one is missing or disabled, icing becomes likely.
1. Exhaust heat riser (big one)
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Routes hot exhaust under the intake
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Warms the carb base and manifold
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Often:
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rusted shut
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wired open
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removed with headers
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No heat = ice.
2. Hot air intake stove
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Pulls warm air from around the exhaust manifold
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Controlled by a thermostatic door in the air cleaner
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Missing hose or stuck door = cold dense air = icing
3. Coolant-heated intake (some models)
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Coolant passages warm the manifold
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Blocked ports or bypassed hoses kill heat transfer
Where the ice actually forms
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Back side of the throttle plate
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Idle discharge ports
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Lower venturi walls
That’s why:
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It won’t idle
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Throttle has to be held open
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Idle mixture changes do nothing
The fuel is literally blocked by ice.
How to confirm carb icing (no guessing)
Do this once and you’ll never forget it:
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Engine starts acting up
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Shut it off
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Remove the air cleaner
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Look down the carb throat
Frost or moisture = icing
If it’s iced, don’t touch a screwdriver.
What actually fixes it (no band-aids)
You must restore heat. Period.
Minimum checklist:
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Exhaust heat riser moves freely
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Hot air stove hose connected and intact
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Thermostatic air cleaner door operates
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Intake heat passages not blocked
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Carb base gasket correct (not insulating spacer only)
Headers + open element air cleaner + cold air = guaranteed icing.
What does not fix carb icing
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Bigger jets
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Richer idle mixture
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New carb
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Electric choke adjustment
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Blaming ethanol
Those only mask symptoms—temporarily.
Bottom line
If an Aisan-carb engine:
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runs fine cold
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runs bad warm
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fixes itself after sitting
That’s textbook carb icing, not a carburetor problem.
