You’re describing a cold-weather, high-speed lean event. Tell it like it is: this is almost never the fuel pump, and the fact it only happens when it’s cold is the biggest clue.
Your 1984 Ford F-250 with the 351W and a Motorcraft carburetor (almost certainly a Motorcraft 2150) is running into one or more of the issues below — listed in order of likelihood.
Carburetor icing (VERY common, often overlooked)
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Happens only in cold or chilly weather
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Occurs at steady highway speed
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Sudden loss of power
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Lean backfire
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Comes back if you:
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Take it out of gear
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Pump the throttle
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Let it rev and clear itself
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Why it happens
At highway speeds, airflow through the carb drops the internal temperature below freezing. Moisture freezes inside the venturi, restricting airflow and fuel delivery. Pumping the throttle temporarily breaks the ice loose.
What to check (do this first)
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Heat stove tube from exhaust manifold to air cleaner
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Missing? Rusted through? Not connected? That’s your problem.
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Thermostatic air cleaner door
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With engine cold, it should pull hot air, not cold.
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Vacuum motor on the air cleaner
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If it doesn’t move, the door stays open ? icing guaranteed.
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A LOT of these trucks lost the hot-air setup over the years. People toss them thinking they’re emissions junk. They’re not.
2?? Choke pull-off / choke opening too fast
When cold:
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Choke opens too early
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Mixture goes lean under load
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Backfire through carb
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Feels like fuel starvation
Check:
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With engine cold, choke plate should start closed
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As engine warms, it should open slowly and smoothly
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The vacuum pull-off diaphragm must HOLD vacuum
If the pull-off leaks, the choke snaps open ? lean stumble at cruise.
Weak float level or borderline fuel delivery (cold exposes it)
Cold fuel is denser and vaporizes worse.
If:
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Float level is slightly low
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Needle & seat is marginal
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Fuel line routing is poor
You’ll only notice it when it’s cold and cruising.
What to verify:
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Float level set to spec (don’t eyeball it)
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Tank vent working (no vacuum forming)
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Rubber fuel lines not collapsing internally
Still: this is secondary, not primary.
EGR valve partially open when cold
Less common, but possible.
If the EGR:
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Leaks exhaust into intake at cruise
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Causes lean misfire/backfire
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Acts worse in cold air
Quick test: temporarily block EGR vacuum line and road test.
What it is NOT
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Bad fuel pump (you replaced it, and failure wouldn’t be seasonal)
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Ignition module (would act up hot, not cold)
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Vapor lock (opposite weather)
Straight answer
You almost certainly have carb icing due to a missing or non-functional hot air intake system.
Second most likely is a choke pull-off issue.
One More Possibility
A small vacuum leak will make the carburetor run lean and create popping. As the engine warms up the metal expands and closes up the leak.
Use carb cleaner and spray around the throttle shaft ends. Any rpm change indicates a leak and they need to be bushed.
