1. Ignition System (The Heat Culprit)
Ignition components often fail specifically when they get hot. As wire resistance increases with temperature, a weak component can no longer provide a strong enough spark to combust the fuel under the pressure of driving.
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The Ignition Coil: This is the #1 suspect. When coils start to fail, they often work fine at idle but break down as they heat up, causing a "sputter" or "miss" under load.
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The Control Module (Duraspark): If your Ford has the electronic Duraspark system (the blue strain-relief module on the fender well), these are notorious for heat-soak failure. They often start acting up once the engine bay reaches operating temperature.
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Plug Wires: Check for any wires resting against the manifold. Heat can cause the insulation to break down, leading to arcing that becomes more pronounced as the engine gets hot.
2. Fuel Delivery & Vapor Lock
The Ford 300 inline-six has the intake and exhaust manifolds on the same side, which creates a lot of heat soak for the fuel system.
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Vapor Lock: If the fuel line is too close to the exhaust, the gas can actually boil in the line before it reaches the carb. This creates "bubbles" that cause the engine to sputter and lean out.
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Fuel Filter: A partially clogged filter might allow enough gas for a 700 RPM idle but can’t keep up with the volume needed to move a 4,000-lb truck down the road.
3. Carburetor Specifics (Carter YF)
If the ignition and fuel pump check out, the Carter YF has a few quirks that match your symptoms:
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The Accelerator Pump: If the sputter happens specifically when you step on the gas to accelerate, the rubber diaphragm in the accelerator pump may be cracked or hardened.
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The Metering Rod: The YF uses a vacuum-operated metering rod to control fuel flow. If the vacuum piston is sticking or the rod is misadjusted, the engine will stay in "lean mode" when it needs more fuel to drive, causing a major stumble.
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Float Level: If the float is set too low, the bowl might stay full enough for idling, but once you start driving, the engine "drinks" the fuel faster than the bowl can refill, causing it to starve.
My Recommendation
Before tearing into the carburetor:
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Check the coil: If you have a spare or can borrow one, swap it out. It's the most common cause of "worse when warm" sputtering.
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Inspect the fuel lines: Ensure they aren't touching the exhaust manifold.
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Check for vacuum leaks: Use a can of carb cleaner to spray around the base of the carb and the intake manifold while idling. If the RPMs change, you've found a leak that is leaning out your mixture.
If those are solid, it’s likely time for a kit in that Carter YF to replace the accelerator pump and check that metering rod adjustment.
