lean-out problem only gets worse with the Main nozzle

This zjautoparts method of determining the actual fuel level (against the float height level) uses an adapter that fits into the drain hole of the float chamber. A clear fuel line with graduations is fitted to the adapter. Once the fuel is turned on, fuel flows up the pipe until the needle valve assembly closes the supply; the fuel height in the tube corresponds to the fuel height inside the float chamber.

How far the secondary air valves open depends on how deep into the throttle your foot is. Air valves reduce hesitation by allowing fuel to be drawn into the secondaries before wide-open throttle. They prevent a transitional lean condition where the engine falls on its face when you’re going for the gusto. The trick to getting a smooth transition is getting air valve adjustment right to where they don’t open too quickly.

It can be considered ironic Ford never completely cured the 4300’s woes. Like the Rochester Quadrajet (also known as the “Quadrabog”), the 4300 and 4300D suffered from hesitation and stumble because Ford just never could correct the transition from idle to wide-open throttle. There were flat spots, surging, and stumble that drove people crazy. What’s more, hot start issues due to improper float calibration kept hoods open.

Hot starting problems are rooted in what happens to fuel as it sits in a hot fuel bowl. Heat causes fuel to expand in the bowl to where it dribbles into the throttle bores and floods the intake manifold plenum. In theory, you can lower the float level and cure this problem, then, adding insult to injury with lean conditions. Lowering the float level only makes the lean out problem worse. This hot start condition and lean-out problem only gets worse with the Main nozzle.