When you’ve synchronized your airflow, checked for external vacuum leaks, and verified that your idle mixture screws are responsive, yet you’re still pulling carboned-up plugs from the rear cylinders, the problem is likely hidden inside the carburetor or downstream in the manifold.
If your engine is "hunting"—fluctuating between 800 and 1100 RPM—it is often a sign of a mixture that is too rich to burn consistently, causing the engine to load up and then clear itself repeatedly.
Here is a systematic approach to finding the "hidden" culprit in your fuel system.
1. Inspect the Power Valve Internals
Even if you have flat-filed the power valve unit to ensure a perfect seal against the body, the internal mechanism may be failing.
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Diaphragm Failure: If the internal diaphragm is ruptured, raw fuel will be pulled directly into the intake tract via the vacuum channel.
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Spring Tension: A weak spring can allow the valve to stay open at idle, even when vacuum is high.
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The Test: Temporarily plug the power valve vacuum channel. If the idle stabilizes and the richness clears, the valve is leaking internally.
2. Verify Fuel Pressure vs. Float Level
Setting the float level is a dry measurement, but the fuel level is a dynamic one. High fuel pressure can overwhelm the needle and seat, forcing the fuel level higher than intended.
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Pressure Check: Most vintage-style carburetors prefer 2.5 to 3.5 PSI. If your pump is pushing 5 PSI or more, it will cause "nozzle drip."
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The Visual Test: Look down the throat of the carb at idle. If you see fuel beading or dripping from the venturi clusters (the brass nozzles), the fuel level in the bowl is too high, regardless of where the float is set.
3. Check for Internal Vacuum "Cross-Talk"
You’ve ruled out external leaks with brake cleaner, but internal leaks can occur where the carb meets the manifold.
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Gasket Matching: If the base gasket is the wrong shape, it can allow vacuum to leak between the idle circuit and the power valve circuit.
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Signal Blockage: If the power valve isn't receiving a clear vacuum signal because of a misaligned gasket, it will remain "open" and dump extra fuel into the rear cylinders.
4. Isolate Vacuum Accessories
The back three cylinders are often the first to be affected by leaks in vacuum-assisted components.
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Brake Booster & PCV: If your brake booster or PCV valve is plumbed into the rear of the manifold, a small leak in those diaphragms or hoses will cause the rear cylinders to behave differently than the front.
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The Test: Disconnect and plug the vacuum lines for the booster and PCV. If the "hunting" idle disappears, the leak is in the accessory, not the carb.
5. Evaluate Ignition Health
If the carburetor is delivering the correct mixture but the spark is too weak to ignite it, the plugs will still carbon up.
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Rear-Bank Weakness: Check the condition of the spark plug wires and the distributor cap terminals specifically for the rear three cylinders. A weak spark mimics a rich condition because the fuel isn't being fully burned.
