It sounds like you're experiencing a classic case of fuel percolation or flooding after shutdown with your triple Holley 94 setup on your '56 Y-block. The gas dripping from the center discharge tube and hard starting are key indicators.
The advice you've received touches on the two most likely culprits: heat soak and excessive fuel pressure.
Understanding the Problem: Fuel Dripping After Shutdown
When you shut down a hot engine, airflow stops, and the heat from the engine and manifold radiates upwards, increasing the temperature of the carburetors.
- Fuel Percolation (Boiling): If the carburetors (specifically the fuel in the bowls) get too hot, the fuel can start to boil and expand. This forces fuel up and out of the discharge nozzles and venturis, causing it to drip into the manifold. This is the "leak onto manifold" you're seeing.
- Flooding: This dripping over time floods the intake manifold. When you try to restart, the engine is overly rich, making it hard to start because there's too much fuel and not enough air.
Addressing the Causes:
1. Heat Soak (Most Probable Cause)
Your source is likely correct in betting on heat soak, especially with an aftermarket manifold that might conduct more heat or position the carbs closer to the engine.
- Why it happens: Modern fuels, particularly those with ethanol, have a lower boiling point than older leaded gasoline. They are much more susceptible to boiling in the carburetor bowls.
- Solutions for Heat Soak:
- Insulating Carburetor Spacers/Gaskets: This is often the most effective first step. Install phenolic or wood spacers (1/2" to 1" thick) between the carburetors and the intake manifold. These act as a thermal barrier, significantly reducing heat transfer from the manifold to the carbs.
- Fuel Type:
- Non-Ethanol Fuel: As suggested, switching to non-ethanol gasoline (if available in your area) can help considerably due to its higher boiling point.
- Premium Fuel: Some premium fuels might have slightly different volatility characteristics that could help, but it's not a guaranteed fix.
- Engine Compartment Heat Management:
- Radiator & Fan Efficiency: Ensure your cooling system is operating optimally (clean radiator, proper fan shroud, good fan clutch or electric fan setup). A cooler running engine overall means less heat radiating upwards.
- Lower Temperature Thermostat: This can help the engine run slightly cooler, reducing overall under-hood temperatures.
- Exhaust Heat Management: If your exhaust crossover in the intake manifold is open, consider blocking it off (if compatible with your manifold and climate) to reduce direct heat transfer. Exhaust wraps or ceramic coating on headers (if applicable) can also help manage under-hood temps.
- Ignition Timing: While generally optimized for performance, excessive initial timing can increase engine heat. Ensure your timing is set correctly.
- Fuel Line Routing: Ensure fuel lines are not routed directly over hot engine components. Insulate them if necessary.
2. High Fuel Pump Pressure
The Holley 94 (and most vintage carburetors) are designed for very low fuel pressure.
- Ideal Pressure: The recommended fuel pressure for a Holley 94 is typically 2.5 - 4.5 PSI (pounds per square inch). Your source's recommendation of "5 lbs. or less" is good advice.
- Why high pressure causes leaks: If the fuel pump provides too much pressure, it can overwhelm the needle and seat in the carburetor's float bowl. This forces the needle off its seat, leading to an overfilled bowl and fuel overflowing (or pushing past the needle) into the venturis, even when the engine is off.
- Solution:
- Install a Fuel Pressure Regulator: If you don't have one, install a high-quality, low-pressure fuel pressure regulator in the fuel line before the carburetors. Adjust it to deliver no more than 4 PSI.
- Check Existing Regulator: If you have one, verify its setting with a fuel pressure gauge connected directly at the carburetor inlet.
Power Valve Selection (2.5 HG)
The power valve (PV) opens at a specific vacuum level to enrich the fuel mixture under load.
- Why 2.5 "Hg (inches of mercury) is often recommended for multi-carb setups:
- Low Idle Vacuum: Engines with multiple carburetors, especially if they have a performance camshaft, often produce lower manifold vacuum at idle and light cruise compared to a single large carb.
- Preventing Premature Opening: A power valve rated at 2.5 "Hg (or sometimes 3.5 "Hg) is designed to stay closed unless manifold vacuum drops significantly. This helps prevent the power valve from opening prematurely at idle or light cruise, which would cause an overly rich condition, rough idle, and potentially surging (though less likely to cause dripping after shutdown unless it's a constant, severe over-richness).
- Diagnosis Aid: While an incorrect power valve can cause drivability issues like surging or richness during operation, it's generally not the direct cause of fuel dripping after shutdown unless it leads to severe flooding during operation that carries over. However, ensuring you have the correct power valve is crucial for overall performance and avoiding other tuning headaches.
Recommended Action Plan:
- Check for Heat Soak (Immediately after shutdown): Touch the carburetors' main bodies immediately after you shut the engine down after a drive. If they are too hot to comfortably touch for more than a few seconds, heat soak is definitely a major contributor.
- Install Insulating Spacers: This is your highest priority if heat soak is confirmed or suspected. Phenolic or wood spacers will make a significant difference.
- Verify Fuel Pressure:
- Install a fuel pressure gauge between your fuel pump and the carburetors.
- Start the engine and check the running fuel pressure.
- If it's over 4.5 PSI, install and adjust a good quality, low-pressure fuel pressure regulator to maintain 3-4 PSI.
- Inspect Floats and Needle/Seats:
- Ensure the float levels are correctly set for all three carbs. Even a slightly high float level can contribute to overflow.
- Check the condition of the needle and seat assemblies. Worn needles, seats, or dirt can prevent proper sealing, allowing fuel to leak past. Consider replacing them if they look worn.
- Review Fuel Type: If you can easily access non-ethanol fuel, try it.
- Consider Cooling System Upgrades: If the above steps don't fully resolve it, look into improving your engine's cooling efficiency.
By systematically addressing both heat management and fuel pressure, you should be able to resolve the fuel dripping and hard starting issues with your triple Holley 94 setup.
