Carburetor Percolation

Have you ever left a pot of coffee on the stove for too long and watched it bubble over? That's similar to what happens during carburetor percolation. When the engine's heat boils the fuel in the carburetor, causing it to percolate or "bubble over," you've got trouble.

Top Causes of Carburetor Percolation

1. Excessive Under-Hood Heat

• High engine temps can radiate heat to the carburetor.

• Poor airflow or restricted heat shields make the problem worse.

• Common in tight engine bays or vehicles without proper venting.
  

2. Modern Ethanol-Blended Fuel

• Ethanol has a lower boiling point than pure gasoline.

• E10 and E15 vaporize more easily, especially in older fuel systems not designed for it.

3. Improper Carburetor Insulation

• Missing or incorrect base gaskets or heat spacers allow more heat transfer from the intake manifold.

• Some carburetors require phenolic spacers to prevent heat soak.

4. Hot Soak After Engine Shutdown

• After turning off the engine, coolant and airflow stop.

• The temperature under the hood can spike, boiling the fuel in the bowl and causing flooding or vapor lock.

5. Fuel Line Routing

• Metal fuel lines that pass too close to the manifold or headers can heat the fuel before it reaches the carb.

• This increases the chance of vapor formation before or inside the carburetor.

6. High Float Level or Leaking Needle & Seat

• If the float is too high or the needle and seat are worn, boiling fuel may overflow into the intake.

• This can cause hard starting or stalling after heat soak.

How to Prevent Carburetor Percolation

• Use ethanol-resistant components in the carburetor

• Install a thick insulating base gasket or phenolic spacer

• Reroute fuel lines away from hot engine components

• Install a heat shield under the carburetor if space allows

• Lower float level slightly (within safe range)

• Consider a carburetor fan or hood vent to improve airflow

• Use non-ethanol fuel if available, especially in hot climates

Conclusion

Carburetor percolation is a heat-related issue that can be solved with proper insulation, correct float adjustment, smart fuel routing, and ethanol-compatible parts. If your classic car experiences hard starting or flooding after shutdown, chances are good percolation is the cause.

Signs of Carburetor Percolation

Carburetor percolation can manifest as hard starting, rough idling, and even flooding. It's like your car is telling you, "Hey, I'm not feeling well. Something's wrong."

Impact of Carburetor Percolation

Performance Issues

Percolation can result in stalling, engine hesitation, and overall poor performance. Imagine running a marathon while having the flu; it's essentially the same scenario for your engine.

Fuel Economy

Uncontrolled percolation can cause fuel wastage and poor fuel economy, akin to a tap leaking precious water drop by drop.

How to Prevent Carburetor Percolation

Regular Maintenance

As with our health, prevention is better than cure. Regular check-ups, clean-ups, and replacements of old parts can ensure your carburetor stays in top form.

Insulation Methods

Strategically placed insulating spacers or heat shields can also help keep excess heat away from the carburetor, much like using an umbrella to shield from the sun.

• One of the best ways, but a lot of work, is to run a 3rd line from the fuel pump back to the gas tank. This extra line will help cool the fuel, allowing the fuel pump to pump gas instead of vapor. There are some fuel filters out there that include a return port. There are also return regulators available.
• Put heat wrap around the fuel line in the engine compartment.
• Put a spacer under the carburetor mount. These are hard to find so you will probably have to make one. Yup, a mill would be handy about now. And you will probably need to put longer studs in the manifold to accommodate the extra height.
• Instead of a spacer add extra mounting gaskets. A couple of 1/32 thick gaskets isn’t going to do much. Hopefully thicker gaskets will be available for your engine.
• Add a helper electric fuel pump. Just before startup, you run the electric pump for a few seconds, then turn it off. Use only a low pressure pump. 7 lbs max. You may end up also adding a regulator because you don’t want the pump to overrun your mechanical pump. Most 1 & 2 bbl carburetor run 4-4.5 lbs. Bigger carburetors might handle 5-6 lbs.
• Cool the engine with a cooler thermostat, bigger fan, or bigger radiator.
• Try different brands of gas. Different octanes won’t help you here.
• Use non ethanol gas if you can find it.
• Remember when we used to put wooden clothes pins on the fuel line? The clothes pins act like a heat sync, drawing heat away from the fuel line and my wife says the wooden clothes pins are still available at the dollar store.