I have carburetor boil-over

April 1, 2010 | By Richard Prince


I have a 1982 Mercury Cougar station wagon with the 200 cid six-cylinder engine and a single-barrel Motorcraft carburetor that has had a boil-over problem for many years.

Typically, after running at full operating temperature the carburetor will dribble fuel out of the venturi after the engine is shut off for about 10 to 15 minutes. The fuel has never dribbled out of the fuel bowl vents, just the venturi.

The dribbling of fuel will last for about two minutes and, as expected, it results in flooding causing hard hot starting conditions and a big puff of black smoke out of the exhaust.

I’ve tried numerous corrective actions including lowering the float 1/8” below specifications, which is even with the top of the fuel bowl.

I’ve changed the timing both retarded and advanced along with the 10-degrees base timing. I’ve manufactured a heat shield and ensured that the choke is adjusted properly, all emissions systems are functioning properly and the cold start heat valve in the air cleaner throat is working properly.

I’m at a loss as to how to fix this problem. These rich starts cannot be good for

the catalytic converter. I’ve even performed the vacuum checks to check the converter for blockage and all is normal and at 2000 feet in altitude the engine vacuum is at 19-inches at 700 rpm. The car otherwise runs like a new car.

Is this something that I have to live with? I’m sure this was not acceptable as a new car from Ford. The car will run all day at 70-75 mph and still delivers acceptable fuel mileage. It gets 18 mpg in town and 23 mpg on the highway. Is this an ethanol fuel problem or a heat problem? The temperature under the hood doesn’t feel excessive at all.

What am I missing?


It sounds like you’re experiencing a classic case of fuel vaporizing in the carburetor. The volatility or tendency of the fuel to vaporize is influenced by a number of factors, including ambient air pressure, fuel temperature, and the volatility of the specific fuel you’re using.

All else being equal, gasoline is more likely to vaporize when ambient air pressure is lower.

So, for example, a given fuel is more likely to vaporize at higher altitudes than it is at lower altitudes because ambient air pressure decreases as altitude increases. Similarly, gasoline is more likely to vaporize as the temperature of the gasoline increases. Different gasoline blends have different boiling points, with winter blends typically having a lower boiling point that makes them more likely to vaporize.

The addition of ethanol to gasoline raises its Reid Vapor Pressure, which means it makes the gas more volatile.

From a practical perspective you probably can’t do much to improve the quality of the fuel in your area or the altitude where you drive the car.

However, you can do something about the fuel’s temperature, which is good considering that excessive fuel temperature is the most likely cause of the problem anyway. You state that the temperature under the hood “doesn’t feel excessive at all,” which leads me to believe that you haven’t actually measured the engine temperature. This is certainly something that you should do. Even if the engine temperature is not excessive, the fuel temperature still might be.

Make certain that all fuel lines are as far as possible from exhaust pipes and other sources of heat. If necessary, wrap the lines with insulating material available from any race equipment supplier and some auto parts stores. If possible, measure the temperature of the intake manifold adjacent to and beneath the carburetor. This can easily be done with an infrared thermometer.

Try installing an insulation block between the carburetor and intake manifold to shield the fuel from engine heat.