Carburetor icing causes stalling

In cold, damp weather, around 35 to 40 degrees F (or colder), you may experience this problem; your engine starts up just fine and runs well for about five minutes. Then it loses power and no matter what you do you can't make it accelerate. Sometimes it just plain stalls out, you pull over and try to start it, but it won't start. You may smell gasoline like the engine is flooded - that's because it is. You wait five or ten minutes and try it again and it starts just fine and runs like a top for the rest of your journey (if you are lucky). Under extreme conditions it may also continue to act up for the rest of the day. It depends on atmospheric conditions and the degree to which your system is faulty. Here's the solution to your problems.

Examine the air inlet to the carburetor. Look for a diverter tube, about an inch and a half in diameter, that goes from the carb inlet to the exhaust manifold. There is also a tube that runs to the front of the car - these tubes split at a "Y" or "T" intersection. At the split of the "Y" or "T" there is a vacuum operated valve that diverts the air intake to come from the rapidly heating exhaust manifold area rather than the cold, wet air from the front of the car. This is done to prevent icing inside the throat of the carburetor or the intake manifold.

In light weight reciprocating engine powered aircraft, Cessna, Pipers and the like, we have a knob to pull that is labeled "carb heat" which is used under exactly the same conditions of temperature and humidity, and particularly on the decent from altitude with power reduced, to prevent icing in the carburetor. That is the manual version of the vacuum operated diverter valve in your car.

What happens is this: The air entering the carburetor expands rapidly when going into the venturi in the carburetor which super cools it (Boyle's or Charles' Law of adiabatic expansion or something like that). If there is sufficient moisture in the air, it freezes and deposits ice on the metal surfaces inside the throat of the carb. That chokes the motor causing the stalling and excessive fuel consumption. Wait ten minutes and the residual heat in the engine might melt the ice and you are on your way again (if you are lucky). Sometimes the cycle repeats itself as ice once again builds up inside the intake system. That diverter valve will continue to ensure that the air entering the engine is at the correct temperature for proper function of the engine. If it never lets warm air into the carburetor then you will suffer performance problems, at a minimum poor gas mileage and at worst case a totally flooded out engine which will not start up until the ice melts.

Make sure the diverter valve is functioning!! Make sure the tubing going from that diverter valve to the exhaust manifold is there, and not split open or torn. If it is missing completely or broken, or if the vacuum line to the diver valve is disconnected, or if the diaphragm in the diverter valve is skrogged, then that nice warm air around the exhaust manifold isn't sucked into the carb and icing occurs. Also make sure that the thermostat is functioning so that the engine warms up to the correct operating temperature.

One final note; there is always the exception to the rule. I just got a mailnote from a reader who was experiencing what seemed like carb ice even after the engine had reached operating temperature. Same thing - power loss, pull over, wait a few minutes and it started up and ran again. The only difference was that this was after a long time driving down the highway at 75 MPH. It turned out that he had a leak in the vacuum line to the diverter valve, which was therefore stuck in the "cold" position, and when it was repaired there was no longer a problem. This was an 84 Honda Civic. It appears that the engine in that car uses a modulated diverter and measures the air temperature in the intake manifold constantly and then adds warm air if the conditions dictate. So, the problem may happen even after the engine has reached operating temperature.

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