A. Some aircraft engines create carburetor ice more than others. Your Continental O470 aircraft engine in the Cessna 182 forms carburetor ice in conditions where a Lycoming O-320 engine in a Piper Cherokee 140 doesn't.

 In a carburetor there are two places where ice typically forms; in the venturi where the air pressure is decreased and the fuel introduced, and the second place is down stream of the throttle butterfly.

Carburetor venturi

 As the aircraft carburetor vaporizes fuel, it cools the intake air by evaporation. The carburetor venturi also cools the air by adiabatically expanding the intake air, known as the Joule-Thomson Effect. If the air temperature drops below the dew point, moisture in the air condenses into water droplets. Thus, water can be present in the carburetor even when flying in clear skies.

 Ice forms near the aircraft carburetor butterfly when water droplets strike parts of the carburetor (typically the butterfly and venturi) that are freezing. Freezing is determined by: the outside air temperature, the temperature drop, and heat absorption from the engine. With the throttle partly closed, such as in a low power descent, you may have a 10" Hg or more pressure drop across the throttle butterfly.

 The more gasoline the carburetor evaporates, the colder the carburetor. As your engine idles at the end of the runway, little fuel is evaporating and the carburetor's temperature may be above freezing. Adding takeoff power increases the rate of fuel evaporation. Now the carburetor may be cold enough to form carburetor ice. Your carburetor can get pretty cold. The heat loss from evaporation of gasoline at the stoichiometric ratio creates a theoretical temperature drop of 40 degrees F. The adiabatic expansion of gas across the carburetor's venturi also lowers the temperature. Thus, you could drop the carburetor temperature to freezing and form carburetor ice at an ambient temperature of 72 degrees or higher. Stoichiometric is the leanest possible mixture. At richer mixtures the ambient temperature at which carburetor ice forms is even higher - Lycoming publishes a temperature range of twenty to 90 degrees F. for carburetor ice.

 Adding alcohol to gasoline dramatically increases the ambient temperature where ice forms. Evaporating methyl alcohol creates a temperature drop of 300 degrees F. As a result, carburetor ice occurs over a wide range of temperatures.

 Your Continental aircraft engine forms carburetor ice at a higher outside air temperature than your Lycoming engine since the carburetor on a Continental engine absorbs less engine heat. Continental mounts the carburetor to the intake pipes away from engine heat. Lycoming mounts the carburetor to the oil pan where it absorbs heat from the engine oil. One side effect of forming carburetor ice at a higher air temperature is that the warm air holds more moisture than cold air; therefore, you have a potential for greater carburetor icing with Continental engines than with Lycoming engines.

 In Bendix pressure carburetors such as the PS series used on Lycoming aircraft engines,  the fuel is injected into the cylinder's intake manifold so you don't get the cooling effect of the fuel vaporizing and possibility of carburetor ice is reduced.

Pilot Description of Carburetor Ice

I used to fly behind an O-300 (big brother to your O-200) and carb ice was a regular occurrence.  My typical experience was similar to your training, a gradual rpm drop which was eventually followed by roughness if not corrected.  The rpm drop was often disguised if I were climbing or maneuvering since that tends to vary the rpm with a fixed pitch prop.

  Upon application of carb heat, I usually got some increased
roughness after about 10 sec. as the engine ingested the ice, then smooth operation within 20-30 seconds.  After turning the heat off, RPMs rose back to normal.

  The only time I had an instant improvement it turned out to be a
blocked intake (ice on the filter).  The engine smoothed out
immediately when carb heat was applied, then immediately back to roughness when heat was turned off. There are two things happening when you select carb heat.  Your sending hot, less dense air to the carb (richening the mixture) and you're also bypassing the air filter.
  An immediate improvement usually indicates that either the richer mixture or the bypassed intake solved the problem

There is a product that helps prevent fuel system icing called "EGME sold under the trade name  "Prist". Prist is well known in the jet world where aircraft without heated fuel tanks are generally required to use Prist Hi-Flo (or generic equivalent) to prevent fuel system icing. Prist for piston engines is called "Lo-Flo". The only difference between the products is the dosage rate. Recently Prist Lo-Flo has been discontinued by the manufacturer.  Contact Sky Ranch for sales details.

Lycoming Service Letter L172B dated January 25, 1980 says in part "...tests conducted with EGME used as an ice inhibiting fuel additive have shown to be satisfactory for use in all Avco Lycoming engines with no adverse affects on engine operations when used in concentrations up to 0.15% by volume in Aviation Gasoline. The use of EGME as a fuel additive in Avco Lycoming engines is approved by Avco Lycoming and the FAA." 

Continental Service Bulletin M81-11 Rev. 1 states in part "...Under certain ambient conditions of temperature/humidity, water can be supported in the fuel in sufficient quantities to create restrictive ice formation along various segments of fuel system." "... ethylene glycol monomethyl ether compounds conforming to military specification MIL-L-27686E, if approved for use in the aircraft fuel system by the aircraft manufacturer, can be added for this purpose. The ethylene glycol monomethyl ether compound must be carefully mixed with the fuel in concentrations not to exceed 0.15 percent by volume."

Continental issues the following warning that is important to understand:

"Mixing of the EGME compound with the fuel is extremely important because concentration in excess of that recommended (0.15 percent volume maximum) can have a harmful effect on engine components. Use only blending equipment that is recommended by the manufacturer to obtain proper proportioning."

In Jet aircraft Prist is always blended as it is only partially fuel soluble. Use in aviation gas requires careful blending to guarantee appropriate concentration.