O-200 cold weather starting

[Aviatard]

So for cold weather starts I suggest carb heat on, primer armed to help keep it going and at least 1000 RPM. The carburetor is low in the cowling with very little engine heat to help with atomization of the fuel. So with carb heat on you reduce the amount of cold air blasting into the carburetor.

I’ve seen this suggestion before but I don’t really understand how having carb heat on would help in starting a cold engine. Once it starts and runs for a bit, and generates some exhaust heat I can see maybe it helping to smooth out but before that how can it have any effect?

[AirFrame]

I’ve seen this suggestion before but I don’t really understand how having carb heat on would help in starting a cold engine. Once it starts and runs for a bit, and generates some exhaust heat I can see maybe it helping to smooth out but before that how can it have any effect?

For getting the first bang in the first cylinder, you're right, it probably doesn't help. But as soon as there's any combustion in any cylinder, the exhaust pipes start to heat up, and the carb heat will start to do something. And the longer any combustion continues, the more heat there will be. So it rapidly contributes to keeping the engine going, even if it doesn't help with the first few blades. Hence, keep a shot in the chamber (of the primer) and get it past the first few fires, and it'll rapidly improve.

[2R]

Cold Starting Liquids and solids don’t burn; only vapors burn. This is true whether a fuel is in the atmosphere or confined in the cylinder of an engine. So, starting a cold engine requires a fuel with a high enough volatility to form a vapor-air mixture in the flammable range. The lower the ambient temperature, the higher the volatility of the fuel must be, or starting aids, such as engine heaters or primers, must be used.
Smooth Running For an engine to run smoothly, the fuel-air mixture must be distributed evenly among all the cylinders. Even-mixture distribution is an inherent characteristic of fuel-injected engines but not of carbureted engines. In both types of engines, the volatility of the fuel must be high enough for it to evaporate and mix with air before combustion. In a carbureted engine, the fuel must evaporate and mix with air to form a uniform mixture in the fraction of a second it spends in the carburetor and inlet manifold before it is drawn into a cylinder. If the fuel is only partially vaporized, some cylinders, depending on engine geometry, will receive a mixture of liquid fuel, fuel vapor, and air that is too rich, while others will receive a mixture of fuel vapor and air that is too lean. In addition, because tetraethyl lead is less volatile than the fuel,2 the anti- knock additive will tend to be concentrated in the liquid fuel.
The uneven distribution of fuel and antiknock additive among the cylinders can result in rough running and poor fuel economy. It may even lead to knocking and engine damage. And if droplets of liquid fuel reach the cylinder wall, they will dilute the lubricating oil film, which may result in increased piston ring and cylinder wall wear.

Quote from the Chevron fuel book . Just something to think about

Also if you are bored or having trouble sleeping ; Stoichiometric air-to-fuel ratios of common fuels is worth a look .

[J31]

So for cold weather starts I suggest carb heat on, primer armed to help keep it going and at least 1000 RPM. The carburetor is low in the cowling with very little engine heat to help with atomization of the fuel. So with carb heat on you reduce the amount of cold air blasting into the carburetor.

I’ve seen this suggestion before but I don’t really understand how having carb heat on would help in starting a cold engine. Once it starts and runs for a bit, and generates some exhaust heat I can see maybe it helping to smooth out but before that how can it have any effect?

When operating with carbureted Continental's in very cold weather you may have to use carb heat to keep them going. Below -25 in the C-150 you might want to question whether you really want to fly, as that little engine does not make much heat to keep it running.