Engines always run best when they are fully warm.  However, it is the intake manifold that is the specific part of the engine that needs to be warm for the engine to run smoothly.

Gasoline enters the air stream to the engine via the venturi(s) in the carburetor.  Liquid gasoline is atomized in the venturi(s), meaning that it becomes extremely fine droplets.  Some (depending up manifold temperature and vacuum) of this gasoline mist will vaporize, meaning that it will draw heat from the air and evaporate into gasoline fumes.  This vaporization causes the air stream temperature to become colder and it's possible to the intake manifold to be covered in condensation or even frost.

Intake Manifold Frost

 

Heat must be added to the intake manifold to replace that absorbed by the vaporizing gasoline and this is the reason that the manifold heat control system (aka, heat riser) provides a hot spot under the carburetor.

The following 7:57 minute video explains this process at 4:41:

 

Some of the gasoline droplets will not make the 90° turn from the carburetor into the intake manifold and will drop out of the air stream to form a puddle on the intake manifold floor.  The hot spot below the carburetor vaporizes the puddled gasoline back into the air stream, which helps to ensure the engine gets the correct fuel mixture.

When the engine is first started, the intake manifold is cold and a greater proportion of the atomized fuel does not vaporize. To compensate, a carburetor has a choke to enrich the fuel mixture until the intake manifold warms up.  Until it does, gasoline will continue to puddle and will flow as liquid film to the cylinders.  Some of this liquid gasoline will flow into the combustion chambers and will then wash the oil off the cylinder walls as it creeps into the crankcase.  Over time, the lack of oil at the cylinders will cause them to gradually lose compression as they wear.  The cylinders getting the largest proportion of start-up liquid gasoline flow will show the most wear.

Some of the gasoline that reaches the crankcase will boil off when the oil becomes hot enough.  However, not all is boiled-off and the amount of residual gasoline in the oil will increase over time.  It generally takes longer for the oil to reach its operating temperature that the coolant.  Gasoline is a poor lubricant and will reduce the oil's viscosity and dilute the additives.  To reduce the oil's gasoline contamination, you should:

  • make sure that your manifold heat control system is working properly
  • make sure that your choke is working properly
  • make sure that heated air system (if so equipped) is working properly
  • plan your trips so that your engine remains at operating temperature for extended periods (at least 30 minutes)