What could cause an overly rich mixture during slow running?

An overly rich mixture during slow running in a reciprocating engine can occur when the priming pump is left open. When the priming pump is inadvertently left open, it can introduce excess fuel into the carburetor, which results in a richer air-fuel mixture than required for efficient combustion. This can lead to various issues, including poor engine performance, increased emissions, and fouling of spark plugs.

The other options highlight different conditions that would generally not lead to an overly rich mixture. Low fuel pressure typically results in a lean mixture rather than a rich one, as there wouldn’t be enough fuel to mix with the air. A float chamber level that is too low would also cause the mixture to lean out, since there would be insufficient fuel available for proper vaporization and combustion. Finally, a partially blocked main jet could restrict fuel flow, which might result in a lean condition rather than an overly rich mixture. Understanding these distinctions is essential for troubleshooting and maintaining the proper functioning of an engine.