What is a possible cause of an engine having a too rich mixture that prevents it from idling?

A prime line that is open can lead to a too rich mixture in the engine because it allows excess fuel to flow into the combustion chamber. When the primer line is left open, fuel may continually enter the engine, overwhelming the air-fuel mixture needed for proper combustion. This results in an overly rich mixture, which means there is too much fuel compared to air in the mixture. An engine that is idling requires a precise balance of fuel and air; when this balance is disrupted by an excess of fuel, the engine struggles to maintain a stable idle and may stall.

Other factors in the choices, such as a dirty air filter or a faulty fuel regulator, can also affect fuel delivery and air intake, but the specific issue of an open primer line leads directly to a rich mixture scenario that is particularly detrimental to the engine's ability to idle effectively. In terms of spark plug gaps, incorrect gaps can cause misfiring or poor ignition but do not directly correlate to a rich fuel condition affecting idle stability in the same way as an open primer line does.