What is a likely cause of a reciprocating engine backfiring through the induction system at low RPM?

A likely cause of a reciprocating engine backfiring through the induction system at low RPM is a lean mixture. When the air-fuel mixture in a reciprocating engine becomes too lean—meaning there is not enough fuel relative to the amount of air—the combustion process can become unstable. At low RPM, the lower level of fuel can lead to incomplete combustion or misfires.

This incomplete combustion can result in unburned fuel igniting in the intake manifold, causing a backfire. It is important for engines to maintain the correct air-fuel mixture for smooth operation, particularly at low RPMs where the fuel delivery might not be as consistent.

In contrast, an idle mixture that is too rich would typically lead to rough running but not necessarily backfiring through the induction system. Similarly, while a clogged derichment valve might affect the mixture, it does not specifically address the lean condition primarily responsible for backfiring. Lastly, excessive fuel atomization generally facilitates better combustion rather than causing backfiring, as it allows for a more uniform and stable combustion process.