FAA Aviation Mechanic Powerplant (AMP)Ignition and Starting SystemsMedium
A technician is installing an ignition harness and connecting leads to the distributor block terminals of a magneto on a nine-cylinder radial engine. If the leads are connected to the wrong distributor block terminals, the most likely result is that the engine will
- Aexperience only a slight loss of power with otherwise normal operation
- Brequire the magneto's internal E-gap timing to be readjusted to compensate
- Coperate normally, since distributor block terminals are not firing-order specific
- Drun rough, backfire, or fail to start because cylinders fire out of the correct sequence
Show answer & explanationAnswer & explanation
Correct answer: D. run rough, backfire, or fail to start because cylinders fire out of the correct sequence
The distributor block terminals are numbered to match the engine's firing order, and each ignition lead must be routed from the correct terminal to the correct cylinder. Misconnected leads cause cylinders to fire out of sequence, producing severe roughness, backfiring through the intake or exhaust, or a failure to start.
Why the other options are wrong
- A. Firing out of sequence causes far more than a slight power loss; it disrupts the entire combustion sequence.
- B. E-gap timing is an internal magneto adjustment unrelated to which terminal a harness lead is connected to.
- C. Distributor block terminals are specifically numbered to match firing order; misconnection is not harmless.
Distributor Block Firing Order
The distributor block routes high-voltage pulses to spark plugs in the exact sequence of the engine's firing order; each numbered terminal corresponds to a specific cylinder's ignition event.
- Distributor rotor passes each terminal in the same sequence as the engine's firing order
- Ignition harness leads must be traced and connected to match this sequence
- Mismatched leads cause rough running, backfiring, or a no-start condition
Memory trick: Wrong wire, wrong turn — cylinders fire out of order and the engine coughs.