Early aircraft engines were not required to have dual ignitions. For example this flathead that powered the 'Arrow'It might be aircraft. Planes are required to have a twin plug setup.
It would be a BIG project to set up a twin firing setup, when an automobile didn't really need it. Probably, that is why both plugs are not wired.
MIKE![]()

Perhaps, in the early days, it was not required. I have read, where it is required, now. I'm sure that a dual system has saved many lives. What was the year of the "ARROW" ?
SHADETREE - You are a student of flying, please step in and educate us.
MIKE![]()
...The engine that 51 Merc pictured is definitely an aircraft or air boat engine. Note the propeller flange and the gear reduction...
Bud
They appear to be Fenton, see the casting just under the radiator hose.
What radiator hose. Boat or airplane both are/were air cooled. JMO
The 'tilt' is because of the angle the picture was taken, the engine is indeed out of an aircraft, not a boat.Shadetree, The carburetor in 51 Merc's picture appears to be tilted backward. Would this mean that it more likely was a boat engine than a plane engine?

Not very many air-cooled boat engines out there. DD