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The Franklin is usually a very smooth and sweet sounding engine. Looked after they can give good service life.
Things to watch for: 14 mm threads on spark plugs instead of the usual 20 mm and the meat can let go and blow a spark plug out – thread lube / anti seize compound, proper torque – don’t over torque.
Intake seals will leak and engine will run lean on that side and eventually backfire blowing intake elbow out a quarter inch or more – with full throttle, full rich and carb heat (enrichens) may still get 40 to 50 % power until you pull throttle back for landing at which point – at what ever altitude – the engine will quit completely – make sure seals are in good condition and no deformation of structure, wipe with alcohol and run a bead of clear silicon around the outside so the sucking action works with the sealant.
If still have cast iron valve guides, and aircraft sits for a prolonged period – depending on humidity and other factors – they can rust and seize or restrict the valve stems and can cause a valve to break and drop into cylinder – partial power loss and big pucker factor – as your Engineer/A&P, I “can’t” recommend using a can of STP every second oil change as STP is not an approved product …a cup of Marvel Mystery Oil in the gas, however, is approved and apparently helps avoid this problem. Otherwise, good habit if the engine hasn’t run in a long time, pull the valve covers and slowly, genitally rotate prop by hand and watch for rocker arm action and associated valve response after clearance gap is closed.
Fuel starvation; the Frank tends to be a fuel hog compared to near equivalent Lyc or Cont. if you’re not use to the Frank it can surprise a person – on my 165 Hp, I found that doing circuits it would easily burn 10 gallons per hour and with a fine pitch prop, in cruise it wasn’t much better. I always planned for 10 gallons an hour and I never ran out of gas. Having said, as with all light single engine aircraft, best to keep at least one eye out for an open field.