Checked oil breather extension that I put on before last flight. It caught a small amount of oil (Photo #1), and the inside of the cowling was essentially dry, except for a tiny streak emanating from the airbox (UV Photo #2). This is telling me that more than likely most of the oil appearing inside the lower cowl for the last couple of test flights was from blowback from the too-short breather tube. The very small drip from the bottom of the airbox seems to be able to be explained by the article copied below.
Began custom fitting wing root area plugs using foam from old seats. See photo #3. If this works to seal out cold air then I will fabricate cosmetic covers from materials from the first (incorrect) interior kit.
Oil Out the Air Box By John Propst with Technical Review by Bill Pancake Abstract: This article discusses an issue related to engine oil leaking out the carburetor air box on small continental aircraft engines. Background An aircraft with a small Continental engine is restored and the engine has a major overhaul. Following the restoration, the plane and engine performs like a new one. It flies true and the engine performs better than expected with very little oil consumption (say 1 to 2 quarts in 25 hours). Yet the owner notices engine oil leaking from the air box. This article describes one scenario that explains the cause for this leakage. The new cylinders, pistons, and piston rings available from several manufacturers do a much better job of sealing the piston to the cylinder. This is witnessed by extremely high differential compression tests and very low oil consumption. This results in higher intake vacuum levels, especially when the engine is at idle with the carburetor butterfly valve nearly closed. This high vacuum level during idle in turn results in a high differential pressure across the intake valve guides. During idle, especially during and following landing of the aircraft, this sucks oil into the intake manifold system from the cylinder head. This oil then mig
