Having purchased and repaired 2 of the L70 air-cooled Yanmars to make my own generator I can add some details here.
First as was mentioned some of these engines may have been replaced due to the refurbishment requirements and are in fact fine.
However the 2 I received had issues.
The first one skipped at high speed.
Teardown & inspection revealed filthy internals with a dark brown very hard to remove varnish coating on everything.
The camshaft injection lobe had deep corrosion pits that affected the injection event.
The cylinder bore was polished also.
Other than the camshaft & varnish coating on all internal parts it was serviceable.
I ordered a new camshaft, rings, seals & gaskets and cleaned all parts and honed the cylinder and reassembled.
The engine runs perfectly and is now a backup power source.
The main issue with these engines occurs when operated in cold environments with little to no load. With no load the engine cannot warm up enough to keep moisture from collecting in crankcase oil and it causes corrosion and cylinder glazing which is fatal to it's long term operation.
The corrosion damage appears to happen after the engine is returned to storage and allowed to sit for long periods of time.
The camshaft corrosion occurs at the point where the cam lobes and valve lifters come to rest together.
I would like to add that this scenario is also possible during the warmer summer months if operated in a low load condition for many hours.
The manufacture (Dewy I believe) makes a kit for cold weather operation that keeps a constant minimum load on the engine to keep the unit warm and helps minimize these situations.
Air-cooled engines are at the total mercy of the ambient temperatures and can prevent them from warming up to an acceptable level for proper operation.
The 2nd engine had similar issues although not as severe, it is currently being used as a backup generator.
90cummins
