I have two OS 25S engines, both originally RC.  One works great -- starts easily, runs consistently, probably delivers more power than it should, etc. 

The other one appears to be a turd, and I can't tell why.  It may just be clapped out, but I'm seeing symptoms that confuse me.  The biggest confusion is that when the engine is cold, it has plenty of compression -- it flips over with a nice satisfying "happy engine" noise, it takes a few seconds to leak down if you hold it at TDC, all that stuff.  Compression is down significantly when it's hot, but still not too much, AFAIK.  Even when cold, though, it's exceedingly hard to start, and when I do get it running it'll go like gangbusters for a while and then just suddenly flame out -- it doesn't sag, or show signs of being too lean or too rich, it just flat dies, and suddenly.

The backplate gasket seems to be in good shape.  The crankshaft fit seems to be good, although it has about 18 thou of end play vs. 13 on my good engine, and when you pull it out it "clacks", which the good engine does not do.  The only thing odd is that with the head off, as I turn it through a stroke, it doesn't seem to wait to "puff" air up from the crankcase in the normal way -- instead, it starts "pre leaking" air a few degrees before the piston gets down to the transfer port, then gives a weak "puff" once it gets down.

So -- is the thing just clapped out, only in an unusual place in the liner?  I don't have any great love for the engine, but it'd be nice to get it working if I can do so without great expense.