I recently replaced 2 x GMS47 engines in my planes with 2 x OS47AX's for similar reasons as above.

The GMS's were near new, run in, but were causing increasingly slow take-offs and flight. They started suffering from underpower, dead stick landings with burnt plugs and coming in hot and smelly. No they were not set lean - this is where the plot thickens.

As I richened the needle from peak and the rpm's dropped, I found the more I richened the needle, it didn't appear to go into 4 stroke mode as most engines do. This made me suspicious that perhaps somehow it was leaning back out again although I was richening it. Now, I wasn't setting it too rich either for flight like slobering rich as this is not good for an engine either.

I also found the GMS's noisy and as noise is an issue at our field, it was easier just to junk them. The new OS46AX's run smoothly, quietly, with plenty of power, no dead sitcks, not too hot after landing and can go into 4 stroke mode if desired.

I originally went away from OS when I had a problem with a dodgy 46FX, but now I am back.